BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, Fungi, Plants

4. Class: Diatoms, or Siliceous Algae (Bacillariophyceae = Diatomeae)

Diatoms, comprising over 10,000 species from about 200 genera, are a group of extremely diverse coccoid unicellular Algae, sometimes forming ribbon- or star-shaped colonies. The brown METABOLISM/14.html">Chloroplasts, sometimes occurring singly or in pairs per Cell, contain largely the same pigments as golden algae. The storage products also coincide. Assimilation products are deposited outside the chloroplasts: chrysolaminarin in The Cell sap (in golden algae, in special vacuoles), and oil in specific vacuoles. Only the male Gametes of some species from the order Centrales possess a single, forward-directed tinsel flagellum; other flagellated stages are unknown.

Diatoms occupy a special position in the system due to the presence, within the outer cytoplasmic layer, of two siliceous halves of the frustule, of which one (epitheca), like the lid of a box, fits over the lower one (hypotheca) (Fig. 11.76, B). Adjacent to the lateral margins of the Valves is a girdle consisting of two overlapping bands (girdle bands). The cell looks different depending on whether it is viewed from the valve (A) or girdle (B) perspective. Incomplete partitions, or septa (G), may sometimes extend from the girdle into the cell interior.

Class="center">Fig. 11.76. Bacillariophyceae: A — G — Pennales: A — C — Pinnularia viridis, A — valve view, with raphe (600x), B — girdle view (600x), C — cross-section (200x), D — Licmophora flabellata (200x), E — Synedra gracilis (200x), F — Asterionella formosa (200x), G — Tabellaria flocculosa (400x); H — L — Centrales: H — Coscinodiscus pantocseki (200x), J — Triceratium distinctum (200x), K — Odontella (Biddulphia) aurita (400x), L — Chaetoceros castracanei (250x)

The silicate (siliceous) frustule has, especially on the valve surfaces, extremely complex structures, often arranged in rows; they frequently consist of tiny chambers that can be open or closed either from above or below, and are then penetrated by extremely fine pores or slits (Fig. 11.77). The silicate is partly crystalline, partly amorphous, and polarization-optically isotropic. In the valves of fossil diatoms, the crystalline lattice acquires an amorphous Structure. Along with silicate, Polysaccharides ("Pectins"), Proteins, and fat-like substances have also been detected in The Cell wall, but Cellulose has not been found. The components for building the frustule are formed in flat vesicles beneath the Plasmalemma. These vesicles probably originate from the Golgi apparatus: apparently, several Golgi vesicles fuse to form a silicate-depositing vesicle.

Fig. 11.77. Bacillariophyceae, Pennales: A — D — Pleurosigma angulatum, STRUCTURE OF THE siliceous valve: A — general view of the middle part of the valve with the raphe, B — raphe and pores, C — pores, D — reconstruction of the valve structure based on an electron micrograph; E — Gomphonema parvulum, cross-section through a cell at the end of division (10000x); CM — cytoplasmic membrane; D — dictyosomes; M — mitochondrion; N — nucleolus; O — oil droplet; P — pyrenoid in the chloroplast; R — raphe

Vegetative Reproduction. Diatoms reproduce vegetatively by binary Cell Division. During this process, the two halves of the frustule (thecae) are pushed apart by the enlargement of the protoplast. Each daughter cell, having inherited only one half of the frustule from the mother cell, synthesizes the missing half, which is always a hypotheca. Those daughter Cells that inherited the original hypotheca (which now becomes the epitheca) and form the corresponding half of the frustule (i.e., a new hypotheca) are smaller than the mother cell. With subsequent divisions, this leads to a progressive reduction in cell size down to a certain minimum size (about half of the original size), at which sexual reproduction occurs, associated with a significant increase in the size of the zygote (auxozygote). In some species, the sizes of the epitheca and hypotheca are equalized due to the elasticity of the girdle.

Sexual reproduction. The life cycle is diplontic with gametic Meiosis; the diatom cell thus contains a diploid nucleus (in contrast to, for example, Zygnematophyceae). During meiosis, haploid gametes arise from diploid cells.

