Phycology - Kostikov I.Yu. - 2009-2013

Chapter 24. Green algae – Chlorophyta

The division of green Algae (Chlorophyta or Chlorophycophyta) is the most species-rich group among algae. According to various authors, it comprises 20,000–25,000 species. The oldest known fossils date back to the Late Precambrian - Proterozoic (approximately 1400 million years ago). Green algae are widely distributed in freshwater and hypersaline Water bodies, seas and oceans, terrestrial habitats, as well as on snow and ice.

Class="center">Taxonomic Features of the division

Pigments and reserve nutrients

Photosynthetic Pigments include almost all pigments characteristic of higher plants: chlorophylls "a" and "b", all known carotenes (α, β, γ, and ε-carotene), and lutein-series xanthophylls (lutein, zeaxanthin, neoxanthin, violaxanthin, antheraxanthin). Specific xanthophylls found in Siphonocladophyceae, Ulvophyceae, and Prasinophyceae include siphonaxanthin, siphonein, loroxanthin, and prasinoxanthin. Some prasinophytes also contain a pigment similar to chlorophyll "c".

Due to the predominance of chlorophylls, most Chlorophyta species are green. Under extreme conditions (e.g., in hypersaline water bodies), some green algae trigger a mechanism of hypersynthesis of carotene and secondary carotenoids, causing the Cells to turn red (Dunaliella salina). During Aging or The formation of resting cells, the Cytoplasm may accumulate oil colored yellow and red by various carotenes and xanthophylls (a mixture known as haematochrome).

The primary assimilation product is starch. It is always deposited within the plastid—either directly in the chloroplast stroma or around the pyrenoid. Extraplastidial assimilates are represented by oil (accumulating mainly during Cell aging) and leucosin. In siphonocladophycean algae, the main assimilate can be the polysaccharide inulin.

Cytological features

The cells of green algae have a typical eukaryotic Structure and are generally similar to the cells of higher plants. Specific features are primarily associated with the Structure of Cell walls, METABOLISM/14.html">Chloroplasts, the flagellar apparatus of monadoid cells, as well as the particularities of mitosis and cytokinesis.

Cell walls. The Cells of the vast majority of green algae are covered by a Cellulose-pectin wall or a wall whose framework is formed by crystalline-ordered Glycoproteins enriched with hydroxyproline. Representatives lacking any coverings other than The Plasma Membrane—i.e., having "naked" cells—are much rarer. In green algae of the class Prasinophyceae, as well as in the motile reproductive cells of certain ulvophyceans and charophyceans, from one to several layers of submicroscopic organic scales are deposited On the surface of the plasma membrane.

The ULTRASTRUCTURE OF THE wall is diverse and determined by the taxonomic position and age Characteristics of Specific representatives. Wall formation begins with The Development of the so-called primary wall, which develops exterior to the plasma membrane and consists of an inner layer W1, several layers of the so-called central triplet (layers W2 - W6), and, in many species, an additional outer layer W7. Subsequently, a secondary wall can develop beneath the primary glycoprotein wall, with a structural framework composed of cellulose microfibrils cemented with pectic substances.

In many unicellular green algae (particularly Chlamydomonas reinhardtii and related genera), the inner layer W1 is formed by a loose layer of glycoprotein microfibrils arranged more or less radially. This is followed by the central triplet, which typically consists of three closely appressed layers: W2, W4, and W6. Layer W2 represents a network of fibrils oriented parallel to The Cell surface (Woessner, Goodenough, 1993).

However, all The Diversity of wall structures represents various modifications of a basic structural plan. This plan is most clearly traceable in young cells of unicellular algae, whose walls consist of three layers.

The first (outer) layer is very thin, formed by sulfated and acidic Polysaccharides, and has a fringed or wavy edge. It is believed to perform receptor Functions. The second layer (the so-called primary wall, or periodically globular layer) is intermediate, formed by regularly arranged subunits of hydroxyproline globules, glycoproteins, and Oligosaccharides. The third layer (secondary wall, or fibrillar layer) is internal, formed by cellulose microfibrils "cemented" with pectic substances. The secondary wall is most developed in mature and old cells. The plasma membrane lies beneath the inner layer.

