Diatoms are the worst enemy of an aquarium. Division of diatoms

Reproduction of diatoms

Division. Most often, diatoms reproduce by vegetative cell division into two halves; this process usually occurs at night or at dawn. Rates of division vary among species and can vary even within the same species depending on season or environmental conditions. In spring and early summer, the maximum development of diatoms is observed as a result of their intensive division. The presence of nutrients in water promotes the division and growth of diatoms.

Experiments have shown that in a culture medium, some planktonic species can divide up to 3-8 times a day. Benthic species divide much less frequently - once every 4 days. There are known cases of even more rare division - once every 25 days. But this information is not absolute, and the rate of division may vary depending on the latitudinal location of the reservoir, its physicochemical regime and, of course, the characteristics of the species.

The process of cell division in diatoms is unique due to the presence of a hard shell. First, droplets of oil begin to accumulate in the protoplast, and the protoplast itself significantly increases in volume, as a result of which the epitheca and hypotheca of the carapace diverge, remaining connected only by the edges of their belt rims. The protoplast is divided into two equal parts, and with it the chloroplasts. If there is one chloroplast, then it is divided in half; if there are many of them, then half of them first enter the daughter cells, and then they divide, as a result of which the original number of chloroplasts is formed in the daughter cells. The nucleus divides mitotically, often with clearly visible chromosomes and a centrosome at each of the resulting poles. After the final division of the cell into two, each of the daughter cells, having received only half of the mother’s shell, immediately “completes” the missing half, but always the internal one, i.e. the hypotheca. Additional education shells - inserted rims, septa and other structural elements - appear soon after the formation of a new hypotheca. Thus, the two daughter cells resulting from division turn out to be dissimilar in size: one cell, which received an epitheca, retains the size of the mother cell, and the other, which received the mother hypotheca, which became an epitheca in the new cell, acquires smaller sizes. As a result, after repeated divisions, a gradual decrease in cell size occurs in half of each given population: in centric diatoms, the cell diameter decreases, and in pennate diatoms, the length and partly the width of the cells decreases. It has been established that in some species, during the process of division, cell sizes decrease by almost 3 times compared to the original ones.

Microspores. In many planktonic diatoms, so-called microspores are found - small bodies that arise in cells in quantities from 8 to 16 or more, and in some species there are more than 100 of them. Microspores with and without flagella, with chloroplasts and colorless ones have been observed. Microspores most often develop in species of the genus Chaetoceros, and even their germination has been observed (Fig. 90).

The process of microspore formation has not been studied cytologically, and their nature has not been precisely established.

Sexual process and formation of auxospores. Auxospores, i.e. “growing spores,” are those that, when formed, grow greatly and then grow into cells that differ sharply in size from the original ones. The ability to form auxospores is characteristic only of diatoms, but it has not yet been possible to fully explain this process and the reasons that give rise to it. The formation of auxospores is likely caused by various reasons. I agree with the most common opinion that it occurs as a result of multiple divisions, leading, as described above, to cell shrinkage. Having reached minimum sizes, the cells develop auxospores, which leads to restoration of their size. However, other researchers believe that auxospore formation is simply associated with cell aging, since it was often possible to observe it even when the cells had not yet reached their minimum size. From these positions, auxospore formation is considered as a process of “rejuvenation” of the cell. In addition, there are observations indicating the development of auxospores when environmental conditions change, for example, with a sharp drop in air or water temperature.

Whatever the reasons that contribute to the emergence of auxospores, the main thing has been established: auxospore formation is always associated with the sexual process. In diatoms, all three types of the sexual process generally known in algae occur - isogamous, anisogamous and oogamous, as well as some forms of the reduced sexual process (Fig. 91). In pennate diatoms, the sexual process in all cases consists of the bringing together of two cells, in each of which the valves move apart and reduction division of the nucleus occurs, after which the haploid nuclei fuse in pairs and one or two auxospores are formed. In centric diatoms, there is no pairwise approach of cells and an auxospore is formed from one cell, in which the maternal diploid nucleus first divides into four haploid nuclei, two of them are then reduced, and two merge into one diploid nucleus and an auxospore is formed.

All diatoms are diploid organisms, and they have a haploid phase only before the fusion of nuclei in the auxospore. In both the first and second cases, after the fusion of the nuclei, a zygote is formed, which immediately, without a resting stage, sharply increases in size and develops an auxospore. According to their position and connection with the mother cell, auxospores are classified as different types: free auxospore, terminal, lateral, intercalary and semi-intercalary (Fig. 92).

After the auxospore matures, a new cell develops in it, which first forms an epitheca and then a hypotheca. The first cell to emerge from an auxospore is called the initial cell. It is significantly larger in size than the original.

Dormant spores. The formation of resting spores is usually preceded by either abundant vegetation of the species or the onset of unfavorable conditions. The protoplast of the cell contracts, rounds, and on its surface first the primary spore valve appears, and then the secondary one, both tightly connected by their edges (they lack a girdle). The valves often differ in structural elements; they are covered with spines, processes and some other formations (Fig. 93, 94). Typically, diatoms develop only one spore per cell. After a certain time, the resting spore, like an auxospore, increases in volume and gives rise to a new cell, twice as large as the original one.

Resting spores are commonly produced by many marine neritic diatoms, as well as some freshwater species. In representatives of many genera they occur periodically as a normal occurrence in the life cycle.

Protoplast. The cytoplasm in diatom cells is located in a wall layer or accumulates in the center of the cell or at its poles. The remaining areas of the cell are filled with many vacuoles with cell sap, which sometimes merge into one large vacuole.

The nucleus is usually spherical and is most often located near the center of the cell in the cytoplasmic bridge or in the peripheral layer of the cytoplasm. In some diatoms it has H-shape. There are from 1 to 8 nucleoli in the nucleus.

Chloroplasts in diatoms are quite diverse in shape, size and number in the cell. In most centric diatoms they are small, numerous, in the form of grains or disks, lacking a pyrenoid. Less commonly, they are larger, one or several per cell, and have the shape of a plate with smooth or rugged lobed edges, with one or several pyrenoids. In pennate diatoms, chloroplasts are usually large, lamellar, often with lobed edges or with perforations; they are not numerous (one or two per cell), occupy almost the entire cavity, usually with pyrenoids. Their number, size and position vary even among representatives of the same genus (Table 10).

The color of chloroplasts in diatoms has various shades yellow-brown color depending on the set of pigments, among which brown ones predominate - carotene, xanthophyll and diatomine, masking chlorophylls a and c in a living cell. After the cell dies, the brown pigments dissolve in water and the green chlorophyll becomes clearly visible.

During the process of photosynthesis, diatoms produce oil in the form of droplets of various sizes, sometimes in significant quantities. It serves as a reserve nutrient, especially during periods when cell division stops or is delayed. The oil extracted from diatom cells has a fish oil odor. In addition to oil, some species are also characterized by the presence in the cells of droplets of volutin, which have a dull bluish sheen. Volutin is insoluble in ether and, when a living cell is stained with methylene blue, acquires a reddish-violet hue. Small droplets of volutin are distributed throughout the cytoplasm, and large droplets (Bütschli bodies) occupy a certain position at the ends of the cell (species of the family Nitzschiaceae) or on both sides of the central cytoplasmic bridge (genus of the family Naviculaceae). Leukosin is also found as a nutrient in diatom cells.

The shell and its structure. The shell of diatoms is produced by the cell itself during its life. It consists of two almost equal parts and is designed like a box, closed with a lid. The outer, larger part of the shell - the epitheca, like a lid, has its edges on the inner half - the hypotheca, corresponding to the box (Fig. 76). The epitheca and hypotheca consist of a valve and a girdle. The valve belonging to the epitheca is called epivalva, and the valve belonging to the hypotheca is called hypovalva. The valve has a front surface, flat or slightly convex, and a curved marginal part, called the valve bend, which sometimes differs in structure. The bend of the valve in some diatoms is low and rather weakly expressed; in others it is quite high and makes up a significant part of the lateral surface of the shell.

The valves come in a variety of shapes: round, elliptical, ovoid, rhombic, lanceolate, triangular, quadrangular, clavate, crescent-shaped, guitar-shaped, wedge-shaped, etc. The ends of the valves are variable and varied: beak-shaped, capitate, retracted, blunt, sharp, etc. (Fig. 77, 78).

Each valve is adjacent to a belt (connecting) rim, which is a wide or narrow ring bordering the bend of the valve, but not fused with it. The girdle rim of the epitheca, with its free edge, moves onto the girdle rim of the hypotheca and tightly covers it, but does not fuse with it. True, in representatives of some genera, search rims are formed only during cell division, and the epivalva and hypovalva are tightly connected to each other directly by the edges of the valve fold. In addition, in many diatoms, intercalary rims, from one to many, are formed between the valve bend and the search rim. Each new, younger insertion rim always appears between the fold of the sash and the previous rim. They are additional formations that differ not only in form, but also in structure. The shape of the inset rims is one of the characteristic features of the genus. They are collar-shaped, ring-shaped, semi-ring-shaped or consist of separate segments shaped like a trapezoid, rhombus or scale (Fig. 79).

