Phycology - Kostikov I.Yu. - 2009-2013

Chapter 17. Diatoms – Bacillariophyta

The phylum of diatoms comprises over 20,000 species¹, ranking second only to green Algae in terms of diversity. Diatoms emerged relatively recently—approximately 130 million years ago during the Cretaceous period of the Phanerozoic, almost concurrently with flowering plants. Currently, diatoms are in a state of biological progress and represent the dominant group in both marine and freshwater habitats.

All diatoms are coccoid unicellular or colonial organisms, ranging in size from 4-2000 µm, with average values between 20-50 µm.

Diatoms belong to the tubular-cristate organisms within the phylum Stramenopiles, possessing secondary endosymbiotic Plastids derived from rhodoplasts. The most striking and specific feature of diatoms is the presence of a siliceous frustule.

Class="center">Taxonomic CHARACTERISTICS OF THE phylum

Pigments and reserve nutrients

Photosynthetic Pigments in diatoms include chlorophylls a and c, along with β- and ε-carotenes. Among xanthophylls, the yellow pigment fucoxanthin is dominant, imparting a brown color to the plastids; diatoxanthin, diadinoxanthin, and neoxanthin are present in smaller amounts.

The primary assimilation product is chrysolaminarin, with oil and volutin serving as secondary reserves. These products accumulate either directly in the Cytoplasm or within vacuoles.

Cytological characteristics

Cell coverings in diatoms are highly specific, consisting of a mineral frustule located exterior to the Plasmalemma. Chemically, the diatom frustule is similar to opal, composed primarily of silica (SiO₂ * nH₂O). Additionally, the frustule incorporates salts of aluminum, iron, and magnesium, as well as minor amounts of Proteins. The specific gravity of the frustule averages 2.07. Interestingly, when the concentration of silica in the surrounding environment drops below 0.5 mg/L, Cells become incapable of forming a frustule and rapidly perish. Furthermore, certain metals (particularly germanium) can inhibit frustule formation.

The frustule in all diatoms consists of two halves: a larger one, the epitheca, and a smaller one, the hypotheca (Fig. 17.1). Each half of the frustule is composed of a valve and a girdle band (cingulum). The margins of the valve are bent at a more or less right angle, forming a region known as the valve mantle. The girdle band closely attaches by one side to the valve margin in this region. The girdle band of the epitheca slips tightly over the girdle band of the hypotheca, giving the frustule the appearance of a box (the hypotheca) fitted with a lid (the epitheca). The region where the girdle bands of the epitheca and hypotheca overlap is called the girdle. Accordingly, the diatom frustule can be viewed in two orientations: in valve view and in girdle view. The frustule exhibits a rather complex Structure, which is specific to each genus and species of diatoms.

Fig. 17.1. Cytology/cytology/92.html">SCHEMATIC STRUCTURE OF a diatom frustule. A - main Structural elements of the frustule; B, C - frustule in different planes (B - valve view; C - girdle view). 1 - epitheca; 2 - hypotheca; 3 - valve of the epitheca (epivalve); 4 - valve of the hypotheca (hypovalve); 5, 6 - girdle bands (5 - epicingulum, 6 - hypocingulum).

The shape of the diatom frustule is quite diverse and is characterized primarily by the Symmetry type of the valve. If multiple axes of symmetry can be drawn through the valve, it is termed radially symmetrical or actinomorphic (Fig. 17.2). When only two, one, or no axes of symmetry can be drawn, the valve is called zygomorphic. Zygomorphic Valves can be symmetrical in both longitudinal and transverse directions (bisymmetrical), symmetrical along only one axis—either longitudinal or transverse (monosymmetrical valves), mirror-symmetrical (S-shaped), or asymmetrical. In girdle view, diatom frustules are usually (though not always) more or less rectangular.

The valve and its main structural elements. The valve possesses homogeneous regions known as hyaline areas, a system of perforations through which the protoplast interacts with the external environment (areolae, raphes, etc.), and a system of internal and external frustule outgrowths and thickenings (costae, processes, etc.).

Areas. Large hyaline areas are called areas (or fields). If an area is located in the center of the valve, it is termed central. An area extending from one pole of the valve to the opposite pole is called an axial area.

Perforations. The most common type of perforation is the areola. An areola is a small pore in the frustule closed on one side by a thin, perforated silica plate called the velum. The velum may be located on either the outer or the inner side of the valve. Occasionally, the velum closes the areola on both sides. The open side of the areola, lacking a velum, is called the foramen. Under light Microscopy, areolae appear as tiny dots. When such dots are arranged in dense, regular rows, the dot system takes on the appearance of a stria. Depending on their orientation relative to the valve margin, striae are classified as parallel, radial, or convergent (Fig. 17.2).

Fig. 17.2. Types of diatom valves: A, B - actinomorphic; C - bisymmetrical; D, E - monosymmetrical; F - mirror-symmetrical (A - Cyclostephanos dubius, B - Cyclotella meneghiniana, C - Navicula trivialis, D - Gomphonema affine, E - Cymbella tumida, F - Gyrosigma acuminatum).

Algae with a zygomorphic frustule very often possess a raphe. Cytoplasm circulates within the raphe, enabling those diatoms that possess a raphe to exhibit active gliding motility. Raphes are subdivided into slit-like and canal-like types.

Under a Light Microscope, a slit-like raphe appears as a distinct line originating at The Cell poles, extending along the axial area, and interrupting in the central region of the valve—within the zone of the central area. The section of the raphe from the pole to the central area is called a raphe branch; thus, a slit-like raphe has two branches. Within the thickness of the valve in the central area zone, each raphe branch terminates in a vertical canal. The canals of both branches, in turn, are connected by a single horizontal canal. This entire system of two vertical and one connecting horizontal canal is located within a valve thickening called the central nodule. At the poles, the external raphe fissure widens and curves, forming the polar fissure. The valve thickening enclosing the polar fissure is termed the polar nodule.