In many species, during cell aging, akinete, and zygote formation, the outer and intermediate layers can break down and disappear, while the inner fibrillar layer thickens and multiplies (Fig. 24.1). Wall thickening is usually associated with the synthesis of cellulose microfibrils, the assembly of which occurs on The surface of the plasma membrane by special terminal enzymatic complexes. Therefore, the wall always grows from the inside out. Occasionally, the inner layer contains additional layers formed of Chitin or sporopollenin.

Fig. 24.1. Basic structural plan of The Cell wall in green algae of different ages: A - young, B - mature, C - old cells. 1 - plasma membrane, 2 - outer layer of sulfated and acidic polysaccharides, 3 - intermediate globular glycoprotein layer, 4 - inner fibrillar cellulose layer (schematized after McLean, 1968; Konstantinova, 1991).

Cell walls can be continuous (in most species) or consist of several fragments (e.g., 2 in Microspora, 3–9 in Treubaria); the wall surface is smooth or bears various ornamentations such as spines, spinules, Ribs, warts, etc., and sometimes possesses a rather complex pore apparatus (especially in Conjugatophyceae). In a significant number of species, the wall is mineralized with iron, manganese, or calcium salts.

Prasinophycean algae lack a cell wall; their plasma membrane surface is covered with submicroscopic scales formed in the cisternae of the Golgi apparatus and secreted outwards near the Base of the flagella.

Nuclear apparatus of green algae has a typical eukaryotic structure. The outer membrane of the nuclear envelope is isolated and has no structural connection with chloroplasts (Fig. 24.2). In telophase, it is usually formed by the fusion of membrane vesicles originating from the Golgi apparatus or The Endoplasmic reticulum. The Homology of these structures can often be observed even in interphase, when the outer nuclear membrane transitions into endoplasmic reticulum channels or is connected to dictyosomes.

Fig. 24.2. General plan of a green algal Cell Structure (using Chlamydomonas as an example): 1 - plasma membrane, 2 - cell wall, 3 - contractile vacuoles, 4 - Golgi apparatus, 5 - endoplasmic reticulum, 6 - double-membrane chloroplast envelope, 7 - eyespot (stigma), 8 - stromal starch, 9 - pyrenoid, 10 - pyrenoid starch sheath, 11 - thylakoids assembled into a granum, 12 - nucleus, 13 - nucleolus, 14 - mitochondrion, 15 - oil droplet, 16 - microtubular flagellar roots, 17 - flagellum, 18 - basal body, 19 - fibrillar connective fiber of basal bodies (after Ettl, 1983).

The Nucleus contains one to several nucleoli, which (except in Conjugatophyceae) disappear during mitosis. Centrioles are usually absent in monadoid cells, their function being performed by the basal bodies of the flagella. In cells of non-motile representatives, centrioles are present (except in Conjugatophyceae and Charales).

Mitosis is closed, semi-closed, or open, and can proceed in several variants, which are discussed below in the descriptions of individual classes.

According to the analysis of various genes, nuclear DNA is most closely related to the nuclear DNA of glaucophytes, rhodophytes, cryptophytes, prymnesiophytes, and higher plants.

Photosynthetic apparatus. The chloroplasts are nearly identical to those of higher plants: they are enclosed by a double-membrane envelope, thylakoids are grouped into lamellae of 3–6 or form grana, and girdle thylakoids are absent. Chloroplasts typically contain one to several pyrenoids, usually bordered with starch.

In most monad green algae, a stigma is present within the chloroplast. It consists of several layers of osmiophilic globules pigmented with carotenoids, located directly beneath the chloroplast envelope. At the site of the stigma, the chloroplast envelope closely adjoins the Plasmalemma. This specific region of the plasmalemma is believed to function as a photoreceptor, while the stigma acts as a screen regulating The amount of light striking the photoreceptor. The stigma of green algae is not directly connected to the flagellar apparatus.