The presence of intercalary rims in the shell is of great biological importance, since they contribute to an increase in cell volume and its growth.

The part of the carapace between the epivalva and the hypovalva, i.e., the girdle rim of the hypotheca and the girdle rim of the epitheca located on it, and if there are intercalated rims, are called the girdle of the carapace (Fig. 80).

The shape of the shell depends on the outline of the valve. It can be spherical, rod-shaped, saddle-shaped, in the form of an orange slice, a low or high cylinder, a parallelepiped or other geometric figure. On the girdle side it is usually rectangular in shape.

A characteristic feature of the shell is the geometric regularity of its structure, and therefore, to understand its shape, it is very important to take into account the relationship between the axes and planes of symmetry (Tables 11, 12).

With bilateral symmetry in pennate diatoms, several axes and planes of symmetry are determined, which divide the carapace into symmetrical halves. Three main mutually perpendicular axes of symmetry are known: longitudinal, transverse and central, passing through the center of the two shell valves.

The length of the longitudinal axis determines the length of the valve or shell, the length of the transverse axis determines its width, and the length of the central axis determines the height of the shell. In addition to the axes of symmetry, three planes of symmetry are distinguished: longitudinal, running along the shell perpendicular to the valves and dividing it into two equal halves; transverse, running across the carapace perpendicular to the longitudinal plane and to the valves and not always dividing the carapace into two equal halves (if the carapace is heteropole and not isopole, that is, if the ends of the valves are unequal); valve, perpendicular to the two previous ones, but parallel to the valves, that is, passing through the girdle of the shell (Fig. 81).

In centric diatoms, which have radial symmetry, the shell has only two axes and two planes of symmetry. One axis is the diameter of the sash, the other is the central one. A plane of symmetry passing through the center of the valve in any direction always divides the carapace into two equal parts; the second plane of symmetry is valve, like in pennate forms, running perpendicular to the first.

The shape of the valves and shell as a whole, as well as the relationship between the axes and planes of symmetry, are important in the taxonomy of diatoms. However, the main feature in constructing their system is the structure of the silica shell, which poses the greatest difficulty in studying. The structure of the shell, visible in light and electron microscopes, refers to its external and internal patterns, specific to different taxa. The structural elements on the valves of centric diatoms have a radial and tangential arrangement, while in pennates they have a bilateral, or transverse, arrangement, i.e. their structure is symmetrical with respect to the longitudinal and transverse axes. Less commonly, in representatives of some genera the shells are asymmetrical and do not have a single plane of symmetry, and sometimes the asymmetry is expressed only in the structure of the valves.

The main feature of the walls of the shell is that they are pierced by regularly repeating tiny holes - areoles, usually covered outside or inside with a thin perforated film, which received the Latin name velum” (Fig. 82).

Holes in the wall of the shell are necessary for communication of the cell protoplast with the environment. When studying diatoms in a light microscope, it seemed that some species had a structureless shell, and only the introduction into practice of algological studies of an electron microscope showed that these shells also have an extremely thin porous wall. The holes piercing the valve usually occupy 10-75% of its area, and the location of these holes and their number are specific to different genera and species. But there are also areas on the doors that are devoid of holes, for example the central, axial and lateral margins at the seam (see below) and some sculptural details of the structure. In centric diatoms, the areoles are located radially and tangentially; in pennates - in transverse rows, either parallel to each other, or slightly diverging towards the edges of the valve (radial rows) or, conversely, converging (convergent rows).

Sometimes the areoles are located in such a way that, in addition to transverse ones, longitudinal or mutually intersecting oblique rows are also formed (Table 11).

A notable feature of pennate diatoms is the presence of an axial field, which is a structureless narrow or wide strip along the longitudinal axis of the valve. In some diatoms, the axial field expands in the middle of the valve, forming a middle field, which can be round, rhombic, quadrangular, sometimes reaching the edges of the valve.

Most pennate-type diatoms are characterized by another feature - the presence of a suture, which is a short or long slit or two slits (suture branches) cutting through the valve wall and running along the valve from its ends to the middle. The structure of the seam is very different - from simple slit-like to the so-called canal-like. The primitive slit-like suture is represented by two short isolated slits that do not reach the middle of the valve. In representatives of some genera such a suture is located on only one valve, sometimes only at one of its ends, in others - on both valves. Well-developed slit-like suture, characteristic of algae from family naviculaceae(Naviculaceae), is represented by two long slits, or branches, of the suture, passing along both valves and connecting in the middle of each valve with a central node, and at the ends of the valve ending in terminal, or polar, nodes (Fig. 83). The seam slits in the thickness of the valve are crank-curved, so that in cross section they look like a lying letter V (Fig. 84). The suture gap that opens into the cell is called internal, and the gap that opens outward is called external. In the central node, both branches of the suture are connected to each other, ending here with a central pore, and at the ends of the valve with a terminal pore.

The central nodule is an internal thickening of the valve wall, a convexity on its inner surface, and the terminal nodules are the internal and external thickenings of the valve wall.

The most complex device has a so-called canal suture - a channel located in the fold of the valve wall. WITH external environment it communicates with a narrow gap, and opens into the cell cavity with a series of holes with silica partitions - fibulae. The channel-shaped seam is characteristic of algae epithemic family(Epithemiaceae), Nitzschiev(Nitzschiaceae) and Surirelaceae(Surirellaceae) (Fig. 85, 86). It also has a central node, but its position on the valve differs in representatives of different genera. In algae from kind of epitemia(Epithemia) branches of the canaloid suture connect at an angle and are close to the ventral edge; in species genus rhopalodia(Rhopalodia) suture stretches along the dorsal edge; from representatives sort of Nietzschian(Nitzschia) is located in the keel located along one of the edges of the valve (Fig. 87, 1, 2), and in species kind of denticle(Denticula) runs more or less eccentrically to the longitudinal axis of the valve. In algae from genera surirella(Surirella) and campylodiscus(Campylodiscus) the canaloid suture lies on the edge of the valve wing, which is located on the border with the bend of the valve and encircles it. Therefore, when examining the shell from the valve side, it is not visible (Fig. 87, 3, 4). And only from representatives genus Cylindrotheca(Cylindrotheca) suture spirally surrounds the carapace.

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The biological significance of the seam in the life of diatoms is very great: in addition to the communication of the cell protoplast with the external environment, with the help of the seam the cells move quite quickly along the substrate and in the water column. In phylogenetic terms, the appearance of a suture is a progressive feature; it is characteristic of younger species, which in modern seas and oceans make up over 70% of the total number of diatoms.

In addition to the above-mentioned structures, most diatoms have various formations on the external and internal surfaces of the valves in the form of hollow or solid outgrowths, bulges, horns, bristles, spines, spines, grooves, chambers, ribs, etc., which perform certain functions: secrete mucus , unite cells into colonies, increase the surface of the shell in planktonic species, providing cell buoyancy in water, etc.

In some diatoms that have elongated valves, silica septa or septa are formed on the inner surface of the inserted rims, protruding into the carapace cavity parallel to the plane of the valves. Septa appear either along the entire inner surface of the intercalary rim, or only at one of its ends. They are usually clearly visible from the side of the belt, vary in position, shape and size and have one or more holes. In diatoms with a heteropole carapace, septa most often appear only at its wide end (genus Licmophora), with an isopole carapace - at either or both ends (genus Tetracyclus, Tabellaria). The septa can be narrow, or they protrude deeply into the cavity of the shell, right up to its middle (Fig. 88).

In a small number of diatoms, another type of septum is formed, the so-called pseudosepta, which develop on inside the valve itself and protruding into the cavity of the shell in the form of a short and rather rough partition, visible from the valve and from the girdle. Unlike septa, pseudosepta are always perpendicular to the valve and arise simultaneously with it. Representatives genus Mastogloya(Mastogloia) are characterized by special formations - chambers representing polygonal, less often elongated voids in the wall of the shell, open into the cell or outward with round holes (Fig. 89).

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Latin name Bacillariophyta

- this is a completely special group of unicellular organisms, sharply different from other algae: the diatom cell is surrounded on the outside by a hard silica shell called the carapace. The shape of this shell is so varied, intricate and bizarre, and its structure is so thin, elegant and beautiful that it can be mistaken for the work of a skilled artist. Some diatoms can rival the beauty of their shells with the jewelry of an inventive craftsman.
The existence of diatoms first became known at the beginning of the 18th century, when Leeuwenhoek microscopes and a highly magnifying magnifying glass were introduced into research practice.
This group of algae has several scientific names: diatoms (Diatomeaea), silica (Kieselalgae) and bacillariophyta. They received the first name due to their reproduction by dividing the shell into two halves, the second is associated with the presence of a silica shell in the cell, and, finally, the last name was given by the first genus, scientifically described in 1788 - bacillaria, which means “rod-shaped”. In Russian literature, the name “diatoms” or “diatoms” has been established; the most modern scientific Latin name is Bacillariophyta.
Thanks to the transparent shell and brown color of the chloroplasts, diatoms are easy to distinguish under a microscope from other unicellular lower algae.
Diatoms are single-celled microscopic organisms that live alone or in colonies. various types: chains, threads, ribbons, stars, bushes or slimy films. Colonies are usually microscopic, but sometimes macroscopic and then visible to the naked eye. Cell sizes range from 4 to 1000 microns, and in some representatives - up to 2000 microns.
The study of diatom cells began at the beginning of the last century. With the help of a light microscope, even then it was possible to obtain a fairly complete and correct understanding of the structure of the cell and the structure of the shell. The introduction of electron microscopes - transmission and scanning - into the practice of algological research over the past 10-15 years has made it possible to significantly supplement our knowledge of the structure of cell organelles and the fine structure of the shell, many of the details of which were unknown.