A canal-like raphe appears as a continuous line running uninterrupted from one cell pole to the other. This type of raphe consists of a narrow external fissure and a tubular canal that is analogous to the internal fissure of a slit-like raphe. The tubular canal opens into the cell cavity via prominent pores known as fibulae. Unlike the slit-like raphe, which is typically positioned along the valve axis, the canal-like raphe is generally located along the valve margin within its internal thickening (the keel) or within an external outgrowth (the ala or wing).

Valve thickenings. In a significant number of diatoms, the valve features a system of thickened regions that give the valve surface a washboard-like appearance. These thickenings are called costae (or Ribs). In valve view, costae somewhat resemble striae; however, the distinction between costae and perforations becomes clearly apparent when examining the frustule in girdle view.

In addition to costae, the valve may bear other thickenings, such as: strutted processes (fultoportulae), labiate processes (rimoportulae), short spines, and long setae, etc.

Girdle bands of the frustule are always paired: one band (the epitinctum/epicingulum) connects to the margin of the epitheca valve, and the second (hypocinctum/hypocingulum) connects to the margin of the hypotheca valve. The girdle band of the epitheca, on the side opposite to the valve margin, always overlaps the girdle band of the hypotheca, forming the junction zone between the epitheca and hypotheca, known as the girdle.

The girdle band of each half of the frustule can be simple or complex. A simple band consists of only a single ring-closed strip. This type of band is the most common among diatoms. Complex bands occur in a significantly smaller number of species. They are formed by several successively nested ring-shaped strips. The ring directly adjacent to the valve is called the valvocopula, or valve ring. The ring connected to the valvocopula, as well as all subsequently attached rings of the girdle band, are called copulae or intercalary bands. The intercalary bands of the frustule halves, through which the epitheca is connected to the hypotheca, are called connecting bands.

Intercalary bands are most commonly continuous; less frequently, each band is formed by two interconnected semi-rings or consists of numerous densely interconnected segments.

In some diatoms, the girdle band can grow into the cell cavity, forming an incomplete partition—a septum—located parallel to the valve.

The plasmalemma is tightly pressed against the inner side of the frustule.

Many species of diatoms are capable of secreting mucilage. Using mucilage, the algae form mucous tubes (e.g., Cymbella), various mucous stalks (Rhoicosphenia, Gomphonema), and aggregate into colonies (Tabellaria, Fragilaria). In algae possessing a raphe, mucilage plays an extraordinary role in the active gliding movement of the cell. Furthermore, during the sexual process, mucilage facilitates the approach of cells and holds them together until their cytoplasms fuse—plasmogamy.

The nuclear apparatus of diatoms is typically eukaryotic. In interphase, there is always a single nucleus located in the center of the cell. Like in other stramenopile algae, The Nucleus is structurally linked to the chloroplast envelope, as the outer nuclear membrane is continuous with the outer membrane of the chloroplast Endoplasmic reticulum. Microtubular parts of tripartite flagellar mastigonemes (retronemes) may form between the nuclear envelope membranes, but due to the absence of flagellated stages in many diatoms, this process is rarely observed. Typically, 1–2 Golgi complexes are located near the nuclear surface.

Mitosis in diatoms is open, and centrioles are absent. Instead, during interphase on the lateral side of the nucleus, nearly adjacent to the girdle, There is a small body tightly pressed against the nuclear envelope, which Functions as the microtubule-organizing center (MTOC).

In prophase, a small layered body appears near the MTOC, through which the MTOC presumably divides into two subunits, with each subunit acquiring a disk-like shape. Subsequently, spindle microtubules form between the disks, while the disks themselves migrate to opposite poles of the nucleus and position themselves near the valve surface: one beneath the epitheca, and the other beneath the hypotheca. In late prophase, the nuclear envelope breaks down, and the spindle microtubules move into the nuclear zone.

In metaphase, the spindle differentiates into the so-called central and radial spindles. The central spindle passes through the center of the nuclear zone and connects the two disks. The microtubules of the central spindle are numerous, arranged circumferentially, and form two overlapping semi-cylinders. Each semi-cylinder is attached at its terminal part to the disk. The radial spindle consists of individual, relatively few microtubules that are attached by one end to the disk and by the opposite end to a chromosome.

In anaphase, the microtubules of the central spindle elongate in the zone of semi-cylinder overlap, pushing the disks with their radial spindle microtubules toward opposite valves. Along with the radial spindle microtubules, the Chromosomes also segregate toward the opposite valves.

In telophase, the central spindle disassembles, the chromosomes detach from the radial spindle fibers, and the nuclear envelope reforms. In a plane parallel to the valves, the plasmalemma forms a Cleavage furrow that divides the mother cell into two daughter cells, with each cell receiving one disk bearing a bundle of chromosome-free radial microtubules.

Next, each polar disk, along with its microtubules, migrates to the opposite side of the nucleus—toward The surface of the future new valve. At this time, the Golgi complex and endoplasmic reticulum begin to pinch off A large number of membrane vesicles filled with silica. These vesicles migrate to the plasmalemma, beneath which they begin to fuse with one another, forming the silicalemma—a large single-membrane cisterna where The formation of the new valve takes place. The movement and ordered fusion of silica-containing vesicles with the silicalemma are "guided" by the microtubules left over from the radial spindle.

Thus, in diatoms, the Formation of the siliceous frustule is closely linked to the nuclear apparatus, specifically to the microtubule-organizing center. It is also noteworthy that nuclear division is always preceded by chloroplast division, which predominantly occurs in late interphase.

The photosynthetic apparatus of diatoms consists of secondary-symbiotic plastids of the red algal (rhodophytan) type. METABOLISM/14.html">Chloroplasts are predominantly parietal in position, and their number and shape are specific to different genera. Depending on illumination, chloroplasts can slightly alter their position, moving deeper into the cell or shifting toward its surface. This mobility is enabled by contractile microfilaments that connect the outer membrane of the plastid to the plasmalemma.

The chloroplast envelope possesses four membranes, of which the two outer ones form the chloroplast endoplasmic reticulum, which is continuous with the nuclear envelope. A periplastidal space is situated between the outer and inner chloroplast membranes (Fig.).