Chloroplast DNA is closed into a ring and consists of 100–300 kbp. The genophore is dispersed. Based on molecular phylogenetic reconstructions, the chloroplast DNA of green algae originates from cyanobacterial DNA, with the chloroplast DNA of glaucophytes and red algae forming its sister lineages, and that of higher plants forming its descendant lineage.

Green algae are considered to be organisms with primary endosymbiotic chloroplasts. The latter arose through the divergence of the photosynthetic apparatus of a hypothetical photoautotrophic ancestor that acquired a plastid via an endosymbiosis of the "heterotrophic eukaryote + cyanobacterium" type.

The mitochondrial apparatus is typically represented by a branched mitochondrion with lamellar cristae. Mitochondrial DNA consists of 16–80 kbp (up to 220 kbp in marine siphonous algae) and, according to molecular phylogenetic reconstructions, is close to the mitochondrial DNA of other eukaryotes that possess Mitochondria with lamellar cristae.

Flagellar stages. The vast majority of monad cells in green algae possess two isokont and isomorphic flagella. Four- or multi-flagellate (stephanokont) forms occur less frequently (Fig. 24.3). Among primitive Chlorophyta, there are species with one, two to three unequal, or six to eight flagella of equal length.

The flagellar surface is usually smooth or covered with organic submicroscopic scales formed in specialized cisternae of the Golgi apparatus. Some genera possess simple tubular mastigonemes.

Fig. 24.3. Common (1–4) and relatively rare (5–9) body plans of monad stages in green algae: 1, 2 — bi- and quadriflagellate isokont; 3, 4 — stephanokont with annular (3) and spiral (4) flagellar arrangement; 5 — uniflagellate; 6, 7 — bi- and triflagellate heterokont; 8, 9 — hexa- and octaflagellate isokont.

The transition zone of the flagellum exhibits a specific stellate structure found exclusively in the monad cells of green algae and higher plants (Fig. 24.4). In addition to the stellate structure, primitive green algae of the class Prasinophyceae may have a spiral structure and an intermediate plate in the transition zone.

The system of flagellar roots consists of microtubular and microfibrillar roots. The Organization plans of The ROOT System vary across different classes of the division.

Other Organelles. The vacuolar apparatus is represented by true vacuoles containing cell sap and bounded by a tonoplast. The cells of many monad and heminomad freshwater and terrestrial algae typically contain two, and occasionally several, contractile vacuoles. Certain prasinophyte algae feature trichocysts and mucous bodies structurally resembling those of cryptophyte and raphidophyte algae.

Fig. 24.4. General body plan of the flagellar apparatus in green algae: 1 — peripheral microtubules of the axoneme, 2 — central microtubules, 3 — stellate structure in the transition zone, 4 — basal body, 5 — lower part of the basal body ("cartwheel"), 6 — upper connecting fiber of the basal bodies, 7 — plasmalemma, 8 — microtubular flagellar roots, 9–12 — STRUCTURE OF THE axoneme (9), transition zone with the stellate structure (10), upper part of the basal body (11), and the "cartwheel" zone (12).

Types of Body Structure

In terms of Morphology, Chlorophyta is the most diverse algal division. Green algae can be micro- or macroscopic, unicellular or multicellular, and some possess a non-cellular (siphonous or siphonocladous) organization. Nearly all types of morphological body structure are represented within this division: monad, heminomad, coccoid, sarcinoid, trichal, heterotrichal, parenchymatous, siphonous, and siphonocladous. Structural type serves as one of the primary criteria for classifying green algae into orders in classical systems. Both unicellular and multicellular algae can form various colonies and lead a free-living or attached lifestyle.