Structure of Diatom Cells

The diatom cell consists of a protoplast surrounded by a silica shell called the carapace. The protoplast, with its outer compacted layer (plasmalemma), is closely adjacent to the shell and fills its internal cavities. There is no cellulose shell found in most algae. Chemical analysis of the shell showed that it consists of an amorphous form of silica, reminiscent of opal in composition, with a density of 2.07. The thickness of the walls of the shell depends on the concentration of silicon in the medium and varies within significant limits: in thin-walled forms - from hundredths to tenths of a micrometer, and in thick-walled forms it reaches 1-3 microns. The walls of the shell are pierced with tiny holes that ensure the exchange of substances between the protoplast and the environment. They are also equipped with various shaped elements that make up the structure of the shell and serve as the main taxonomic characters in constructing a system of diatoms. The shell and its structure are visible even at low microscope magnifications. According to the shape of the shell, all diatoms are divided into two groups: centric - with a radially symmetrical shell and pennate - with a bilaterally symmetrical shell.
Protoplast. The cytoplasm in diatom cells is located in a wall layer or accumulates in the center of the cell or at its poles. The remaining areas of the cell are filled with many vacuoles with cell sap, which sometimes merge into one large vacuole.
The nucleus is usually spherical and is most often located near the center of the cell in the cytoplasmic bridge or in the peripheral layer of the cytoplasm. In some diatoms it has an H-shape. There are from 1 to 8 nucleoli in the nucleus.
Chloroplasts in diatoms are quite diverse in shape, size and number in the cell. In most centric diatoms they are small, numerous, in the form of grains or disks, lacking a pyrenoid. Less commonly, they are larger, one or several per cell, and have the shape of a plate with smooth or rugged lobed edges, with one or several pyrenoids. In pennate diatoms, chloroplasts are usually large, lamellar, often with lobed edges or with perforations; they are not numerous (one or two per cell), occupy almost the entire cavity, usually with pyrenoids. Their number, size and position vary even among representatives of the same genus (Table 10).
The color of chloroplasts in diatoms has different shades of yellow-brown color depending on the set of pigments, among which brown pigments predominate - carotene, xanthophyll and diatomine, which mask chlorophylls a and c in a living cell. After the cell dies, the brown pigments dissolve in water and the green chlorophyll becomes clearly visible.

The color intensity of chloroplasts and their size are different and depend on the lifestyle of algae: in planktonic species they are golden-yellow, small, disc-shaped, and in bottom species and those attached to the substrate they are large lamellar, dark brown, therefore large accumulations of diatoms acquire a well-defined brown or dark brown color.
During the process of photosynthesis, diatoms produce oil in the form of droplets of various sizes, sometimes in significant quantities. It serves as a reserve nutrient, especially during periods when cell division stops or is delayed. The oil extracted from diatom cells has a fish oil odor. In addition to oil, some species are also characterized by the presence in the cells of droplets of volutin, which have a dull bluish sheen. Volutin is insoluble in ether and, when a living cell is stained with methylene blue, acquires a reddish-violet hue. Small droplets of volutin are distributed throughout the cytoplasm, and large droplets (Bütschli bodies) occupy a certain position at the ends of the cell (species of the family Nitzschiaceae) or on both sides of the central cytoplasmic bridge (genus of the family Naviculaceae). Leukosin is also found as a nutrient in diatom cells.
The shell and its structure. The shell of diatoms is produced by the cell itself during its life. It consists of two almost equal parts and is designed like a box closed with a lid. The outer, larger part of the shell - the epitheca, like a lid, has its edges on the inner half - the hypotheca, corresponding to the box (Fig. 76). The epitheca and hypotheca consist of a valve and a girdle. The valve belonging to the epitheca is called epivalva, and the valve belonging to the hypotheca is called hypovalva. The valve has a front surface, flat or slightly convex, and a curved marginal part, called the valve bend, which sometimes differs in structure. The bend of the valve in some diatoms is low and rather weakly expressed; in others it is quite high and makes up a significant part of the lateral surface of the shell.
The valves come in a variety of shapes: round, elliptical, ovoid, rhombic, lanceolate, triangular, quadrangular, clavate, crescent-shaped, guitar-shaped, wedge-shaped, etc. The ends of the valves are variable and varied: beak-shaped, capitate, retracted, blunt, sharp, etc.
Each valve is adjacent to a belt (connecting) rim, which is a wide or narrow ring bordering the bend of the valve, but not fused with it. The girdle rim of the epitheca, with its free edge, moves onto the girdle rim of the hypotheca and tightly covers it, but does not fuse with it. True, in representatives of some genera, zonular rims are formed only during cell division, and the epivalva and hypovalva are tightly connected to each other directly by the edges of the valve fold. In addition, in many diatoms, intercalary rims, from one to many, are formed between the valve bend and the girdle rim. Each new, younger insertion rim always appears between the fold of the sash and the previous rim. They are additional formations that differ not only in form, but also in structure. The shape of the inset rims is one of the characteristic features of the genus. They are collar-shaped, ring-shaped, semi-ring-shaped or consist of separate segments shaped like a trapezoid, rhombus or scale.
The presence of intercalary rims in the shell is of great biological importance, since they contribute to an increase in cell volume and its growth.
The part of the carapace between the epivalva and the hypovalva, i.e., the girdle rim of the hypotheca and the girdle rim of the epitheca located on it, and if there are intercalated rims, are called the girdle of the carapace.
The shape of the shell depends on the outline of the valve. It can be spherical, rod-shaped, saddle-shaped, in the form of an orange slice, a low or high cylinder, a parallelepiped or other geometric figure. On the girdle side it is usually rectangular in shape.
A characteristic feature of the shell is the geometric regularity of its structure, and therefore, to understand its shape, it is very important to take into account the relationship of axes and planes of symmetry.
With bilateral symmetry in pennate diatoms, several axes and planes of symmetry are determined, which divide the carapace into symmetrical halves. Three main mutually perpendicular axes of symmetry are known: longitudinal, transverse and central, passing through the center of the two shell valves.
The length of the longitudinal axis determines the length of the valve or shell, the length of the transverse axis determines its width, and the length of the central axis determines the height of the shell. In addition to the axes of symmetry, three planes of symmetry are distinguished: longitudinal, running along the shell perpendicular to the valves and dividing it into two equal halves; transverse, running across the carapace perpendicular to the longitudinal plane and to the valves and not always dividing the carapace into two equal halves (if the carapace is heteropole and not isopole, that is, if the ends of the valves are unequal); valve, perpendicular to the two previous ones, but parallel to the valves, i.e. passing through the belt of the shell.
In centric diatoms, which have radial symmetry, the shell has only two axes and two planes of symmetry. One axis is the diameter of the sash, the other is the central one. A plane of symmetry passing through the center of the valve in any direction always divides the carapace into two equal parts; the second plane of symmetry is valve, like in pennate forms, running perpendicular to the first.
The shape of the valves and shell as a whole, as well as the relationship between the axes and planes of symmetry, are important in the taxonomy of diatoms. However, the main feature in constructing their system is the structure of the silica shell, which poses the greatest difficulty in studying. The structure of the shell, visible in light and electron microscopes, refers to its external and internal patterns, specific to different taxa. The structural elements on the valves of centric diatoms have a radial and tangential arrangement, while in pennates they have a bilateral, or transverse, arrangement, i.e. their structure is symmetrical with respect to the longitudinal and transverse axes. Less commonly, in representatives of some genera the shells are asymmetrical and do not have a single plane of symmetry, and sometimes the asymmetry is expressed only in the structure of the valves.
The main feature of the walls of the shell is that they are pierced by regularly repeating tiny holes - areoles, usually covered outside or inside with a thin perforated film, which received the Latin name "velum". Holes in the wall of the shell are necessary for communication of the cell protoplast with the environment. When studying diatoms in a light microscope, it seemed that some species had a structureless shell, and only the introduction into practice of algological studies of an electron microscope showed that these shells also have an extremely thin porous wall. The holes piercing the valve usually occupy 10-75% of its area, and the location of these holes and their number are specific to different genera and species. But there are also areas on the doors that are devoid of holes, for example the central, axial and lateral margins at the seam (see below) and some sculptural details of the structure. In centric diatoms, the areoles are located radially and tangentially; in pennates - in transverse rows, either parallel to each other, or slightly diverging towards the edges of the valve (radial rows) or, conversely, converging (convergent rows).
Sometimes the areolas are arranged in such a way that, in addition to transverse ones, longitudinal or mutually intersecting oblique rows are also formed.
A notable feature of pennate diatoms is the presence of an axial field, which is a structureless narrow or wide strip along the longitudinal axis of the valve. In some diatoms, the axial field expands in the middle of the valve, forming a middle field, which can be round, rhombic, quadrangular, sometimes reaching the edges of the valve.
Most pennate-type diatoms are characterized by another feature - the presence of a suture, which is a short or long slit or two slits (suture branches) cutting through the valve wall and running along the valve from its ends to the middle. The structure of the seam is very different - from simple slit-like to the so-called canal-like. The primitive slit-like suture is represented by two short isolated slits that do not reach the middle of the valve. In representatives of some genera such a suture is located on only one valve, sometimes only at one of its ends, in others - on both valves. A well-developed slit-like suture, characteristic of algae from the naviculaceae family, is represented by two long slits, or branches, of the suture running along both valves and connecting in the middle of each valve with a central node, and ending at the ends of the valve with terminal, or polar, nodes ( Fig. 83). The seam slits in the thickness of the valve are crank-curved, so that in cross section they look like a lying letter V (Fig. 84). The suture gap that opens into the cell is called internal, and the gap that opens outward is called external. In the central node, both branches of the suture are connected to each other, ending here with a central pore, and at the ends of the valve with a terminal pore.
The central nodule is an internal thickening of the valve wall, a convexity on its inner surface, and the terminal nodules are the internal and external thickenings of the valve wall.
The most complex device has a so-called canal suture - a channel located in the fold of the valve wall. It communicates with the external environment through a narrow gap, and opens into the cell cavity with a series of holes with silica partitions - brooches. The channel-shaped suture is characteristic of algae of the epithemiaceae, Nitzschiaceae and Surirellaceae families. It also has a central node, but its position on the valve differs in representatives of different genera. In algae of the genus Epithemia, the branches of the canaloid suture are connected at an angle and close to the ventral edge; in species of the genus Rhopalodia, the suture stretches along the dorsal edge; in representatives of the genus Nitzschia it is located in the keel located along one of the edges of the valve, and in species of the genus Denticula it runs more or less eccentrically to the longitudinal axis of the valve. In algae from the genera Surirella and Campylodiscus, the canaloid suture lies on the edge of the valve wing, which is located on the border with the bend of the valve and encircles it. Therefore, when examining the shell from the valve side, it is not visible. And only in representatives of the genus Cylindrotheca the suture spirally surrounds the carapace.
The biological significance of the seam in the life of diatoms is very great: in addition to the communication of the cell protoplast with the external environment, with the help of the seam the cells move quite quickly along the substrate and in the water column. In phylogenetic terms, the appearance of a suture is a progressive feature; it is characteristic of younger species, which in modern seas and oceans make up over 70% of the total number of diatoms.
In addition to the above-mentioned structures, most diatoms have various formations on the external and internal surfaces of the valves in the form of hollow or solid outgrowths, bulges, horns, bristles, spines, spines, grooves, chambers, ribs, etc., which perform certain functions: secrete mucus , unite cells into colonies, increase the surface of the shell in planktonic species, providing cell buoyancy in water.
In some diatoms that have elongated valves, silica septa or septa are formed on the inner surface of the inserted rims, protruding into the carapace cavity parallel to the plane of the valves. Septa appear either along the entire inner surface of the intercalary rim, or only at one of its ends. They are usually clearly visible from the side of the belt, vary in position, shape and size and have one or more holes. In diatoms with a heteropole carapace, septa most often appear only at its wide end (genus Licmophora), with an isopole carapace - at either or both ends (genus Tetracyclus, Tabellaria). The septa can be narrow, or they protrude deeply into the cavity of the shell, right up to its middle.
In a small number of diatoms, another type of partition is formed, the so-called pseudosepta, developing on the inside of the valve itself and protruding into the cavity of the shell in the form of a short and rather rough partition, visible from the valve and from the girdle. Unlike septa, pseudosepta are always perpendicular to the valve and arise simultaneously with it. Representatives of the genus Mastogloia are characterized by special formations - chambers representing polygonal, less often elongated, voids in the wall of the shell, open into the cell or outward with round holes.
Some structural details are not visible in a light microscope, but are detected only at high magnifications using an electron microscope. All of the listed structures have a clear, regular shape and a certain number of elements per unit surface. Most of them perform a specific function, ensuring the adaptability of diatoms to living conditions.