Thylakoids are grouped in threes to form lamellae; a girdle lamella runs directly beneath the chloroplast envelope. Quite frequently, the chloroplast contains a naked pyrenoid, which, unlike in many other stramenopile algae, may be traversed by pairs of thylakoids. At the poles of the plastids, a ring-closed chloroplast DNA molecule is compactly localized, containing approximately 120–130 kbp of nucleotide pairs.

Mitochondria are usually several in number. They are located in the peripheral region of the cells and possess exclusively tubular cristae.

The vacuolar apparatus comprises 4 types of vacuoles: those containing cell sap, volutin, chrysolaminarin, and oil. In algae with actinomorphic frustules, cell sap vacuoles are generally small and numerous. If the frustule is zygomorphic, there are most commonly two such vacuoles. They are positioned along the longitudinal axis of the cell, displacing the cytoplasm toward the periphery, although a cytoplasmic bridge housing the nucleus remains in the center of the cell. Volutin vacuoles—Bütschli bodies—serve as a reserve depot for polyphosphates. They exhibit a pale blue coloration, and their position is specific to various diatom families.

The flagellar apparatus. Motile stages are represented exclusively by naked, uniflagellate spermatozoa and are known in only a few diatom genera. The flagellum is covered with retronemes produced within the chloroplast Endoplasmic reticulum and the perinuclear space of the nuclear envelope. The axoneme is atypical, as it comprises only nine peripheral doublet microtubules, while the central pair of microtubules is absent (9+0 formula). There are also differences in The structure of the basal body: instead of triplets, it is formed by doublet microtubules and, similarly to dictyochophyte algae (see Chapter 18), is pressed against the nuclear membrane.

Locomotion. Three fundamentally Different types of movement have been identified in diatoms: a) flagellar movement, performed exclusively by spermatozoa in certain centric diatoms; b) amoeboid movements of gamete protoplasts during the sexual process; and c) gliding movement in vegetative cells of species that possess developed raphes. The mechanisms of the first Two Types of movement are analogous to those found in many other unicellular algae, Protozoans, and fungus-like organisms. The Mechanism of gliding movement is unique.

Gliding movement of vegetative cells takes place through the participation of mucilaginous fibrils secreted via the external slit of the raphe. In this process, one end of the mucilage fibril attaches to the substrate, while the opposite end remains connected to transmembrane structures embedded in the plasmalemma within the raphe zone. These transmembrane structures comprise a protein and an ATPase, and are in turn associated with bundles of contractile Actin microfilaments located beneath the plasmalemma along the raphe. Near the middle part of the raphe branch, adjacent to the transmembrane structures, submicroscopic vesicles containing fibrillar material of a polysaccharide nature, known as crystalloid bodies, are situated.

During movement, the Contents of the crystalloid bodies are extruded into the raphe slit near the central nodule, condensed, and structured, transforming into a mucilage fibril composed of acidic mucopolysaccharides. Calcium is required for the attachment of the mucilage fibril to the substrate. By means of contractile microfilaments, the transmembrane structures with their attached mucilage fibrils are pulled along the plasmalemma parallel to the raphe. Consequently, the cell moves in a direction opposite to the Displacement of the mucilage fibril. When the fibril reaches the polar fissure, it detaches from the plasmalemma and remains on the substrate. Using methylene blue, the tracks of cell movement can be reconstructed at high microscopic magnifications based on THE POSITION OF the detached mucilage fibrils left behind on the substrate.

The shapes of the tracks correspond to the Morphology of the raphe: in algae with a straight raphe (e.g., Navicula, Pinnularia), the tracks are straight; with a curved raphe (Nitzschia), they are broad and arcuate; and with an S-shaped raphe (Gyrosigma), they are sigmoid. In monoraphid algae with concave valves (Cocconeis), the tracks are finely wavy. Specific movements are performed by raphe-bearing algae attached to the substrate by mucilaginous stalks: for example, Gomphonema periodically spins rapidly around its stalk.

The direction of movement can be governed by both phototaxis and chemotaxis, although the direct photo- and chemoreceptors remain unidentified. Locomotion in diatoms is reversible: during movement, mucilaginous fibrils are alternately secreted by different Branches of the raphe, causing the cell to move forwards and backwards, with the track in one direction being longer than the other.

Types of morphological body structures

Diatoms exhibit exclusively a coccoid type of morphological body structure, although in some cases, filamentous colonies of diatoms may be mistakenly identified as filamentous forms (e.g., Melosira).

Reproduction and life cycles

Representatives of the division reproduce via vegetative Cell Division (binary fission) and sexually. All diatoms share a single type of life cycle—a diplontic cycle with gametic reduction, without ALTERNATION OF GENERATIONS, and featuring a specific growth stage known as the auxospore.

Vegetative division (binary fission) is the primary mode of reproduction and is characteristic of all diatom species. Prior to division, the cell swells slightly due to lipid accumulation or active Water uptake. As a result, the epitheca and hypotheca begin to push apart. Next, the chloroplasts divide, and mitosis occurs. Following mitosis, each daughter cell inherits one half of the parent frustule and then builds the second half using the silicalemma. The maternal half always becomes the epitheca of the daughter cell because the new valve, formed within the silicalemma, is constrained by the edges of the cingulum (girdle band) inherited from the parent cell.

Upon completion of valve formation, silica-bearing vesicles migrate to the zone located beneath the girdle band of the maternal frustule half and form the valvocopula of the hypotheca. The valvocopula joins with the hypothecal valve, completing the formation of the missing frustule half.

Thus, as a result of vegetative binary fission, the cell that inherits the maternal epitheca retains the size of the parent cell. In the second cell, the frustule half that functioned as the hypotheca in the parent cell becomes the epitheca. In other words, the size of the second cell decreases. Since silica is rigid and cannot expand, cell sizes within a population would theoretically decrease rapidly. However, this does not actually happen because diatoms possess specific mechanisms for maintaining constant cell size. Among the most prominent are mechanisms for restoring optimal size through the auxospore stage, predominantly as a result of the sexual process.