Reproduction and Growth of Multicellular Thalli

Green algae exhibit all the principal modes of asexual reproduction and sexual processes known in eukaryotic algae. Overall, within the division, There is a trend toward replacing motile reproductive cells with non-motile ones, and reducing asexual reproduction via spores in favor of intensified vegetative and sexual reproduction. In certain evolutionary dead-end lineages (e.g., Chlorococcales or Scenedesmales), there is a tendency toward the secondary loss of the sexual process and a transition to apogamy.

Green algae exhibit cell Divisions of the schizotomy, schizogony, and cytokinesis types (Fig. 24.5). The latter two types drive the growth of multicellular thalli, with cytokinesis involving a cell plate occurring exclusively in the division Chlorophyta.

Fig. 24.5. Modes of Cell Division in green algae: A — schizotomy, B, C — schizogony, D — cytokinesis (1, 2, 6 — schizogony resulting in sporulation; 3 — schizogony leading to

the formation and growth of multicellular sarcina-like aggregates; 4, 5 — schizogony driving thallus growth in filamentous and heterotrichal forms; 7, 8 — cytokinesis involving an annular furrow and a cell plate, which invariably promotes thallus elongation).

Life Cycles

Green algae are characterized by various types of life cycles. About a quarter of known species are agamontic or apogamous, with cyclomorphosis-type life cycles.

Most species of the division belong to eugamophytes, and their life cycles are represented by haplophasic, haplodiplophasic, and diplophasic cycles with zygotic, gametic, sporic, and somatic reduction, accompanied by isomorphic and heteromorphic ALTERNATION OF GENERATIONS. Life cycles may be uniform across all representatives of a taxon at the class or order rank, or they may differ even among various species within the same genus.

System of Green Algae

Green algae are a subject of intensive research. The vast amount of new information, along with The Need for its generalization and systematization, places the Taxonomy of Chlorophyta in an extraordinarily dynamic state.

Today, there are numerous systems of green algae. Based on the underlying principles, all these systems can be divided into: a) classical morphological-ontogenetic, b) cytological, and c) molecular-phylogenetic systems.

Morphological-Ontogenetic Systems

Systems of this type are based on principles developed in the early 20th century by the prominent phycologist Adolf Pascher. The division of green algae into high-rank taxa (classes, orders) is carried out primarily according to the types of morphological structures and the Specific features of the sexual process. Among such systems, the system by B. Fott (Fott, 1971, 1973) and its later modifications have gained the widest recognition. In particular, the system outlined in the textbook "Freshwater Algae of the Ukrainian SSR" (Topachevsky, Masyuk, 1984) has become widely used in Ukraine.

Classical systems are artificial; they do not reflect the true Phylogenetic relationships among taxa and are therefore unsuitable for addressing A number of tasks (for instance, in comparative floristics, historical analysis, and phylogeny). Nevertheless, they offer several advantages over newer cytological and molecular-phylogenetic systems: classical systems are convenient for identifying specific genera and species, as well as for determining the placement of novel taxa within the system. Very importantly, modern identification keys for green algae are still based precisely on classical systems.

According to classical morphological-ontogenetic systems, the division Chlorophyta is divided into three classes: Chlorophyceae (or Euchlorophyceae), Conjugatophyceae, and Charophyceae. The Classification into classes is based on the type of sexual process, the presence or absence of flagellate stages in ontogeny, and The structure of reproductive Organs. The division of classes into orders is carried out primarily based on the types of somatic morphological structure.

Class Chlorophyceae unites unicellular, multicellular, and coenocytic algae capable of asexual reproduction via spores and/or hologamous or merogamous sexual reproduction. The class is divided into 11 orders. Their main features and Examples of characteristic representatives are given below.

Order Pedinomonadales comprises monadoid algae with an atypical structure of the flagellar apparatus (uni-, bi-, or triflagellate heterokont species). Characteristic representatives are Pedinomonas and Monomastix.

Order Volvocales comprises monadoid algae with a typical structure of the flagellar apparatus. Characteristic representatives are Dunaliella, Chlamydomonas, Haematococcus, Gonium, Pandorina, Volvox.

Order Tetrasporales comprises hemimonadoid algae, for example, Tetraspora.

