Phycology - Kostikov I.Yu. - 2009-2013

Chapter 24. Green algae – Chlorophyta

24.3. Streptophyte lineage

24.3.2. Class Zygnematophyceae (=Conjugatophyceae)

Conjugates are one of the most speciose classes of Chlorophyta, comprising about 10,000 species. Representatives of the Class are predominantly microscopic freshwater Algae. A very small number of conjugate species occur in saline waters and subaerial habitats (soils and aerophytic communities). Together with charophycean algae, conjugates represent the phragmoplast-based evolutionary line of green algae that gave rise to higher land plants. Specific features of the class include the complete absence of any flagellated (monad) stages and the presence of a sexual process known as conjugation.

Cytology/cytology/25.html">General characteristics and Features of the Class

Biochemical Features

The composition of pigments and assimilation products is typical of green algae as a whole (chlorophylls "a" and "b", all types of carotenes, xanthophylls of the lutein series, with starch as the primary storage product); thus, there are no specific biochemical peculiarities unique to this class.

Cytological Features

Cell walls consist of a Cellulose-pectin layer, with the synthesis of cellulose microfibrils mediated by rosettes of terminal enzyme complexes located in the Plasmalemma. Each complex consists of six cellulose synthase subunits arranged in a rosette.

Overall, The Cell wall of conjugates is three-layered: an outer pectin layer capable of producing massive mucilage. The middle and inner layers are cellulosic. The cellulose microfibrils in the middle layer are arranged as randomly oriented bundles, whereas those in the inner layer form regular parallel arrays.

In many unicellular representatives, The Cell walls feature large or small pores. Mucilage is secreted through the large pores, enabling Cells to exhibit a slow gliding movement; hence, these large pores are sometimes referred to as mucilage pores. Small pores facilitate the exchange of metabolites between the cell and its environment. In desmid and gonatozygalean algae, the cell walls are intricately sculptured, bearing various thickenings, outgrowths, spines, and bristles.

Nuclear apparatus and mitosis. The nuclei of conjugates are usually clearly visible under a Light Microscope due to the presence of a large nucleolus. At the onset of mitosis, the nucleolus does not disappear, but divides as an independent organelle, which is why it is sometimes referred to as an endosome.

Mitosis in conjugates is semi-open, centrioles are absent, and the spindle persists throughout telophase. Interestingly, the nuclear envelope remains intact during prophase, metaphase, and anaphase, although large "windows" form within it or it partially fragments. In telophase, the nuclear envelope breaks down and is reassembled only after the completion of karyokinesis through the fusion of membrane vesicles derived from the Golgi apparatus and Endoplasmic reticulum.

Chromosomes in conjugates are generally small and numerous, and centromere positions are not fixed. In many species, the chromosome number is variable even within a single clone.

Fig. 24.68. Selected features of the class Zygnematophyceae (=Conjugatophyceae). A, B - diagram of the rosette terminal enzyme complex; C - cell wall Structure; D - general cell Organization plan; E - mitotic telophase; F - Formation of the transverse cell plate during telophase (D-F based on Spirogyra); 1 - plasmalemma, 2 - cellulose microfibril synthesized by the rosette complex, 3 - central globule of the cellulose synthase complex, 4 - impression of a cellulose microfibril, 5 - rosette of the cellulose synthase complex, 6 - pectin layer of the cell wall, 7 - middle wall layer composed of randomly oriented cellulose microfibrils, 8 - inner wall layer composed of ordered cellulose microfibrils, 9 - mitochondrial profile, 10 - chloroplast, 11 - Golgi apparatus, 12 - tonoplast, 13 - nucleus, 14 - endosome (nucleolus), 15 - cell plate in formation, 16 - plasmalemma in the Cleavage furrow region, 17 - mitotic spindle, 18 - phragmoplast microtubules, 19, 20 - small and large membrane vesicles containing cell plate precursor material, 21 - tonoplast membrane (A-C after Giddings, Brower, Staehelin, 1980; Kordyum, 1989; Mix, Manshard, 1977; D - orig., E, F - Fowke, Pickett-Heaps, 1969).

Cytokinesis occurs via the simultaneous growth of an annular cleavage furrow and a cell plate. The latter is formed with the participation of a phragmoplast: during telophase, new microtubules and Actin microfibrils assemble, orienting parallel to the spindle microtubules. Vesicles containing cell plate precursor material pinch off from the Golgi apparatus and migrate to the plane of Cell Division. The movement of these vesicles is "guided" by the microtubules and microfilaments of the phragmoplast. The cell plate forms and expands through the fusion of these vesicles; it lacks pores, and therefore daughter cells are not interconnected by plasmodesmata.