Diatoms nutritional methods

Mostly photoautotrophic organisms that form organic matter during photosynthesis. Nine pigments were found in the chloroplasts of diatoms: chlorophylls a and c, - and -carotenes and five xanthophylls - fucoxanthin, diatoxanthin, neofucoxanthins A and B and diadinoxanthin. The composition and quantity of pigments is not constant and depends on the intensity of light, its quality, the content of nutrients in water, as well as on the age of the cell and the characteristics of its vital activity. The amount of chlorophyll decreases in old cells, and a lack of nitrogen and phosphorus sharply reduces the content of chlorophyll a. A lack of nutrients in water, even with high light intensity, leads to a decrease in the amount of pigments, and an abundance of nutrients, even with low light intensity, promotes their formation. The end product of photosynthesis is fats, not carbohydrates.
The intensity of photosynthesis per unit of biomass is not the same in planktonic and benthic diatoms. In benthic forms it is much higher, since their chloroplasts are larger and have a more intense color. In addition, in mobile forms, photosynthesis is more active than in immobile forms, and is significantly enhanced during the period of cell division. Conditions for photosynthesis at the water surface are quite close to those air environment, but change sharply as the algae descend to depth.
Planktonic diatoms that live in the pelagic zone of the seas can exist at a depth of 100 m or more with high water transparency. However, not only the intensity of illumination changes with depth, but also the quality of light due to different absorption of solar spectrum rays of different wavelengths, which affects different species differently.
Among planktonic and benthic diatoms, there are light-loving and shade-loving species that differ in the intensity of photosynthesis and the coefficient of solar energy utilization at the same radiation. In light-loving species, maximum photosynthesis occurs at noon, and in shade-loving species, in the morning and afternoon hours.
The study of diatoms in cultures revealed the great plasticity of diatoms in the assimilation of both mineral and organic substances.
Silicon plays a special role in the life of diatoms, which they need to build their shell. Its absorption occurs in accordance with the rhythm of cell division and depends on the chemical and physical properties of the environment. Diatom cell division occurs normally if there is at least 5 mg/l of silicon in the water, and when its content is about 0.5 mg/l, division stops.
Silicon is absorbed by diatoms in the form of silicic acid and organic compounds silicon The need for silicon in diatoms varies and depends on the habitat and physiological state of the cells. For example, benthic species with a thick-walled shell need more silicon compared to planktonic forms with a thin-walled shell. During the period of abundant reproduction, which usually occurs in the spring, and in some species in the fall, diatoms experience the greatest need for silicon: its insufficient content in water causes a slowdown in the rate of division and leads to a decrease in the thickness of the shell.
Besides inorganic substances, diatoms also require small amounts of organic matter to grow and develop. Vitamin B has a very good effect on them. A study of the organic nutrition of diatoms has shown that they need B vitamins more than other algae.

Some diatoms can generally switch from autotrophic to heterotrophic nutrition. There are even known forms with colorless chloroplasts or without them at all - these algae are already obligate heterotrophs.