Order Chlorococcales comprises coccoid algae. Characteristic representatives are Chlorococcum, Trebouxia, Hydrodictyon, Chlorella, Scenedesmus, Ankistrodesmus.

Order Chlorosarcinales comprises sarcinoid algae. Characteristic representatives are Chlorosarcinopsis, Tetracystis, Chlorokybus.

Order Ulotrichales comprises trichal algae with bi- or quadriflagellate reproductive cells. Characteristic representatives are Ulothrix, Uronema, Klebsormidium, Stichococcus, Raphidonema, Ulva, Enteromorpha, Microspora.

Order Chaetophorales comprises heterotrichal algae with bi- or quadriflagellate reproductive cells. Characteristic representatives are Stigeoclonium, Chaetophora, Draparnaldia, Pleurastrum, Coleochaete.

Order Oedogoniales comprises trichal and heterotrichal algae with stephanokont reproductive cells and a specific type of cell division involving the formation of caps. Characteristic representatives are Oedogonium, Bulbochaete.

Order Bryopsidales comprises siphonous algae. Characteristic representatives are Bryopsis, Codium, Acetabularia.

Order Siphonocladales comprises siphonecphalin/siphonocladous algae characterized by an isogamous or heterogamous sexual process. Characteristic representatives are Rhizoclonium, Cladophora, Siphonocladus.

Order Sphaeropleales comprises siphonocladous algae featuring an oogamous sexual process. During development, the zygote germinates into a specialized multinucleate germling. A characteristic representative is Sphaeroplea.

Class Conjugatophyceae unites coccoid and trichal algae with a sexual process of the conjugation type. Any monadoid stages in the developmental cycle are absent. Based on structural types, the class is divided into 4 orders.

Order Mesotaeniales comprises coccoid algae with a smooth cell wall, in which the cell is not differentiated into semicells. Characteristic representatives are Mesotaenium, Cylindrocystis, Spirotaenia.

Order Gonatozygales comprises trichal algae with fragile filaments and sculptured walls. A characteristic representative is Gonatozygon.

Order Desmidiales comprises coccoid algae with a sculptured cell wall and a complex pore apparatus. Cells are predominantly differentiated into semicells. Characteristic representatives are Penium, Closterium, Cosmarium, Micrasterias.

Order Zygnematales comprises trichal algae with smooth walls. Characteristic representatives are Spirogyra, Zygnema, Mougeotia.

The class Charophyceae comprises heterotrichous algae with a specific articulated-whorled structure, in which the sexual process is a highly specialized oogamy, and the flagellate stages are represented exclusively by spermatozoa. It includes a single order, Charales. Typical representatives are Chara and Nitella.

Cytological systems

In the 1970s, electron microscopic studies revealed that green algae possess two fundamentally distinct types of flagellar root organization—cruciate and asymmetric unilateral; Three types of cytokinesis—involving a Cleavage furrow, phycoplast, or phragmoplast; three types of mitosis—closed, semi-closed, and open; several different variants of spindle structure; various types of chromosome behavior in metaphase; and different Variants of the ultrastructure of cell coverings, among others. Given the existence of such a variety of ultrastructural features, several phycologists working in the field of comparative Cytology of green algae—such as K.D. Stewart, K.R. Mattox, M. Chadefaud, T. Christensen, and others—attempted to restructure the classification of green algae on a cytological basis. The division of green algae was divided into the classes Pedinophyceae, Micromonadophyceae, Prasinophyceae, Chlorophyceae, Ulvophyceae, and Charophyceae.

Later, the classification of classes was constantly refined, new class characteristics were introduced, the classes themselves were subdivided, and their number increased¹. For example, in the most modern textbook by C. van den Hoek, D. Mann, and H. Jahns (van den Hoek, Mann, Jahns, 1995), based on ultrastructural features, Chlorophyta are divided into 11 classes. However, the taxonomic weight of certain ultrastructural features remained problematic. The situation began to change significantly in the 1990s.