Features of the photosynthetic apparatus. Conjugates are dominated by species possessing central star-shaped or plate-like METABOLISM/14.html">Chloroplasts, or peripheral (parietal) chloroplasts in the form of spiral ribbons. Reticulate, disc-shaped, and cup-shaped parietal chloroplasts are not characteristic of conjugates, unlike other classes.

Flagellated stages are completely absent.

Types of Morphological Organization

Within the class, only Two Types of morphological organization are represented: coccoid and filamentous. In coccoid forms, cells are solitary or united into chain-like colonies. Desmid cells are constricted in the middle, consisting of two semi-cells. In mesotaenialean algae, cells are cylindrical, straight or curved, and not differentiated into semi-cells.

Filamentous forms appear as long or short unbranched uniseriate threads. Sometimes filaments easily break down into individual cells.

Reproduction and Life Cycles

Reproduction occurs via vegetative and sexual means. The primary mode of Vegetative Reproduction in unicellular forms is cell division by binary fission, and in filamentous forms, by filament fragmentation; some species are capable of producing akinetes. The sexual process is conjugation.

The life cycle is haplontic, without Morphology/12.html">ALTERNATION OF GENERATIONS, and features zygotic reduction. The zygote germinates after a resting period, producing 1-4 germlings.

Class System

Based on structural types, the presence of Cell Differentiation into semicells, wall ornamentation, the presence of a pore apparatus, and the number of zygote germlings, the Conjugatophyceae are traditionally divided into 4 orders: Mesotaeniales, Desmidiales, Gonatozygales, and Zygnematales.

However, this division only partially aligns with the results of molecular phylogenetic reconstructions, particularly based on the nuclear Gene encoding the 18S ribosomal RNA subunit. Thus, According to the molecular tree, the class includes three evolutionary lineages: 1) coccoid and filamentous algae with a smooth cell wall lacking a pore apparatus, where cells in coccoid forms are not differentiated into semicells. This Lineage corresponds to the monophyletic order Mesotaeniales and the paraphyletic order Zygnematales; 2) coccoid algae with ornamented walls or walls possessing a pore apparatus, yet not differentiated into semicells. This lineage unites the order Gonatozygales and the families Peniaceae and Closteriaceae, which are classified under Desmidiales in the classical system; 3) coccoid algae whose cells are differentiated into semicells and feature ornamented walls with a well-developed pore apparatus. This lineage corresponds to the family Desmidiaceae within the order of the same name. The phenotypic CHARACTERISTICS OF THE orders are presented in Table 24.8.

Table 24.8. Main taxonomic features of the orders of conjugate algae correlating with molecular phylogenetic reconstructions

Zygnematales includes about 1,600 species of coccoid and filamentous conjugates. The cells of zygnematalean algae are cylindrical or spindle-shaped, with smooth walls devoid of a pore apparatus. Consequently, unlike other orders, zygnemataleans are incapable of active movement. In filamentous forms, the filaments are simple, unbranched, single-rowed, and typically covered with a thick layer of amorphous mucilage. In Water bodies, these algae form soft, cotton-like algal mats that are slippery to the Touch.

Reproduction occurs via transverse cell division in half, filament fragmentation, and conjugation (Fig. 24.69). During conjugation in coccoid forms, two cells become enveloped in common mucilage, after which a short conjugation tube grows from each cell toward the other. At the contact point of the conjugation tubes, the cell wall dissolves, the protoplasts fuse, and a zygote is formed.

In filamentous zygnemataleans, two types of conjugation are distinguished: scalariform and lateral. In scalariform conjugation, protoplasts of cells from different filaments fuse: two filaments become enveloped in a common mucilage, and a conjugation tube begins to grow from each cell toward the partner cell. When the conjugation tubes meet, the walls in the contact zone dissolve, forming a conjugation tube (conjugation canal). Next, either both protoplasts move toward each other and fuse within the conjugation canal (this process is considered physiological isogamy), or the protoplast of one cell flows into another, in which case the zygote is formed in the receptive cell (physiological anisogamy). This type of conjugation is called scalariform due to the resemblance of the two filaments with their fused conjugation tubes to a ladder. Lateral conjugation occurs between two adjacent Cells of the same filament (Fig. 24.69).

Fig. 24.69. Reproduction in zygnematalean algae. 1 - successive stages of cell division in Mesotaenium; 2, 3 — conjugation (2) and zygote (3) in Cylindrocystis; 5, 6 - scalariform conjugation (5) and zygote germination (6) in Spirogyra; 7 - lateral conjugation in Mougeotia (1, 4, 6 - after Rundin, 1977; 5, 7 - orig.).