An auxospore (or growing spore) is a specialized cell capable of growth. The cell that transforms into an auxospore sheds its frustule and begins to increase rapidly in size. Upon completion of growth, the protoplast of the auxospore forms an epitheca and a hypotheca, giving rise to the so-called initial cell, the dimensions of which typically approach the maximum possible for the given species. Although auxospores can form through various processes (e.g., when cells reach their minimum possible size or upon frustule damage), they most frequently arise following the sexual process—from a germinating zygote.

Sexual process. During the sexual process, two cells become enveloped in a common mucous sheath, their protoplasts increase in size, and they push the frustule halves apart. Subsequent development proceeds via different pathways in various species, determining the manifestation of a specific type of sexual process. Nevertheless, what remains common in all cases is that the nuclei of each cell undergo one to several divisions, with one of these divisions obligatorily being meiotic.

In algae with actinomorphic valves, oogamous sexual reproduction is most common. In algae with zygomorphic valves, the most widespread types of sexual processes are isogamy and heterogamy. Isogamy results in the formation of two zygotes, whereas heterogamy yields either two or one.

During oogamy, Meiosis occurs in one of the cells—the oogonium, and three of the four haploid nuclei degenerate. The protoplast with the remaining viable nucleus transforms into a single, haploid egg cell. In the male cell—the antheridium, often referred to as a spermatogonium—one to several divisions take place, the last of which is meiotic. Consequently, from two to many small male Gametes are formed within the spermatogonium. In different species, these gametes may possess motile or immotile flagella, or lack flagella altogether. Motile male gametes are called spermatozoa, while immotile ones are called spermatia. A spermatozoon or spermatium fertilizes the egg cell. A zygote is formed, which germinates directly into an auxospore without a resting period.

In isogamy, each parent cell divides meiotically to produce four haploid nuclei, two of which degenerate. Next, the protoplast of the gametangium divides without nuclear division, resulting in the formation of two uninucleate, immotile gametes in each gametangium. Copulation occurs between pairs of gametes from different gametangiums. Since the gametangia are held in close proximity by a shared mucous layer and are in contact via their girdle bands, a slight enlargement of the gamete protoplasts is sufficient to ensure physical contact between the gametes. Following copulation, each pair of gametes forms a zygote, which develops into an auxospore.

Heterogamy resulting in two zygotes occurs in much the same way as isogamy, except that the gametes from one gametangium (considered male) are motile, while those from the other are immotile. Driven by amoeboid Movements of the protoplast, the male gametes migrate to the female ones, fuse with them, and likewise form two zygotes that germinate into auxospores.

In heterogamy resulting in a single zygote, the protoplasts of the prospective gametangia divide meiotically. In each gametangium, three of the four haploid nuclei degenerate, and only a single gamete is formed. Copulation occurs through the fusion of a motile gamete from the male gametangium with an immotile female gamete. As a result, only one zygote is produced, and consequently, only a single auxospore.

A special type of Sexual process in diatoms is autogamy. Only a single cell participates in this process. Its nucleus undergoes meiotic division, after which two haploid nuclei degenerate, while the other two fuse, restoring a diploid nucleus and forming a zygote that develops into an auxospore.

Thus, the sexual process in diatoms not only provides a certain level of recombination Variability, but also performs the function of maintaining a specific range of cell sizes within the population, thereby counteracting potential population degeneration caused by the reduction in cell size during vegetative division.

The life cycle in diatoms is diplontic, featuring gametic reduction, lacking alternation of generations, and possessing a unique developmental stage—the auxospore.

Resting stages are represented by bivalved siliceous cysts and are known only in certain marine centric diatoms. Cyst formation begins with the division of the cell protoplast. Next, the maternal frustule is shed, and each daughter protoplast becomes enclosed within two convex valves, while girdle bands fail to develop. The valve that forms first bears various surface protuberances (ridges, spines, spinules). The second valve remains smooth.

Systematics of the Division

The leading diagnostic features upon which The system of Bacillariophyta is built relate to the STRUCTURE OF THE siliceous frustule. Correlated with this complex of features are numerous characteristics pertaining to protoplast Organization and the Specifics of the sexual process. Today, two main groups of diatom Classification systems exist: classical morphological and morpho-cytological systems.

The principles of the classical morphological system were developed in the 19th century by the German phycologist F. Hustedt, who proposed distinguishing Bacillariophyta taxa based on morphological Features of the frustule visible under an optical microscope. In the 20th century, various authors repeatedly proposed modifications to Hustedt's system. For instance, the modification proposed by O.V. Topachevsky and O.P. Oksiyuk is widely used in Ukraine. In Western Europe and the Americas, one of the most popular variants of Hustedt's system was developed by H. Lange-Bertalot and K. Krammer.

The morpho-cytological system was proposed in the second half of the 20th century by British phycologists F. Round, R. Crawford, and D. Mann, based on data obtained from studying the ULTRASTRUCTURE OF THE frustule using a Scanning Electron microscope.

In the late 20th century, the first attempts were made to test both types of classification systems using molecular Taxonomy Methods. However, phylogenetic trees constructed from 18S ribosomal RNA Gene sequencing data demonstrated that neither classical nor morpho-cytological systems show sufficient concordance with molecular dendrograms.

Thus, the question of a classification system that adequately reflects the Phylogeny of the division remains open. Below, the material is presented following the morpho-cytological system of F. Round, R. Crawford, and D. Mann, which shows slightly better alignment with molecular data than variants of Hustedt's classical system.

According to this system, Bacillariophyta includes three classes, which are distinguished by valve symmetry and the presence of a raphe: Coscinodiscophyceae, Fragilariophyceae, and Bacillariophyceae.