Reproduction of Diatoms

Division. Most often, diatoms reproduce by vegetative cell division into two halves; this process usually occurs at night or at dawn. Rates of division vary among species and can vary even within the same species depending on season or environmental conditions. In spring and early summer, the maximum development of diatoms is observed as a result of their intensive division. The presence of nutrients in water promotes the division and growth of diatoms.
Experiments have shown that in a culture medium, some planktonic species can divide up to 3-8 times a day. Benthic species divide much less frequently - once every 4 days. There are known cases of even more rare division - once every 25 days. But this information is not absolute, and the rate of division may vary depending on the latitudinal location of the reservoir, its physicochemical regime and, of course, the characteristics of the species.
The process of cell division in diatoms is unique due to the presence of a hard shell. First, droplets of oil begin to accumulate in the protoplast, and the protoplast itself significantly increases in volume, as a result of which the epitheca and hypotheca of the carapace diverge, remaining connected only by the edges of their belt rims. The protoplast is divided into two equal parts, and with it the chloroplasts. If there is one chloroplast, then it is divided in half; if there are many of them, then half of them first enter the daughter cells, and then they divide, as a result of which the original number of chloroplasts is formed in the daughter cells. The nucleus divides mitotically, often with clearly visible chromosomes and a centrosome at each of the resulting poles. After the final division of the cell into two, each of the daughter cells, which received only half of the mother’s shell, immediately “completes” the missing half, but always the inner one, i.e. hypotheca. Additional shell formations - intercalated rims, septa and other structural elements - appear soon after the formation of a new hypotheca. Thus, the two daughter cells resulting from division turn out to be dissimilar in size: one cell, which received an epitheca, retains the size of the mother cell, and the other, which received the mother hypotheca, which became an epitheca in the new cell, acquires smaller sizes. As a result, after repeated divisions, a gradual decrease in cell size occurs in half of each given population: in centric diatoms, the cell diameter decreases, and in pennate diatoms, the length and partly the width of the cells decreases. It has been established that in some species, during the process of division, cell sizes decrease by almost 3 times compared to the original ones.
Microspores. In many planktonic diatoms, so-called microspores are found - small bodies that arise in cells in quantities from 8 to 16 or more, and in some species there are more than 100 of them. Microspores with and without flagella, with chloroplasts and colorless ones have been observed. Microspores most often develop in species of the genus Chaetoceros, and even their germination has been observed (Fig. 90).
The process of microspore formation has not been studied cytologically, and their nature has not been precisely established.
Sexual process and formation of auxospores. Auxospores, i.e. “growing spores” are those that, when formed, grow greatly and then germinate into cells that differ sharply in size from the original ones. The ability to form auxospores is characteristic only of diatoms, but it has not yet been possible to fully explain this process and the reasons that give rise to it. The formation of auxospores is likely caused by various reasons. According to the most common opinion, it occurs as a result of repeated divisions, leading, as described above, to cell flickering. Having reached a minimum size, the cells develop auxospores, which leads to restoration of their size. However, other researchers believe that auxospore formation is simply associated with cell aging, since it was often possible to observe it even when the cells had not yet reached their minimum size. From these positions, auxospore formation is considered as a process of “rejuvenation” of the cell. In addition, there are observations indicating the development of auxospores when environmental conditions change, for example, with a sharp drop in air or water temperature.
Whatever the reasons that contribute to the emergence of auxospores, the main thing has been established: auxospore formation is always associated with the sexual process. In diatoms, all three types of the sexual process generally known in algae occur - isogamous, anisogamous and oogamous, as well as some forms of the reduced sexual process (Fig. 91). In pennate diatoms, the sexual process in all cases consists of the bringing together of two cells, in each of which the valves move apart and reduction division of the nucleus occurs, after which the haploid nuclei fuse in pairs and one or two auxospores are formed. In centric diatoms, there is no pairwise approach of cells and an auxospore is formed from one cell, in which the maternal diploid nucleus first divides into four haploid nuclei, two of them are then reduced, and two merge into one diploid nucleus and an auxospore is formed.
All diatoms are diploid organisms, and they have a haploid phase only before the fusion of nuclei in the auxospore. In both the first and second cases, after the fusion of the nuclei, a zygote is formed, which immediately, without a resting stage, sharply increases in size and develops an auxospore. According to their position and connection with the mother cell, auxospores are of different types: free auxospore, terminal, lateral, intercalary and semi-intercalary.
After the auxospore matures, a new cell develops in it, which first forms an epitheca and then a hypotheca. The first cell to emerge from an auxospore is called the initial cell. It is significantly larger in size than the original.
Dormant spores. The formation of resting spores is usually preceded by either abundant vegetation of the species or the onset of unfavorable conditions. The protoplast of the cell contracts, rounds, and on its surface first the primary spore valve appears, and then the secondary one, both tightly connected by their edges (they lack a girdle). The valves often differ in structural elements; they are covered with spines, processes and some other formations. Typically, diatoms develop only one spore per cell. After a certain time, the resting spore, like an auxospore, increases in volume and gives rise to a new cell, twice as large as the original one.
Resting spores are commonly produced by many marine neritic diatoms, as well as some freshwater species. In representatives of many genera they occur periodically as a normal occurrence in the life cycle.

The department is called Diatoms (from the Greek. di - two, tome - cut, dissection), or Bacillaria ( bacillum– stick). Includes single-celled solitary or colonial organisms, almost always microscopic in size; forms visible to the naked eye and reaching 2–3 mm. The presence of a bivalve silica shell is characteristic. About 6–10 thousand species are known.

Cell structure.

Chloroplasts of diatoms of various shapes, usually wall-shaped, contain pigments - chlorophylls a and c, carotenes, fucoxanthins.

In centric diatoms the chloroplasts are numerous and small, while in pennate diatoms they are large and often lobed. The color of chloroplasts is brown, yellowish or golden. It is due to the fact that green pigments - chlorophylls - are masked by additional yellow-brown xanthophylls, of which fucoxanthin predominates. There may be one or several pyrenoids; they protrude beyond the chloroplast and are sometimes penetrated by thylakoids.

There are many drops of oil in the cytoplasm. Volutin occurs in the form of large drops with a characteristic blue sheen.

Mitochondria Diatoms have various shapes (spherical, oval, rod-shaped, filamentous). Golgi apparatus located next to the nucleus, it consists of several dictyosomes (up to 20), which contain from 4 to 12 cisternae.

On top of the plasma membrane in diatoms, a special cell cover is formed - a shell. It is composed of amorphous silica, similar in composition to opal, which is why diatoms are often called the “gems” of the seas (“opal” means “precious stone” in Sanskrit). In addition to silica, the shell contains an admixture of organic compounds and some metals. It is covered inside and out with a thin organic layer consisting of pectin substances. After the death of the algae, the contents of the cell are destroyed and disappear, while the siliceous skeleton of the shell remains unchanged. It does not rot.

The shell consists of two halves: the upper, larger - epithecus and lower smaller – hypotheca. The epitheca is put on the hypotheca like a lid on a box (Fig. 20). In turn, the epitheca consists of epivalYou(top flap) and epicingulum(girdle rim of epitheca); hypotheca consists of hypovalvae(lower flap) and hypocingulum(girdle rim of the hypotheca). Two search rims, overlapping each other, form a belt. In diatoms, two projections of the shell are distinguished and used for identification: the view from the valve and from the girdle. The valve is usually flat. Its curved edge is called the fold of the sash; it can be low or high. In some genera ( Melosira, Hyalodiscus) the shell valves are closed directly by the edges of the valve folds, and the girdle is formed during cell division.

The insertion rim appears between the edges of the fold of the valve and the belt rim (Fig. 23). There can be many inset rims, but the youngest will always be near the fold of the sash, and the oldest near the girdle. The significance of these rims is to ensure the growth of the shell and increase the volume of the cell. In a number of species, thin silicon incomplete partitions grow from the inner walls of the intercalary rims into the cell cavity - septa. They always have one or more holes and partition the cell into semi-insulated chambers, which increases the surface area of the cells.

There are two main types of valves: actinomorphic, through which three or more axes of symmetry can be drawn (such valves are characteristic of centric diatoms), as well as zygomorphic, through which no more than two axes of symmetry can be drawn (such valves are characteristic of pennate diatoms).

Rice. 23. Diatom Pinnularia(By:): A– view from the side of the belt; B– view from the seam side; IN- lengthwise cut; G– cross section; D– vegetative propagation; 1 – epitheca, 2 – hypotheca, 3 – suture, 4 – nodule, 5 – chromatophore, 6 – pyrenoids, 7 – cytoplasm, 8 – nucleus, 9 – vacuole, 10 – valve, 11 – belt

The shell of diatoms is permeated with perforations, which serve to communicate the cell protoplast with the external environment. Perforations on the sash occupy 10–75% of its area. They can be folded into rows that are visible as strokes. The strokes can be radial, parallel, convergent. The strength of the shell is given by thickenings protruding above the outer or inner surface of the valve, called ribs. On the surface of the valve, spines, bristles, protrusions, and spines are often formed, which are involved in the formation of colonies.

Some pennate diatoms have a suture system. Slit seam consists of a pair of longitudinal slits (seam branches), which located on the sash. Channel suture has the form of a tube located in the thickness of the valve, its comb-like thickening is the keel , or pterygoid outgrowth surrounding the valve along the edge - the wing . The channel-shaped suture communicates with the external environment through a thin slit, and with the internal cavity of the cell through holes. Diatom sutures provide communication between the protoplast and the external environment and also take part in movement.

The nucleus is large, contains 1–8 nucleoli; they disappear during mitosis. There are no centrioles. The center of microtubule organization is the plates (Fig. 24). They are located at the spindle poles. Spindle microtubules form outside the nucleus, then pass into the nucleus through destroyed sections of its shell; the nuclear membrane gradually disappears. Thus, in diatoms, mitosis is open. In the early stages, microtubules move from pole to pole. Chromosome kinetochores appear to be attached to pole microtubules. In anaphase, chromosomes move towards the poles; in late anaphase, the spindle lengthens. Cytokinesis is carried out due to the formation of the cleavage furrow by invagination of the cell membrane from the periphery to the center. Cytokinesis ends with the formation of the shells of daughter nuclei. The plane of cell division in diatoms always runs in a plane parallel to the valve.