In both coccoid and filamentous forms, the zygote becomes encased in a thick, three-layered, typically ornamented wall and transforms into a zygospore, which enters a resting state. Upon the completion of the resting period, The Nucleus undergoes meiotic division. In unicellular forms, a portion of the Cytoplasm generally segregates around each of the four haploid nuclei, forming its own cell wall, thus producing four new vegetative cells. In many filamentous species, three haploid nuclei degenerate, and the zygote then germinates with only a single germling. The tendency toward a reduction in the Number of viable nuclei after Meiosis in conjugates is considered a progressive phenomenon, as the viable germling receives a greater amount of nutrients necessary for its development.

According to molecular data, early divergence within the order occurred along the morphological types of Plastids. This led to The formation of three evolutionary groups of taxa: a) with 1-2 central stellate chloroplasts; b) with an axial plate-like chloroplast; c) with parietal spiral chloroplasts. Each of these lineages gave rise to filamentous forms with the corresponding type of chloroplast, which in the classical system were regarded as an independent order Zygnematales, whereas coccoid forms were grouped into the order Mesotaeniales.

The algal lineage with an axial plate-like chloroplast is exemplified by the coccoid alga Mesotaenium and the filamentous alga Mougeotia. Species of the genus Mesotaenium inhabit predominantly damp rocks and are epiphytes on mosses and Lichens. The cells are cylindrical, solitary, with a single chloroplast in the form of an axial plate. The filamentous algae of the genus Mougeotia are directly derived from this genus.

The thallus of Mougeotia appears as long or short unbranched filaments that in many species easily break apart into single cells. The chloroplast, similarly to Mesotaenium, resembles an axial plate with numerous pyrenoids. Interestingly, in both genera, the chloroplast is capable of changing its position depending on light intensity. Thus, under weak illumination, it turns "face-on", perpendicular to the incident rays; under intense illumination, conversely, it turns in profile, into a plane parallel to the light source.

Examples of coccoid and filamentous zygnemataleans whose cells contain 1-2 stellate chloroplasts are Cylindrocystis and Zygnema, respectively. The first genus unites primarily inhabitants of damp soils, while the second comprises freshwater forms.

The lineage of algae with spiral ribbon-like chloroplasts is represented by the coccoid Spirotaenia and the filamentous Spirogyra. In Spirotaenia, the cells are solitary, cylindrical, with one to several parietal, ribbon-shaped, spirally coiled chloroplasts.

Representatives of this genus are most commonly found in bogs and on damp non-aquatic substrates.

The genus is closely related to the freshwater filamentous alga Spirogyra, which is also characterized by chloroplasts in the form of one or more spiral ribbons. The cells of Spirogyra are quite large, with the greater part of their volume occupied by a vacuole containing cell sap. In the center of the vacuole lies a small area of cytoplasm with a clearly visible nucleus. This area is connected to the peripheral part of the cytoplasm by several cytoplasmic strands.

Unicellular zygnematalean algae are found predominantly in damp aerophytic ecotopes, whereas filamentous ones inhabit freshwater continental water bodies, typically standing or slow-flowing, where they usually form soft, slimy mats (felts). Small quantities of such algal mats are used to manufacture high-quality paper; in the north, dried mats of zygnematalean algae are used as insulating material.

Fig. 24.70. Unicellular and multicellular representatives of Zygnematales: 1 - Spirotaenia, 2 - Spirogyra, 3 - Cylindrocystis, 4 - Netrium, 5 - Zygnema, 6 - Mesotaenium, 7 - Mougeotia (top views in positions 2, 5, 7 show transverse sections through the cell) (1, 2, 5, 7 - after Rundin, 1977; 3, 4 - after Topachevsky, Masyuk, 1984; 6 - orig.).

Gonatozygales. Encompasses about 100 species, the vast majority of which belong to the genus Closterium. The order unites coccoid unicellular and colonial algae whose cells are not differentiated into semicells, yet possess either a developed pore apparatus or ornamented walls, typically of wavy outlines, or both combined. The zygote germinates with one or two germlings. Within the order, three evolutionary lineages are represented, corresponding to three families: Peniaceae, Gonatozygaceae, and Closteriaceae.