Molecular biological systems

To resolve the question of which specific features most realistically reflect the main evolutionary trends of green algae (and, accordingly, which variants of the Chlorophyta system are closest to The Natural System), molecular biological Methods began to be applied In the second half of the 1980s. Initially, research was based primarily on the analysis of nucleotide sequences of the nuclear genes encoding the 5S and 18S ribosomal RNA subunits. Later, similar work was carried out on other genes, not only nuclear but also chloroplast and mitochondrial (e.g., genes for tubulin elongation factor, cytochrome "c", Actin, RUBISCO, etc.). To analyze the results of nucleotide sequence sequencing of certain genes in the early 1990s, numerous variants of constructing molecular phylogenetic trees and evaluating their information content were developed.

Already the first phylogenetic trees constructed on The basis of molecular data demonstrated the monophyly of Chlorophyta, the heterogeneity of the traditional class Chlorophyceae, the broader scope of the class Charophyceae (compared to the traditional one) and its phylogenetic link to higher plants, as well as the need to distinguish several additional classes. The obtained results also made it possible to weigh various morphological and cytological features and isolate those that agreed with molecular phylogenetic reconstructions. In the late 1990s, the Discussion on the main evolutionary trends of Chlorophyta was summarized, and a classification of green algal classes closest to the natural system was developed.

Molecular phylogenetic reconstructions, on the one hand, confirmed the high phylogenetic weight of such cytological features as the structure of the flagellar apparatus, the specifics of mitosis and cytokinesis, and also showed that within Chlorophyta there are currently 7 large and well-defined groups of taxa that should be considered as independent classes. Interestingly, these classes agreed not only with cytological features, but also with the ecological CHARACTERISTICS OF THE species included in them.

According to molecular phylogenetic reconstructions, green algae are divided into the classes Prasinophyceae, Chlorophyceae, Trebouxiophyceae, Ulvophyceae, Siphonophyceae, Zygnematophyceae (=Conjugatophyceae), and Charophyceae. A review of the features underlying this division is given below.

Taxonomic features of green algal classes

MAIN TYPES OF cell coverings

The plasmalemma in green algae can be covered by two fundamentally distinct types of cell coverings—either submicroscopic scales formed in the Golgi apparatus, or a cellulose-pectin wall whose fibrils are synthesized on the surface of the plasmalemma, for example, with the participation of terminal enzyme complexes. The first type of covering is characteristic of vegetative cells of prasinophyte algae, while the second is characteristic of representatives of all other classes. Interestingly, in charophyte and ulvophyte algae, zoospores and Gametes are also covered with submicroscopic scales similar to those of prasinophyte algae, although vegetative cells possess a cell wall. According to the biogenetic law, this may indicate THE ORIGIN OF Charophyceae and Ulvophyceae from hypothetical flagellate ancestors similar to modern Prasinophyceae.

Types of flagellar root systems

In a simplified form, the flagellar root systems of green algae are represented by either a cruciate or an asymmetric type.

In algae with a cruciate root system, four crucially arranged microtubular roots extend from the basal bodies of the flagella. Opposite roots have the same number of microtubules. The basal bodies of the flagella are interconnected by a connecting fiber. In some representatives, two rhizoplasts are also directed deep into the cell.

In a generalized form, variants of the cruciate flagellar root system are denoted by a formula of the type n₁-n₂-n₁-n₂, where n₁ is the number of

microtubules of the first and third roots, and n2 is the number of microtubules of the second and fourth roots. For the vast majority of species with a cruciate system, this formula is (5-6)-2-(5-6)-2, meaning that the number of microtubules in the first and third roots varies slightly, while the second and fourth roots usually consist of two microtubules. Other variants are also known. For example, in trentepohliacean algae, the formula is 6-4-6-4, and in ulvalean and cladophoralean algae, it is 3-2-3-2.