Rice. 24. Mitosis in pennate diatoms (according to: S. Hoek van den et al., 1995): A – interphase, almost before prophase; B – prophase; B – metaphase spindle; G – anaphase spindle; 1 – microtubule center; 2 – core shell; 3 – prophase spindle; 4 – polar plate; 5 – Golgi apparatus; 6 – kinetochore; 7 – chromatid; 8 – chromosomal microtubule; 9 – interpolar microtubule

Movement.

Free-living bacillaria move along the substrate, as if crawling on the side of the valve. The movement occurs evenly or in jerks in the direction of the longitudinal axis of the cell, and alternately, now in one direction, then in the other, directly opposite to it. Diatoms that have a seam are capable of active gliding movement. This movement occurs at a speed of 0.2–25 µm/s. A number of hypotheses have been put forward regarding the mechanism of their movement. One hypothesis associates the movement of diatoms with the release of mucus, which includes fibrillar polysaccharides, through the seam. Transforming into cords that are thrown forward along the substrate, they ensure the movement of diatoms. It is believed that the proteins kinesin and/or dynein are the driving force that leads to the ejection of these strands. Another hypothesis relates the movement of diatoms to the friction of the cytoplasm circulating in the suture. The friction of the flowing cytoplasm against the substrate develops a motor force that moves the cell in the direction opposite to its flow. The third hypothesis explains the movement by alternately taking in water and throwing it out from the opposite end of the cell. The release of the water flow changes the distribution of hydrostatic pressure in the body of the algae, due to which the latter moves in the direction opposite to the flow.

Reproduction. In diatoms, vegetative and sexual reproduction occurs (isogamy, heterogamy and oogamy).

Sexual process in centric and some pennate diatoms oogamous .

One or two eggs; they are fertilized inside the oogonia or, less commonly, after they enter the water. After fertilization, a diploid zygote is formed, which develops into a growing auxospore. It is covered with a shell, which gradually acquires a structure characteristic of this species, turning into a vegetative cell. In most pennate diatoms, the sexual process is isogamous, but gametes lack flagella (Fig. 25, A). Before isogamy, meiosis occurs, resulting in the formation of 1–2 haploid gametes. Fusion occurs in such a way that the gamete from one cell crawls into another. Motile gametes can be considered male, and those remaining in place can be considered female.

Vegetative propagation. Most often, diatoms reproduce vegetatively, by dividing the cell in two. During division, both valves move apart until their edges touch each other, then the cellular contents are divided into two symmetrical halves in the plane of the girdle. Each of the two newly emerged cells produces the missing half of the shell (flap), pushing it into the old half, which it inherited. In free-living diatoms, young cells quickly separate and disperse, while in colonial diatoms they remain next to each other. Since, with further repetitions of the division process, new valves are always inserted into the old ones, and therefore smaller than them, then after a number of divisions the generation of diatoms must become significantly smaller. Cells, having reached a minimum of growth, return to their original size in various ways. First, the smaller of the resulting cells may no longer divide. Secondly, in some diatoms the shell belts are more elastic. Thirdly, it is possible to move apart parts of the shell and equalize the difference in size between the epitheca and hypotheca. Fourthly, the decrease in cell size in diatoms is contrasted with their increase as a result of the sexual process, through the formation of auxospores.

When an auxospore is formed, a copulation process occurs, leading to the formation of a zygospore. Two cells come closer to each other, shed their valves, their protoplasts merge together, are surrounded by a dense cellulose membrane, then increase in volume and turn into an auxospore, giving rise to a new individual, noticeably larger than its parents.

Reduction division in diatoms occurs before the formation of gametes, so vegetative individuals are diploid organisms, and the life cycle The diatom is diplobiont with gametic reduction.

Resting stages. When unfavorable conditions occur, diatoms can form spores and dormant cells. These structures are rich in reserve substances that will be required during germination. Resting cells are morphologically very close to vegetative cells, while the spore carapace becomes thicker, rounded, and its ornamentation changes. Dormant cells usually arise in an environment with a low content of dissolved silicon, while spores, on the contrary, require a sufficient amount of silicon to build their own thick membrane. Resting cells are formed more often by freshwater centric and pennate diatoms, while spores are formed by centric marine diatoms. Both resting cells and spores can survive for decades.

Rice. 25. Scheme of the sexual process and the formation of auxospores of pennate diatoms (A) - using an example Homphonemes and centric diatoms (B) using the example Melosirs(according to: L.L. Velikanov et al., 1981): 1 – sperm development; 2 – development of the egg; 3 – fertilization; 4 – formation of auxospore

WITHsystematics

The peculiar movement and silica shell gave reason more than once to classify the bacillaria as animals. It is now generally accepted that diatoms are algae. Most researchers classify them in a special department. It is believed that the Diatoms department includes about 6-12 thousand species, but some authors are convinced that the true number of diatom species can reach 1 million. In most systems, diatoms are considered in the rank of the Bacillariophyta department with two classes: Centric - Centrophyceae and Pennate - Pennatophyceae.

Class Pennate diatoms– Pennatophyceae

Pennate diatoms usually include mobile unicellular and colonial representatives, through the valve of which one or two axes of symmetry can be drawn; the valves have a suture. The sexual process is isogamous.

Genus Navicula has single cells, usually motile, less often enclosed in gelatinous sheaths, simple or branched, within which the cells retain mobility (Fig. 26, IN). The valves are longitudinally symmetrical, in shape from linear to elliptical. The structure of the valves consists of transverse strokes located parallel and radially. Two large lamellar chromatophores are located along the cell and adjacent to its girdle. The species are widespread in fresh and brackish waters, less common in the seas. Mainly benthic and, rarely, planktonic forms.

Rice. 26. Diatoms (by:): A – Pleurosigma; B – Cymbella; IN – Navicula; G – Synedra; D – Tabellaria; E – Diatom; AND – Meridion; Z – Cyclotella: 1 – seam, 2 – knot

Genus Diatom– cells are connected into ribbon-like and zigzag colonies, sometimes stellate (Fig. 26, E). The carapace from the girdle is linear, with right angles, inserted rims are sometimes present, septa are absent. The valves are elliptical to linear, the structure of the valves is made of transverse coarse ribs and delicate transverse strokes. The axial field is thread-like, barely noticeable. Rimoportulae are located at the ends of the valve. Chromatophores are small, granular, and numerous. Freshwater species, mainly benthic.

Genus Tabellaria– cells rectangular from the girdle, elliptical to elongated linear (Fig. 26, D, rice. 27, G). There are insert rims with septa. There is no seam. Freshwater epiphytes.

Genus Nietzsche– cells from rod-shaped to elliptical, straight, less often curved, solitary, very rarely connected into filamentous or branched colonies. The valves are linear, less often lanceolate and elliptical, with carinae and transverse striae. There is a canal-like suture in the keel. One lamellar chromatophore is located along the cell diagonally or adjacent to one of the girdle sides. Marine, brackish and freshwater, often benthic species (Appendix 3B).

Rice. 27. Seamless Diatoms: A – Synedra; B – Fragilaria; IN – Asterionella; G – Tabellaria: 1 – shell from the valve, 2 – shell from the girdle

Class Centric diatoms –Centrophyceae

Centric diatoms are unicellular and colonial forms, through the valve of which three or more axes of symmetry can be drawn, which lack active motility, do not have a seam on the shell, and an oogamous sexual process is observed. Species of centric diatoms are very widely represented in the plankton of the seas and oceans as one of the main producers of organic substances.

Genus Coscinodiscus– cells are disc-shaped, less often lens-shaped, always solitary. The carapace is usually rough, often with inset rims (Fig. 28, B). The valves are round, flat, convex, sometimes wavy. By

Rice. 28. Centric diatoms: A – Cyclotella; B – Coscinodiscus; IN – Melosira; G – Aulakozira; D – Chaetoceros from the belt, part of the chain: 1 – view from the sash, 2 – view from the belt

The edge of the valve often has a ring of small spines located at a certain angle. Chromatophores are numerous plates located over the entire surface of the cell. The nucleus is central, adjacent to one of the valves, less often suspended on cytoplasmic cords in the center of the cell. Marine species, mainly planktonic. They are often found in large numbers in desalinated marine areas and in inland brackish seas.

Genus Rizosoleniya has cylindrical cells. The cells are often very elongated in height, in the form of a long rod, straight, rarely slightly curved, solitary, less often connected into thread-like colonies. The cross-section of the carapace is from round to elliptical, very thin. There are numerous inset rims, ring-shaped, trapezoidal, rhombic and scale-shaped. The valves are in the form of a cap with an elongated apex, the end of which continues in the form of a spine, bristles or outgrowth. Chromatophores are numerous, granular or disc-shaped, often accumulate in the center of the cell around the nucleus. Marine, planktonic species (Appendix, 3B).