An example is the Centrales (other features are discussed below). Sex Determination is environmental. In cells determined as male, they usually transform directly into a spermatogonium, producing 4 spermatozoa (Fig. 11.78, d — f), each with a single flagellum. In other cells, usually larger ones that transform into oogonia, flagella-less female gametes (egg cells) are formed. The details of gamete formation vary considerably, and their number can differ depending on the species. Spermatozoa swim to the egg cells using their tinsel flagellum. After Fertilization inside or outside the oogonium, the zygote becomes enclosed in a wall in which silicate scales are deposited. It soon germinates, growing to 2 to 4 times the size of the original cell due to the expansion of its wall, and transforms into an "auxozygote"¹. The old halves of the frustule are pushed apart, and new halves are formed inside the wall of the auxospore. This gives rise to a diploid "initial cell," from which, as described above, new daughter generations are subsequently produced vegetatively, accompanied by a progressive reduction in size of most of the offspring.

¹ In Russian literature, the term "auxospore" is used for this structure, although the term "auxozygote" given here is apparently more appropriate. — Translator's Note.

In general, The formation of the frustule halves is associated with mitoses. Even the zygote nucleus undergoes mitotic division during the formation of both initial frustule halves, after which one of the daughter nuclei degenerates.

Occurrence. Diatoms are widespread in freshwaters and seas of all climatic zones. They develop particularly intensively in spring and autumn, while their numbers decline in summer. Many forms live in damp soil and on rocky substrates. In the tropics, they can live together with blue-green algae on leaves (epiphyllous species).

Systematics. Based on valve Symmetry, the Bacillariophyceae are divided into two orders: Centrales and Pennales¹. In the former, the valve is radially symmetrical, while in the latter, it is bilaterally symmetrical. In addition to valve structure, these two orders differ significantly in their type of sexual reproduction.

¹ According to modern molecular systematics data, supported by information on valve ultrastructure, the Centrales represent two evolutionary groups, one of which is closer to the Pennales than to the other group. — Translator's note.

1. Order: centric diatoms (Centrales). The valve outline is circular or triangular with rounded corners (Fig. 11.76, H — L). The ornamentation elements of the valve are arranged radially or concentrically. In contrast to most Pennales, the vegetative cells of Centrales are non-motile. However, male gametes are motile due to a single tinsel flagellum (cf. Fig. 11.20, A, left), which lacks both central microtubules. Sexual reproduction (see above) has been studied in particular detail in Stephanopyxis and Melosira varians.

Distribution. Centrales inhabit mainly the seas, where they constitute a significant part of the phytoplankton (ranking first among primary producers in the World Ocean; see Box 11.7). Many of them have special appendages that facilitate flotation (see Fig. 11.76, L), while others are joined by mucilage into chains or Other types of colonies (K).

Species of the genus Melosira, which form filamentous colonies of short-cylindrical cells (Fig. 11.78), are distributed in both seas and freshwater, while numerous species of the genera Coscinodiscus and Hemiaulus occur only in the seas. The genus Triceratium, also marine (see Fig. 11.76, J), is characterized by valves with triangular to polygonal outlines. Resembling a tin can, the cells of Ethmodiscus gazellae (an inhabitant of warm seas), with a diameter of almost 2 mm, are the largest cells among diatoms. Cells of Stephanodiscus are equipped with a ring of spines along the margin of their circular valves. The genus Rhizosolenia is distributed mainly in the seas.

2. Order: pennate diatoms (Pennales). Cells are rod- or boat-shaped, less frequently wedge-shaped (see Fig. 11.76, A — G); thus, their center of symmetry is elongated along a line from which the ornamentation elements of the silicate wall extend pinnately. In very many forms, a slit — the "raphe" — runs along the longitudinal axis of the valve, the Fine Structure of which varies greatly among the genera belonging to this taxon (see Fig. 11.76, A; 11.77). The raphe is believed to play a role in the gliding movement found only in Pennales; in sessile forms, the raphe is absent. In the center, the raphe is interrupted by a central nodule, and at the ends of the valves, it terminates in polar nodules (see Fig. 11.76, A).