The most primitive representative of the order, bearing some resemblance to mesotaenialean algae, is the genus Penium (Fig. 24.71). The alga has cylindrical cells covered by a wall whose surface is ornamented with fine granules or longitudinal or spiral striae and lacks pores. The chloroplast is axial, with several longitudinal Ribs and multiple pyrenoids, usually slightly constricted in the middle. Vacuoles containing small gypsum crystals are sometimes observed at the cell poles. Species of the genus typically inhabit damp rocks and stones and are epiphytic on mosses.

Fig. 24.71. Gonatozygalean algae: 1 - Penium; 2 - 4 - Closterium (2 - vegetative cell, 3 - mucilage secretion, slide stained with India ink solution, 4 - pore structure in cross-section and top view); 5, 6 - Gonatozygon (5 - vegetative cell, 6 - zygote) (1 - orig., 2, 5, 6 - after Topachevsky, Masyuk, 1984; 3, 4 - after Palamar-Mordvintseva, 1989).

In the genus Closterium, cells are also solitary and fusiformly curved. The cell wall is smooth or faintly striated and features numerous small pores alongside two large ones. The latter are located at the cell poles and function as locomotory Organs, through which the cell secretes mucilage. This mucilage excretion either induces slow reactive movement or forms rapidly growing mucilaginous stalks that allow the cell to change its spatial position. Under low light conditions, cells exhibit positive phototaxis, whereas under high light intensity, it becomes negative. The chloroplasts resemble axial cones with radial longitudinal ribs; several pyrenoids are arranged along the chloroplast axis. As in Penium, vacuoles containing gypsum crystals form at the cell poles. In both genera, the zygote germinates into two germlings.

The typical genus of the order, Gonatozygon, possesses cylindrical cells that join at their ends to form short, few-celled filaments that readily break apart into single cells. The cell walls are typically ornamented with minute granules or spines and contain pores. Chloroplasts are axial, containing pyrenoids. Vacuoles with gypsum crystals, similar to those in Closterium, are occasionally observed at the cell ends. The zygote typically produces a single germling. Species of this genus are frequently found in various freshwater habitats, particularly in the spring.

Desmidiales. According to various estimates, this order includes 4,000–8,000 species of coccoid algae. Desmid cells are either solitary or united into ribbon-like colonies. Each cell consists of two symmetrical semicells connected by a broad or narrow isthmus. The angle between the semicells is termed the sinus. The cell walls of desmids are typically ornamented with warts, spines, or denticles, are frequently impregnated with iron salts, and possess a complex pore apparatus comprising numerous small pores and two to four large mucilage pores located at the cell poles. These mucilage pores facilitate cell motility through a mechanism identical to that in Closterium. Each semicell contains one or two chloroplasts with pyrenoids, while a single nucleus resides in the isthmus region.

Fig. 24.72. Desmid algae. A - cell division: 1 - formation of the transverse septum, 2 - elongation of the isthmus and initiation of young semicell development, 3, 4 - gradual growth of the new semicells, 5, 6 - Separation of the fully formed semicells accompanied by the shedding of their temporary wall. B - pore structure: 7 - in cross-section, 8 - top view (after Palamar-Mordvintseva, 1989).

Fig. 24.73. Desmid algae: 1, 2 - selected species of the genus Cosmarium (1 - C. bigemma; 2 - C. hornavense), 3 - Micrasterias, 4, 5 - selected species of the genus Desmidium (4 - D. aptogonum; 5 - D. graciliceps) (Gontcharov, 1997; 1998).

Desmids reproduce via vegetative cell division or through conjugation. During vegetative division, a cell partition forms in the isthmus via phragmoplast formation. Growth occurs in the isthmus zone of each daughter cell, resulting in The Development of a second semicell. Beneath the wall of the young semicell, a typical three-layered desmid cell wall is synthesized, after which the outer wall layer of the new semicell is shed, and the cells separate (Fig. 24.72).

During sexual reproduction, the zygote (zygospore) typically germinates into two germlings because two of the four nuclei resulting from meiosis undergo degeneration.

The most widespread genera of desmid algae are Cosmarium and Micrasterias. Cosmarium cells are solitary, deeply constricted, and do not form colonies. The semicells are more or less hemispherical and contain stellate chloroplasts (Fig. 24.73).

Cells of Micrasterias are flattened, more or less rounded in top view, and deeply constricted. The semicells are typically divided into three large lobes—one polar and two lateral. Each semicell contains a single lobed chloroplast.

In the genus Desmidium, cells unite into ribbon-like colonies that are usually somewhat spirally twisted. The constriction dividing the cell into semicells is shallow. Chloroplasts are massive, stellate, and deeply lobed, with one per semicell.

Representatives of Desmidiales inhabit various types of fresh waters, yet they reach their greatest Abundance in sphagnum bogs.