Genus Chaetoceros– cells are low-cylindrical, connected in chains, less often single (Fig. 28, D). The valves are elliptical, flat, convex or concave. One thin seta extends from the poles of the valve, with the help of which the cells are connected into colonies. In this case, the bristles of the valves of adjacent cells in the chain come into contact with one another, bend and cross, quite often merging at the point of contact. The terminal bristles of the chain usually differ from the others in length, thickness and direction. The openings between adjacent sashes in a chain are called windows, the outlines of which can be very different. Genus includes a large number of exclusively planktonic species, which make up the bulk of coastal phytoplankton of the seas and oceans.

Ecology and significance

Species of diatoms are widespread in nature, they are found in all kinds of biotopes. Diatoms live in marine, brackish and various fresh water bodies: both standing (lakes, ponds, swamps) and flowing (rivers, streams, irrigation canals). They are common in soil, isolated from air samples, and form rich communities in Arctic and Antarctic ice. Diatoms dominate other microscopic algae in aquatic ecosystems all year round. They are abundant both in plankton and in periphyton and benthos. In the plankton of the seas and oceans, centric forms of diatoms predominate, although some pennate forms are also mixed in with them. In the plankton of fresh water bodies, on the contrary, pennate diatoms predominate. Benthic (bottom) communities are also distinguished by a large variety and number of diatoms, which usually live at a depth of no more than 50 m. Benthic bacillaria crawl along the substrate or attach themselves using mucous legs, tubes, and pads.

Diatoms appear in large numbers in stagnant waters (ponds, puddles, ditches), forming yellow-brown mucous films at the bottom and near the shore. They are found in mineral springs, in water pipes, even in carafes of water, on the surface of wet rocks, stones, wood, earth (for example, in pots with plants), etc. Some types of epiphytic diatoms attach to marine and freshwater algae, sometimes in such abundance that the algae become simply unrecognizable. After water bodies dry out, diatoms can be picked up along with dust by the wind and transported to large areas. Many species are truly cosmopolitan and are found everywhere. The species composition of bacillaria in water bodies is determined by a complex of abiotic factors, of which great importance First of all, it has the salinity of the water. Types of childbirth Rhizosolenia, Skeletonema, Chaetoceros, Biddulphia, Schizonema, Striatella are typical marine ones. Marine forms are usually larger in size and have stronger shells. Most representatives of the genera live in fresh waters Cymbella, Fragilaria, Navicula, Gomphonema. No less important factors for the development of diatoms are temperature, degree of illumination and quality of light. Diatoms grow in the range of 0–70 0 C, but when dormant they are able to tolerate lower or higher temperatures.

The richest in terms of qualitative and quantitative composition of diatoms are the fouling communities and epibionts. Diatoms occupy a dominant position among epiphytes of higher plants and macroscopic algae in fresh water bodies and seas. Many marine animals can become fouled by diatoms, from crustaceans to whales. Such algae are called epizoites. Among diatoms there are also endosymbionts, which live in brown algae, foraminifera, etc.

Bacillaria are of great importance in nature. Making up a significant mass of phytoplankton, diatoms are the beginning of the food chain. They are eaten by both zooplankton organisms and juvenile and adult fish. Providing about a quarter of the planet's organic matter, they are the most important producers of organic matter in the world's oceans. Bacillaria play a major role in the cyclesilicon, annually absorbing about 3 billion tons from the World Ocean.

Bilinsky (city of Bilin in Bohemia) polishing slate, the deposits of which, 0.6–5 meters thick, became known earlier than others, all consist of the shells of diatoms, and the main mass is made up of only one species that is still living - Melosira distances. A large deposit of Luneburg heath reaches up to 13 m in thickness; it consists mainly of Synedra ulna. The deposit near Berlin reaches 30 m; part of the city itself stands on it. A similar deposit is located near Koenigsberg. All these deposits are of recent origin. But there are also much more ancient ones, dating back to the Tertiary era. For example, the city of Richmond in the United States of America is located on such a deposit. Diatoms are also found in amber (tertiary formation). In Russia, shale deposits have been found in the Ulyanovsk, Penza regions and in Siberia.

The most important role belongs to diatoms in sedimentation at the bottom of the ocean. The shells of planktonic diatoms, deposited after the death of algae at the bottom of marine and freshwater basins, formed thick deposits there diatomite(mountain flour) – a mass of white or light gray color, light, porous and hard. Diatomite consists of 50–80% shells of diatoms.

The thickness of the diatomite layer in some places reaches several hundred meters. Large deposits of diatomite in Russia are found in Tyumen, the Volga region, Primorsky Krai and a number of other places. Only in the Tyumen region, the diatomite deposits discovered in the last century amount to up to 500 × 10 12 m 3. It is believed that this discovery can be put on par with the discovery of oil and gas reserves beyond the Urals, since diatomite is a multi-purpose raw material. It serves as a source of about 100 different products, finding application as a material for the production of optical fiber glass, liquid glass, as a filtering agent in various industries, as a polishing and grinding material. It is also used as a buildingmaterial, and also for making dynamite.

Mountain flour serves as a good polishing agent (tripoli), and is used to prepare light bricks(Fabroni bricks), and sometimes glass; it goes further to make dynamite, for which it is mixed with nitroglycerin. Mountain flour in need eaten among the Laplanders, Tungus and other semi-wild tribes (edible land). Mountain flour was either eaten directly or mixed with regular flour and baked into bread. The nutritional value of mountain meal is, in all likelihood, due to the remains of organic substances that are still preserved in dead algae.

The shells of diatoms remain in a fossil state for a long time, so they used to determine the origin and age of various sedimentary rocks. Diatoms are of particular importance in environmental monitoring, since they serve as good org indicatorsnic pollution of the aquatic environment.

However, the massive development of some species of diatoms may also have negativenove value. Toxins have been found in a number of diatoms. So, representatives of the genera Pseudonitshia And Nitshia form domoic acid, which causes amnesic poisoningtion in humans and animals. This acid is soluble in water and insoluble in organic solvents. Domoic acid was first isolated from the macroscopic red alga Chondria (Japanese for “home” – hence the name of the acid). In 1987, more than 100 cases of human poisoning from this toxin after eating mussels were reported in Canada, and four of the victims died. Symptoms of poisoning include, in mild cases, nausea, vomiting, diarrhea; in severe cases, pain sensitivity disappears, hallucinations appear, and short-term memory is lost. Diatoms can clog river mouths and harbors, often cause "blooming" of water and are the reason for the appearance in it Notpleasant smells. By clogging the gills of animals, diatoms cause their death.

Control questions

Name the common ones characteristic features diatoms

The structure of the shell of diatoms.

What pigments and nutritional types are known in bacillaria?

How do diatoms reproduce? Life cycle of diatoms.

Give characteristics of diatoms of the pennate class.

Give characteristics of diatoms of the centric class.

Name typical representatives of pennate and centric diatoms.

In what habitats are diatoms found?

The importance of diatoms for natural ecosystems.

Economic importance of diatoms.

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Structural features

Only coccoids, the form is varied. Mostly solitary, less often colonial.

Veils

Diatoms are characterized by the presence of a special cover consisting of silica - a “shell”. In addition to silica, the shell contains small amounts of iron, aluminum, magnesium and organic substances. In marine planktonic diatoms, the shell substance contains 95.6% SiO 2 and 1.5% Al 2 O 3 or Fe 2 O 3. In rare cases (for example, Phaeodactylum tricornutum) there is no silica. The surface of the shell is covered with a thin pectin layer.

The structure and nature of ornamentation is an important feature for identifying diatom species; it is clearly visible when the protoplast is removed. The shells required for identification, freed from the organic parts of the cell, are obtained by calcination or washing in strong acids. They examine preparations of shells, enclosing them in a medium with a high refractive index - monobromonaphthalene, styrax, Kolbe's medium.

The shell consists of two halves, larger and smaller, fitting into each other like parts of a Petri dish. During division, the halves of the shell separate and new halves are formed in the cleavage furrow. In both daughter cells, the old half of the shell becomes larger (epithecium, see below), and the smaller one is rebuilt. At the same time, the size of cells in a series of divisions gradually decreases. Size restoration occurs during sexual reproduction or through the stage of spore formation.

According to the type of symmetry, a diatom cell, when viewed from the valve, can be:

radial (actinomorphic), this type of symmetry is characteristic of centric diatoms,
bilateral (zygomorphic), in pennate diatoms. More often, the ends of the valves are the same (isopole valves), sometimes the ends of the valves differ in shape (heteropole valves).

Before the group centric And pennate diatoms were considered in the rank of classes, distinguished on the basis of purely morphological characteristics.

There are also two additional types of symmetry:

trillissoid - in this case, the valve structures are located along arcs and radii of a circle, the center of which is located outside the cell (for example, at Eunotia) And
gonoid, with an angular valve (at Triceratium).

Terminology

When describing the shell, the following terminology is used:

Epitheca- the larger half of the shell, its “lid”, hypotheca- his smaller half. The surface of the epithecal valve is called epivalva, hypotheca - hypovalva. Girdle rim of the epitheca - epicingulum, hypotheca - hypocingulum. Both belt rims, nested inside each other, form belt. The image distinguishes view of the shell from the valve And view of the shell from the belt .