Class Coscinodiscophyceae (Centrophyceae)

Unites algae with actinomorphic frustules that always lack a raphe. In most cases, the frustules of coscinodiscophyceans are round and radially symmetrical, which is why representatives of this class are often referred to as centric diatoms. According to paleoalgological data, other groups of diatoms originate precisely from algae of this class. Oogamous sexual processes and motile spermatozoa are found exclusively in centric diatoms. Representatives of this class primarily inhabit the plankton of marine and freshwater bodies. The best-known representatives of this class belong, first of all, to the orders Thalassiosirales and Melosirales.

Thalassiosirales includes algae that possess fultoportulae on the frustule and areolae covered with a velum on the inner side. The areola systems form tangential or radial rows. Plastids are numerous and disc-shaped. It is believed that the presence of an internal velum is a sign of primitiveness for this order, since this type of velum is known in the most ancient fossil diatoms from the extinct order Pyxidisculales.

Representatives of Thalassiosirales are very widespread in the plankton of marine and continental water bodies. In particular, in seas (including the Black Sea and the Sea of Azov), Thalassiosira is one of the dominants. Thalassiosira cells are solitary or joined by mucus into fragile colonies. The frustules are round, with areolae forming tangential striae.

Another characteristic genus of the order, Cyclotella, includes predominantly freshwater planktonic species. Cyclotella cells are most often solitary; the frustules, like those of Thalassiosira, are round, but the areolae form radial rather than tangential striae, which are especially clearly visible near the edge of the frustule.

Melosirales is considered an evolutionarily more advanced order: fultoportulae are absent, and the velum is located on the outer side of the areolae. The latter generally do not form clear, regular striae. Plastids in melosiraleans are numerous, small, and lobed, located in the peripheral layer of the cytoplasm.

The typical representative of the order, Melosira, is extremely characteristic of planktonic communities in freshwater bodies. Melosira frustules have high mantle flanges and complex girdle bands, which is why in preparations the cells lie predominantly in girdle view and have an elongated-cylindrical shape. Several rimoportulae, very small areolae, and sometimes a ring of small spines are located on the frustule. Mucus is secreted through the rimoportulae, by means of which daughter cells remain connected by their frustules after division, forming pairs and triplets of cells. Remaining together, these pairs and triplets lead to the formation of the filamentous colonies typical of this alga.

The sexual process, represented by oogamy, has been studied in quite some detail in Melosira. At the beginning of the sexual process, meiosis occurs in one of the cells; after the First Division (meiosis I), one of the nuclei degenerates and does not enter the phase of meiosis II; the second, viable nucleus divides, and again one of the nuclei degenerates. Cytokinesis does not occur during meiosis I and II divisions. Thus, a single egg cell is formed in the female gametangium.

In the cell serving as the male gametangium, the protoplast swells, causing the shedding of the frustule. Next, four flagella appear On the surface of the protoplast, meiosis takes place, and a four-nucleate naked spermatogenic cell is formed, retaining all four flagella. Subsequently, four uniflagellate uninucleate spermatozoa bud off simultaneously from the spermatogenic cell. Budding occurs in such a way that plastids do not enter the gametes: they remain in the center of the spermatogenic cell, forming the so-called residual cell, which lacks a nucleus and soon disintegrates. Then the spermatozoa fertilize the egg cell, which transforms into an auxospore.

Class Fragilariophyceae

Unites algae with zygomorphic frustules that lack a raphe but possess an axial area. As a rule, a single rimoportule, which is considered a precursor of the raphe, is located on the frustule of fragilariophyceans. Within this class, unlike the Coscinodiscophyceae, flagellated stages are completely absent. The most widespread representatives of the class belong to the orders Fragilariales, Tabellariales, and Licmophorales, which differ primarily in the position of the rimoportule (polar or shifted toward the center) and the presence of intercalary bands with septa.

Fragilariales are characterized by the presence of a single rimoportule at the pole of the frustule, positioned at an angle to the longitudinal axis; intercalary bands are either absent or present, but in the latter case they lack septa. The cells contain from two large to many small chloroplasts, located predominantly in the peripheral layer of the cytoplasm beneath the frustule. Most representatives of this order form colonies, by the shape of which the algae can be identified at the genus level even without making permanent frustule preparations.

In particular, Fragilaria cells are solitary or form ribbon-like colonies in which the frustules are connected by their frustule faces. In girdle view, an individual frustule has an elongated-rectangular shape; in valve view, the frustule is elongated, bilaterally symmetrical, and more or less beak-shaped at the poles. In permanent preparations made from frustules cleaned of cytoplasm, the axial area and alternately arranged striae are clearly distinguishable on the frustule. Species of the genus are predominantly inhabitants of periphytic and benthic communities in freshwater bodies.

In terms of frustule structure, the freshwater planktonic alga Asterionella is very similar to Fragilaria, yet it is easily distinguished by the structure of its colonies. In these, the cells are joined into star-shaped colonies by means of mucus and small spines located at the pole of the frustule.

In its living state, the genus Diatoma is distinguished by the presence of zigzag colonies. In these, cell connection is also carried out via mucus and polar spines on the frustule. In addition, a striking feature of this genus is the presence of coarse transverse ribs on the inner side of the frustule, which give the cell a characteristic transverse striation. Diatoma can often be found in planktonic and benthic communities of freshwater bodies.

The order Tabellariales differs from the previous one in that the rimoportule is located near the center of the frustule, and by the presence of intercalary bands with a septum that blocks almost half of such a band. As a result, the septal system divides the frustule into several semi-open chambers. Each such chamber contains a single ribbon-like plastid. Therefore, in girdle view, a single cell of a tabellarialean alga resembles a colony of several cells with simple girdle bands connected by their frustules like a small picket fence.

Species of the typical genus Tabellaria are very common in benthic and periphytic communities of freshwater bodies. Typically, Tabellaria cells are connected to each other at the corners by means of mucus and spines at the poles of the frustule, forming zigzag colonies similar to those in Diatoma.