Fig. 11.78. Bacillariophyceae, Centrales, Melosira varians. Sexual reproduction (diagram): a — g — male, a' — g' — female PARTS OF THE filament; a — e, a' — e' — meiosis; f — opened; g — released spermatogonium; d' — male nucleus entering through a special slit; f' — fertilization; g' — young auxozygote

Sexual reproduction in Pennales differs from the "normal type" of Centrales because no motile gametes are formed. Here, isogametes in the form of naked protoplasts fuse (the only exception is oogamy in Rhabdonema, where the ♂ gametes still lack flagella). For mating, two vegetative cells glide together and usually secrete abundant mucilage. The Nucleus of each cell divides meiotically into 4 haploid nuclei, of which, however, 2 degenerate and 2 become gamete nuclei. The epitheca and hypotheca separate slightly. Through the resulting gap between them, gametes from different cells can copulate in pairs, giving rise to two zygotes that immediately grow into auxospores. Each of these secretes a pair of siliceous halves of the frustule and becomes the initial cell for subsequent cells of various sizes. The initial and parent cells are arranged perpendicularly (Fig. 11.79) or parallel to each other. Numerous deviations from this common pattern exist.

Distribution. Most motile Pennales live mainly on the bottom of freshwater, brackish, or saltwater bodies of Water (where their mass development can be observed), epiphytically on aquatic plants, or in the soil, though planktonic forms also exist. Approximately 500 species of the genus Navicula, whose cells resemble small boats, are distributed in all types of water bodies; species of the genus Pinnularia (see Fig. 11.76, A–C) with similar but elongate-elliptical cells prefer fresh water. Species of Pleurosigma, whose cells are slightly S-shaped (see Fig. 11.77), can serve as test objects for testing Microscope objectives. In species of the genera Diatoma, Tabellaria (see Fig. 11.76, G), Fragilaria, etc., which live mainly in standing fresh waters, the cells form long chains; in Asterionella (F), they form star-shaped colonies, and in Meridion, fan-shaped colonies (sometimes closing into a ring). Some species of the genus Synedra float freely in the water, while others attach themselves by mucilaginous pads to larger algal filaments (E). In the also sessile species of the genus Licmophora, the cells remain connected to each other after division, resulting in tree-like colonies of cells sitting on mucilage stalks (D). Other genera mentioned in other sections can also be listed: Surirella (see Box 11.7), Nitzschia (see 13.8), Gomphonema (see Fig. 11.77, E), and Rhopalodia (see Fig. 11.79).

Fig. 11.79. Bacillariophyceae, Pennales, Rhopalodia gibba, sexual reproduction: A — two cells joined by a mucilaginous sheath; B — division of mother cells (the degenerated nuclei have already disappeared); C — zygote formation after fusion of gametes; D — increase in size of the auxospore (A — D 410x); E — final stage and formation of a new frustule (240x)

Phylogeny. Centrales, having flagellated spermatozoa, are an earlier group than Pennales, in which flagella have been completely lost. The ancestors of diatoms were possibly algae of the Chrysomonadales type, whose cells can already bear siliceous scales. Diatoms have mostly remained at the coccoid level of Organization, showing only the first hints of a filamentous organization.

The oldest diatoms, specifically centric forms, are known from the Jurassic. Their great species diversity has been recorded since the Cretaceous.

In the Tertiary and interglacial periods, the massive development of diatoms even led to the formation of corresponding rock deposits (for Applications, see Box 11.6, p. 247).

In contrast to both classes discussed previously, the next class lacks simple forms of organization (for example, there are no unicellular forms or unbranched filaments). Macroscopic tissue-like thalli with highly pronounced differentiation into Organs and Tissues are frequently formed. Various types of Morphology/12.html">ALTERNATION OF GENERATIONS occur. The cell wall consists only of alginates and fucoidan.