The valve is usually flat, its edge is called folding the sash. Between the belt rim and the fold of the valve, additional one or more insert bezels. The number of intercalary rims can increase as the cell grows; the youngest of them is located near the bend of the valve. Insert rims can be circular, collar or consist of several parts - semi-circular, diamond-shaped, scaly. Incomplete septa directed inward to the cell may develop on the intercalary rims - septa. Septa always have one or more openings.

Many pennate diatoms have the seam- a central slot running along the sash. The seam can be S-shaped. There may be thickening of the shell in the suture area: central node And polar nodules. Some pennate diatoms at the suture site have an area devoid of ornamentation - axial field. Here it can form false suture- longitudinal edge of the shell. Diatoms lacking a seam are called seamless .

Perforation

The connection of the protoplast with the external environment is ensured by perforations of the shell. Perforation may be absent only in certain areas of the shell and occupies from 10 to 75% of its area.

The layered structure of the shell contains a large number of tiny holes, which in turn lead to more tiny holes - such structures focus light on the chloroplasts.

Formation of the shell

When dividing, each daughter cell receives half the shell from the parent. The resulting half becomes an epitheca, and the cell completes the hypotheca anew. As a result of division, one of the cells retains the size of the parent one, while the second becomes smaller. The energy required to form the shell is obtained through aerobic respiration; The energy produced by photosynthesis is not directly used.

The presence of dissolved silica in the environment is absolutely necessary condition for dividing diatoms.

Silica in sea and fresh water

In water, silica is present in the form of silicic acid:

SiO 2 + 2H 2 O = Si(OH) 4

When the concentration of a solution increases at a pH less than 9 or when the pH of a saturated solution decreases, silicic acid precipitates as amorphous silica. Although silicon is one of the most abundant elements in the earth's crust, its availability to diatoms is limited by solubility. The average silicon content in sea water is about 6ppm. Marine diatoms quickly exhaust the reserves of dissolved silica in the surface layer of water, and this limits their further reproduction.

Silicon enters diatom cells in the form of Si(OH) 4 through silicic acid transport (SIT) proteins. How transport into the cell occurs is still not known, and there is no clear evidence whether it is active or passive (Curnow et al., 2012); Presumably, in marine diatoms it occurs in symport with sodium ions; in freshwater diatoms, it is possible that it also occurs with potassium ions. In marine species, Si(OH) 4 and Na + are transported in a 1:1 ratio. Several genes related to silicic acid transport have been discovered in different diatom species (GenBank). Germanium disrupts the transport of silicic acid in diatoms.

After the valve is formed, in a similar way, in its own silicalemma, a girdle and intercalary rims are formed.

Chloroplasts

The color of chloroplasts is brown, yellowish or golden. It is due to the fact that green chlorophylls are masked by additional carotenoids (brown pigment diatomine; β, ε - carotenes; xanthophylls: fucoxanthin, neofucoxanthin, diadinoxanthin, diatoxanthin). Most diatoms contain two forms of chlorophyll c: c 1 and c 2. Some forms have chlorophyll c 1 can be replaced by chlorophyll c 3 (also found in Prymnesiophytes and Pelagophyceae). Some species may have all three forms of chlorophyll. c, while others have only one form.

Other structures

Most of the diatom cell is occupied by a vacuole with cell sap; the cytoplasm occupies a wall position. In addition, the cytoplasm accumulates in the center of the cell in the form of a cytoplasmic bridge connected to the peripheral layer of the cytoplasm. The bridge contains the core. There are many drops of oil in the cytoplasm. Volutin is found in it in the form of large drops with a characteristic blue sheen. Chrysolaminerin is present.

Life cycle

Vegetative propagation

Vegetative reproduction of diatoms occurs through simple mitotic division. Cytokinesis has a number of features associated with the presence of a shell (see). Since the half of the shell received from the parent cell becomes an epitheca in the daughter cell, and the hypotheca is completed anew, the dimensions of one of the cells remain equal to the parent, and the second becomes smaller. In a series of successive divisions, the size of cells in the population decreases, and the initial maximum dimensions are restored during the process of sexual reproduction associated with the formation of auxospores. Auxospores can arise autogamously due to the fusion of two haploid nuclei of one cell or apogamously (from vegetative cells). In rare cases, it is possible for the cytoplasm to escape from the shell and form it anew - vegetative enlargement.

Spores and resting cells

When unfavorable conditions occur, some diatoms can form spores and resting cells. These structures are rich in reserve substances that will be required during germination. Resting cells are morphologically close to vegetative cells, while the spore carapace becomes thicker, rounded, and its ornamentation changes. Dormant cells can arise in conditions with a low content of dissolved silicon, while spores, on the contrary, require a sufficient amount of silicon to build their own thick shell. Resting cells are formed more often by freshwater centric and pennate diatoms, while spores are formed by centric marine diatoms. Both resting cells and spores can survive for decades. When they germinate, two mitoses with nuclear degeneration are required to form a normal shell. Marine diatom spores play important role in the transport of organic carbon and silicon into sediments.

When spores form, the cell loses vacuoles, and the size of the spore is smaller than the original cell.

Sexual process

Movement

Many suture pennate and some centric diatoms are capable of crawling along the substrate.

Ecology

Diatoms are widely distributed in all kinds of biotopes. They live in oceans, seas, brackish and various fresh water bodies: standing (lakes, ponds, swamps, etc.) and flowing (rivers, streams, irrigation canals, etc.). They are common in soil, isolated from air samples, and form rich communities in Arctic and Antarctic ice. Such a wide distribution of diatoms is due to their plasticity in relation to various environmental factors and at the same time the existence of species narrowly adapted to the extreme values of these factors.

Diatoms dominate other microscopic algae in aquatic ecosystems year-round. They are abundant both in plankton and in periphyton and benthos. The plankton of the seas and oceans is dominated by centric diatoms, although some pennate diatoms are also mixed in with them. In the plankton of fresh water bodies, on the contrary, pennates predominate. Benthic cenoses are also distinguished by a large variety and number of diatoms, which usually live at a depth of no more than 50 m. The life of benthic diatoms is necessarily associated with the substrate: they crawl along the substrate or attach to it with the help of mucous legs, tubes, and pads.

The fouling cenoses are richest in the qualitative and quantitative composition of diatoms. Diatoms occupy a dominant position among the fouling of higher plants and macroscopic algae in fresh water bodies and seas. Many animals (such algae are called epizoonts) can be subject to fouling, from crustaceans to whales. Among diatoms there are also endobionts that live in other organisms, for example in brown algae and foraminifera.

The species composition of diatoms in water bodies is determined by a complex of abiotic factors, of which water salinity is of great importance. No less important factors for the development of diatoms are temperature, degree of illumination and quality of light. Diatoms grow in the range 0-70 and archaea.

Phylogeny

The valves of diatoms do not dissolve in most natural waters, so they have been deposited over the last 150 million years, since the Early Cretaceous. Thus, there is reason to believe that diatoms appeared before the Cretaceous period. The most ancient diatom fossils were centric, while the oldest pennates were seamless from the Late Cretaceous (about 70 million years ago). The remains of suture diatoms are of a later age. According to fossil remains, freshwater diatoms appeared about 60 million years ago and reached their peak in the Miocene (24 million years ago). Paleontological evidence confirms the presence of more primitive characters in the organization of the centric diatoms as an ancient group, while the sutured pennates represent the pinnacle of the evolution of this group. Using molecular biology methods, it was shown that diatoms are a monophyletic group, but within this group, centric diatoms do not form a monophyletic group, as previously thought.

The presence of tripartite mastigonemes on the flagellum, the structure of chloroplasts, pigment systems, tubular mitochondria, reserve products - all this confirms the undoubted belonging of diatoms to the ochrophyte group. Most often, the question of their proximity to other classes of this division is discussed, since the presence of such features as a silica shell, diplobiont life cycle, reduction of the flagellar apparatus, and features of karyo- and cytokinesis significantly distinguish diatoms from other representatives of ochrophytes. It was assumed that the ancestors of diatoms could have been some ancient sinuridae. Some authors even considered Sinuridae as "flagellated diatoms". However, molecular biology data show that among the straminopiles, diatoms form a fairly separate group, which is further separated from other ochrophyte algae than they themselves are separated from each other, but still closer to ochrophytes than to fungal-like protists. Nucleotide sequence analysis of SSU rDNA genes rbc.

Taxonomy

It is believed that the class of diatoms includes about 300 genera, including 20-25 thousand species, but some authors are convinced that the true number of diatom species can reach 200 thousand. The largest genus, consisting of more than 10 thousand species, is Navicula ( Navicula).

Currently there is no established system of diatoms. In most works that concern the study of diatom floras, taxonomy and classification, the class of diatoms is considered at the rank of department ( Bacillariophyta) with three classes ( Coscinophyceae, Fragilariophyceae, Bacillariophyceae). However, the use of molecular biology methods has shown that Coscinophyceae And Fragilariophyceae- paraphyletic groups and requires further revision of the diatom system.