The order Licmophorales includes predominantly marine species of periphytic and benthic diatoms that have wedge-shaped frustules in girdle view, which attach to the substrate by means of mucous stalks. The rimoportule in these algae is single and located near the cell pole. The typical genus of the order, Licmophora, belongs to cosmopolitan marine algae and is found practically everywhere in the epiphyton on stones, mollusk shells, the thalli of macrophyte algae, and marine higher plants.

Class Bacillariophyceae

This class includes algae with zygomorphic frustules that possess a raphe on one or both frustules. According to Modern views on the evolution of diatoms, the raphe originated from a gradually lengthening rimoportule.

The raphe is a structure associated with the ability of bacillariophycean algae to undergo active gliding movement. The Emergence of such an ability is regarded as a major aromorphosis. Therefore, features associated with the raphe (in particular, its type, number, position on the frustule, degree of development, etc.) form the basis for dividing the Bacillariophyceae into orders.

One of the consequences of the transition to a motile lifestyle has been the gradual reduction of The ability to form colonies from cells that remained together after division. Therefore, star-shaped, fence-like, and ribbon-like colonies, which are very characteristic of Fragilariophyceae, are almost never encountered within the class Bacillariophyceae. Bacillariophycean cells are either solitary or form colonies in which the algae are embedded in a common colonial mucus, reside within mucous tubes, or attach to the substrate by mucous stalks.

In algae of the Bacillariophyceae, flagellated stages are completely absent, and sexual processes are represented by isogamy, heterogamy, and autogamy.

The Bacillariophyceae are divided into ten orders, of which seven are considered below (Table 17.1): Eunotiales, Cymbellales, Achnanthales, Naviculales, Bacillariales, Rhopalodiales, Surirellales.

Table 17.1. Leading features of the main orders of Bacillariophyceae

The order Eunotiales includes algae with dorsiventral valves that are symmetrical relative to the transverse axis. A rudimentary raphe is located on the ventral side, with fissure ends starting at the valve poles. Although eunotialean algae possess a slit-like raphe, its rudimentary nature prevents them from active movement.

Species of the typical genus Eunotia inhabit benthic and periphytic communities of oligotrophic acidic water bodies and are characteristic representatives of wetland algoflora.

Cymbellales unites species characterized by a developed slit-like raphe on one of the valves, enabling active cell locomotion. The opposite valve also bears a raphe, which is rudimentary in some species resembling that of eunotialean algae (e.g., Rhoicosphenia), or fully developed in others (Gomphonema, Cymbella, Encyonema). The valves of cymbellalean algae are asymmetric relative to at least one axis. For instance, in Rhoicosphenia and Gomphonema, the valves are symmetric only relative to the longitudinal axis, resulting in club-shaped (clavate) or wedge-shaped outlines. Conversely, in Cymbella and Encyonema, the valves are symmetric relative to the transverse axis and are therefore dorsiventral.

Species belonging to Cymbellales predominantly inhabit the benthos and periphyton of freshwater bodies. For example, Rhoicosphenia is frequently found in the epiphyton of higher aquatic plants. Its cells, which are clavate in valve view and cuneate and curved in girdle view, attach to the substrate via long mucous stalks. This mucilage is secreted through open pores located at the narrowed end of the valve. Rhoicosphenia possesses a single four-lobed chloroplast, with lobes originating from a pyrenoid pressed against the girdle.

In terms of cell outline, species of the genus Gomphonema somewhat resemble the previous genus—their valves are clavate, and cells in girdle view are cuneate, though unlike Rhoicosphenia, they are not curved. These algae inhabit the periphyton primarily, attaching to underwater substrates by means of mucous stalks formed through the secretion of mucilage via a system of submicroscopic pores located at the pole of the narrowed valve side.

Species of the genus Cymbella resemble crescent moons positioned at the tips of branched mucous stalks. A chloroplast containing a pyrenoid is situated on the curved side of the cell. A raphe runs almost along the middle of the valve, with polar fissures bent toward the dorsal side. Near the central nodule under a light microscope on permanent preparations, one can observe one or several dots representing a special type of perforation known as stigma.

In terms of cell shape, the genus Encyonema appears very similar to Cymbella. However, Encyonema cells are predominantly located inside mucous tubes, the chloroplast is positioned on the ventral side of the cell, and the polar fissures of the raphe are likewise bent toward the ventral side. Stigmata are absent in Encyonema.

The most striking feature of Achnanthales is the presence of a raphe on only one of the valves—either the epitheca or the hypotheca. The restriction of the raphe to a single valve is related to these algae's ADAPTATION TO A periphytic lifestyle. Cells of achnanthalean algae are solitary and reside on substrate surfaces, pressing tightly against them with the raphe-bearing valve.

In benthic marine communities, predominantly on small pebbles of the littoral and supralittoral zones, species of the genus Achnanthes develop massively. In valve view, the cells of this alga are bisymmetric; however, observation from the girdle view reveals that the raphe valve is concave, while the rapheless valve is convex. Moving across the substrate using the raphe, the alga selects areas with optimal illumination and halts. Subsequently, one of the polar raphe fissures secretes a mucous stalk. The plastids in this alga are typically numerous and disc-shaped.

In freshwater bodies, species of the genus Achnanthidium, which are similar to the previous genus, develop massively within periphytic and benthic communities. They also possess bisymmetric valves, of which the raphe valve is concave and the rapheless valve is convex. Through mucilage secretion by the polar fissure, the cells form a stalk by which they attach to the substrate during vegetation. This genus differs from Achnanthes not only in its distribution, but also by the presence of a single plastid extending along one side of the valve, as well as a prominent horseshoe-shaped structure near the central area of the rapheless valve, visible on permanent preparations.

In freshwaters, the most characteristic dominants of epiphyton on higher aquatic plants and green macrophytic algae are species of the genus Cocconeis. Their valves are broadly elliptical, with low girdle bands. Cocconeis cells do not form mucous stalks, but instead adhere firmly to the substrate via their raphe valve. Each cell contains a single lobed parietal C-shaped chloroplast with one to several small elongated naked pyrenoids.

Naviculales is the taxonomically richest order not only within the class, but within the division as a whole. Algae of this order possess developed raphes on both valves. The valves themselves are either bisymmetric or S-shaped (mirror-symmetric). The primary criteria used in delimiting families and genera include valve shape, Structural Features of the raphe fissures, and the structure of striae and ribs. Additional diagnostic features allowing the distinction of genera in a living state are primarily associated with plastid morphology. Representatives of Naviculales are widespread in nearly all types of marine and freshwater communities. Typical Examples of naviculalean algae include the genera Navicula, Craticula, Gyrosigma, and Pinnularia.

Species of one of the most numerous genera—Navicula—have a rather simple structure: the valves are lanceolate in outline, resembling a small boat. A straight slit-like raphe runs along the middle of the axial area. Parallel, radial, or convergent striae, formed by regular rows of areolae, are located on the valve surface. The central area is relatively small and does not extend to the valve margins. In girdle view, cells are elongate-rectangular. Girdle bands are simple and lack intermediate bands. There are two plate-like chloroplasts situated along the girdle bands on opposite sides of the valve, each containing a single rod-shaped pyrenoid.

The genus Craticula is similar to Navicula in frustule shape, the presence of a straight raphe, and striae formed exclusively by areolae. However, the central area in Craticula is nearly absent, and the areolae are arranged in such regular rows that the valve surface appears striated by two systems of lines—parallel and perpendicular. There are two plastids, very similar to those in Navicula, but the pyrenoids are broadly elliptical rather than rod-shaped.

In species of the genus Gyrosigma, the valve perforation system resembles that of Craticula. However, the valves of Gyrosigma are not bisymmetric, but mirror-symmetric, resembling an incomplete letter "S". The central raphe fissures are either bent in opposite directions or terminate in a short T-shaped branching. There are two chloroplasts whose shape and arrangement are identical to those of Navicula and Craticula, though their margins are rather incised and finely lobed. Species of this genus predominantly inhabit the benthos of marine and continental water bodies. This genus plays an especially significant role in benthic communities of hyperhaline waters. For instance, in the Saky healing lake in Crimea, aggregations of Gyrosigma cells form a continuous dense layer up to several centimeters thick on the bottom over the therapeutic mud.

The valves of species of the genus Pinnularia have a shape close to elongate-elliptical. The raphe in Pinnularia is slightly wavy. On the inner side of the valve, the frustule features a system of coarse thickenings—costae (ribs)—that somewhat resemble a washboard surface and can be mistakenly interpreted as striae under a light microscope. Between the ribs, the frustule is perforated by rows of fine areolae. In girdle view, the frustule is regularly rectangular, in which position it is clearly noticeable that the ribs are indeed a system of internal valve thickenings rather than perforations. There are two ribbon-like chloroplasts positioned along the girdles on opposite sides of the valve with highly incised margins. In some species, the opposing chloroplast ribbons are connected beneath the hypovalve by a small bridge, giving the chloroplast an H-shape. In the central part of each ribbon, many species possess a single naked ellipsoidal pyrenoid. Because the frustules of species in this genus are generally large, the cells are unable to maintain themselves suspended in the water Column, and consequently inhabit either benthic or periphytic communities of freshwater bodies.

Bacillariales comprise algae whose valves bear a single relatively straight canal-like raphe. The raphe of bacillarialean algae consists of a narrow external fissure that communicates with a canal situated within an internal valve thickening known as the keel. The canal opens into the frustule cavity via large apertures termed fibulae, or keel puncta. The canal with fibulae is homologous to the internal raphe fissure of raphe-bearing diatoms. Representatives of Bacillariales are easily recognized by the presence of fibulae. Algae of Bacillariales are widespread in water bodies of all types and in numerous soils. Typical soil algae include Hantzschia, whereas aquatic forms include Nitzschia.

The valves in Hantzschia are dorsiventral. The raphes of both valves are located on the ventral side of the cell, close to the valve mantle. There are typically two lobed chloroplasts situated on opposite sides of the cell's transverse plane, which resemble two "H" letters in valve view.

In Nitzschia, the valves are either bisymmetric or S-shaped. The raphes are likewise displaced toward the margin of the valve, but the epitheca raphe is located on the opposite side relative to the hypotheca. Typically, species of this genus possess two plate-like entire or slightly dissected plastids positioned on opposite sides of the cell's transverse plane. In terms of species richness, the genus Nitzschia is one of the most numerous within the division. Species with narrow and small frustules are predominantly distributed in the plankton, whereas those with large and robust frustules occur in the benthos and periphyton of continental water bodies and seas.

Rhopalodiales belong to the group of canal-raphe diatoms in which each valve possesses a single raphe lacking fibulae discernible under a light microscope. Cells of rhopalodialean algae are predominantly dorsiventral, crescent-shaped, and attach to the substrate not by the valve, but by the ventral side of the girdle zone. Due to the expansion of the girdle bands on the dorsal side (sometimes via intercalary bands), the dorsal girdle zone is significantly wider than the ventral one.

The raphe originates at the poles of the ventral side of the valve. Further along, in some genera (Epithemia), it runs along the ventral side and curves onto the dorsal side; in others, it runs along the dorsal side (Rhopalodia) or nearly along the middle of the valve. In the center of the valve, the branches of the external raphe fissure form a central nodule that is nearly imperceptible under a light microscope.

The valves of rhopalodialean algae feature coarse internal ribs, between which lie two to several striae formed by areolae.

Characteristic representatives of the order are the periphytic genera Epithemia and Rhopalodia.

Representatives of Surirellales are sometimes referred to as four-raphe diatoms because each valve possesses two canal-like raphes. These are located on opposite sides of the valve within external folds called alae (wings). It is believed that the formation of the additional raphe results from a gradual shift in frustule polarity due to a reduction in valve length and an increase in width, accompanied by the elongation of the polar raphe fissures. The valves of surirellalean algae possess complex systems of costae that reinforce the alae. Systems of areolae are located between the ribs on the valve.

Due to the large and heavy frustules of the members of this order, these algae predominantly inhabit benthic communities.

A typical genus of the order, Surirella, features ellipsoidal or ovoid valves with well-developed alae containing canal-type raphes. Coarse transverse ribs terminating at the alae are present on the valve surface. The cell contains two large plate-like chloroplasts positioned at the cell periphery such that one is pressed against the epitheca and the other against the hypotheca. Species of this genus inhabit both marine and freshwater environments.

In the genus Cymatopleura, the frustule in valve view is constricted and somewhat guitar-shaped. The valve surface is undulating, and the alae are rather narrow. This alga has a single chloroplast consisting of two plates pressed against the epi- and hypotheca, respectively, and connected at one of the poles by a thin bridge. Species of this genus inhabit freshwater environments, mostly with an alkaline reaction.

Distribution and Ecological Features

The general ecophysiological characteristics of Bacillariophyta include, first and foremost, their psychrophilic nature. The Temperature optimum for most diatom species is below 15° C. Therefore, diatoms proliferate massively in cold seas, develop under ice and on the snow surface, and frequently experience mass development in winter. At the same time, despite their cold-loving nature, many diatom species are tolerant of quite high temperatures, which allows them to continue growing in summer and even thrive in hot springs. Overall, the temperature range for active growth of Bacillariophyta spans from -4° to +50° C, with optima for various species ranging from 0° to 15° C.

In nature, diatoms have colonized nearly all types of aquatic and terrestrial habitats. During the Cretaceous period (130–100 million years ago), algae from Coscinodiscophyceae became the dominant group of marine plankton, replacing the then-declining coccolithophores (Haptophyta, Coccolithales) by the end of the period. Around 70 million years ago, diatoms began successfully colonizing freshwater bodies. In the Neogene, they became one of the leading phyla in continental waters, with representatives of Fragillariophyceae dominating freshwater plankton, unlike in marine environments. Later, following the "invention" of the raphe and The Development of active gliding motility, freshwater benthic and periphytic communities were colonized, where most dominants belong to Bacillariophyta. Land habitats were colonized last (likely during the Pleistocene). The Diversity of strictly terrestrial diatoms is low, and almost all terrestrial species belong to the Naviculales and Bacillariales.

Marine diatoms inhabit three MAIN TYPES OF biotopes: plankton, benthos, and periphyton. Marine plankton developing in open waters far from the coast (the pelagic zone) is predominantly represented by species of Coscinodiscophyceae, which possess fragile, thin frustules that allow them to suspend in the water column; however, when washed into the coastal zone, they are easily destroyed by wave action. Species with sturdier frustules develop in the coastal (neritic) plankton. In particular, the dominants in this biotope type within the Black and Azov Seas are primarily species of the genus Thalassiosira. In addition, numerous benthic and periphytic diatoms resuspended into the water column by coastal waves are found in the neritic plankton.

In marine benthos and periphyton,raphid diatoms of the genera Achnanthes, Cocconeis, and Surirella are abundant, alongside the araphid alga Licmophora.

Freshwater planktonic diatoms are mainly represented by genera of Fragilariales (Fragilaria, Asterionella, Diatoma, etc.) and Nitzschia species with thin, elongated frustules. In periphyton growths, the most numerous are various raphid-slit diatoms (Cocconeis, Achnanthidium, Gomphonema, Rhoicosphenia, Cymbella, Navicula), as well as rhopalodialean algae (Epithemia, Rhopalodia). Canal-raphid species from Surirellales and raphid-slit forms with thick, coarse frustules, notably Pinnularia, predominate in the benthos. Some diatoms (such as Melosira) begin their development in the periphyton, but subsequently detach from the substrate and transition to a planktonic lifestyle.

Significance in nature and Human Life

In the biosphere, diatoms are one of the leading groups of producers, playing a planetary role not only in the Carbon and Oxygen cycles, but also in the silicon cycle. Even in seas with the poorest phytoplankton, the biomass of diatoms reaches 2–5 thousand tons per 1 $km^2$ of water area. On a planetary scale, the organic matter of diatoms accounts for about 50% of the total biomass of seas and oceans, and their annual production makes up about 25% of the global planetary total. Annually, diatoms absorb about 3 billion tons of silicon from the World Ocean.

The frustules of fossil diatoms formed sedimentary rocks known as diatomite or "infusorial earth". Diatomite is a lightweight and porous yet quite durable rock, which is why it is used as a building material and for manufacturing fine filters and abrasives. Due to its high adsorption capacity, it is widely used in the chemical, medical, and food industries. Diatomite soaked in nitroglycerin is used to produce explosives—the well-known dynamite invented by A. Nobel.

Fossil diatoms are used in geology to determine the age of sedimentary rocks (so-called diatom analysis). In freshwater bodies, diatoms drive the accumulation of diatomaceous sapropel, which is used as therapeutic mud.

Water quality assessment is carried out using diatom indices based on diatom algae. This methodology is standardized in European Union countries and is mandatory for any ecological assessments of rivers and lakes in Western Europe.

Instances are known where massive proliferation of diatoms causes "water blooms". Centric diatoms, particularly species of the genus Cyclotella, are especially frequent bloom-formers in freshwater bodies. In such cases, the water acquires a muddy-yellow color and an unpleasant fish-oil odor. In recent years, evidence has emerged that in some cases, diatom-induced water blooms can be moderately toxic.

The Position of Bacillariophyta in the System of the Organic World

Although the affiliation of diatoms with the phylum Stramenopiles is beyond doubt, the phylogenetic relationships of this phylum with other chrysophyte algae remain insufficiently resolved. The closest presumed relatives of diatoms are considered to be golden or yellow-green algae, which clearly exhibit a tendency toward silicification of their cell walls, forming siliceous scales or cysts with silicified coverings.

¹ According to estimates by F. Round and R. Crawford (1990), who adopt the monotypic concept of species in diatoms, approximately 100 thousand species are known in the phylum. As calculated by T. Norton and co-authors (1996), the expected species richness of this phylum ranges from 100 thousand to 10 million species.