According to the orientation of the basal bodies, cruciate root systems are divided into three types: with basal bodies displaced clockwise (1-7 system), counterclockwise (11-5 system), or opposite (12-6 system). To determine the orientation type, the anterior part of the cell is viewed in cross-section, and the flagella are oriented vertically. If an axis is then drawn through the centers of the basal bodies, the clock face hours through which it passes will correspond to the orientation of the basal bodies (Fig. 24.6).

Fig. 24.6. Orientation of basal bodies in green algae: 1 - opposite (12-6), 2 - clockwise displaced (1-7), 3 - counterclockwise displaced (11-5), 4 - unilateral (in an asymmetric root system).

Systems of the 12-6 and 1-7 types are characteristic of algae from the class Chlorophyceae, whereas the 11-5 system is characteristic of Trebouxiophyceae, Ulvophyceae, and Siphonophyceae.

In algae with an asymmetric root system, only a single microtubular root, composed of many (up to several dozen) microtubules, extends from the basal bodies of the flagella deep into the cell. It is connected to the basal bodies via a multilayered microfibrillar structure. Monad cells with this type of root system are usually dorsiventral, the flagella are directed toward the ventral side, and the basal bodies are arranged more or less in parallel. Such a system is characteristic of monad cells of representatives of Charophyceae and higher plants.

In algae of the class Prasinophyceae, root systems can be either cruciate or asymmetric, or form a combined variant—a cruciate system with an additional asymmetric root.

In addition to Symmetry types (cruciate or asymmetric), the diversity of green algal root systems is also evident in the structure of microfibrillar roots, basal body connecting fibers, the presence of terminal caps on basal bodies, the angles of inclination of basal bodies relative to each other, and other features.

Types of mitosis

The diversity of mitosis in green algae is primarily observed through its progression during the prophase and telophase stages.

During prophase, the nuclear envelope either remains intact or becomes perforated or disappears entirely. All these behaviors of the nuclear envelope are known in Prasinophyceae; mitosis is closed in Chlorophyceae, Ulvophyceae, and Siphonophyceae, semi-closed in Conjugatophyceae and Trebouxiohyceae, and open in Charophyceae.

Depending on its behavior in telophase, the spindle apparatus is regarded either as persisting throughout the entire telophase—the so-called persistent spindle—or as disintegrating—the collapsing spindle. In the case of a persistent spindle, the daughter nuclei move a considerable distance apart, whereas with a collapsing spindle, they remain close to each other. A persistent spindle is characteristic of representatives of Ulvophyceae, Siphonophyceae, Zygnematophyceae, and Charophyceae, while a collapsing spindle is typical of Chlorophyceae and Trebouxiophyceae. Representatives of prasinophyte algae may possess either a collapsing or a persistent spindle.

Types of Cytokinesis

Cell division, which occurs upon the completion of nuclear division, is carried out either by a cleavage furrow or through the formation of a cell plate.

Cleavage furrow. In this type of division, the plasmalemma and the inner layer of the cell wall grow inward into the center of the cell, constricting it much like an iris Diaphragm. The inward growth of the cell wall is associated with the centripetal deposition of cellulose microfibrils transported in flattened vesicles of the Golgi apparatus. Division involving a cleavage furrow is found in all classes of green algae except Siphonophyceae, in which cytokinesis is absent because representatives of this class possess a siphonal organization.

Cell plate, by contrast, is formed centrifugally. In the region of the future cell partition, a system of parallel microtubules is established, serving as the cell plate organizer. The microtubules of the cell plate organizer act as guiding tracks along which Golgi vesicles containing cellulose microfibrils move toward the site of partition formation. Depending on the orientation of the microtubules relative to the future cell partition, the cell plate organizer system is divided into two types: the phycoplast or the phragmoplast.

When the organizer microtubules are oriented parallel to the future partition, they are said to form a phycoplast. In monadoid forms, phycoplast microtubules are formed with the participation of the first and third flagellar roots. In non-motile cells, the phycoplast develops at the onset of telophase from the central zone of the spindle As a result of the partial breakdown and reorientation of its microtubules. It is believed that the collapsing type of spindle is a consequence of the utilization of its microtubules in the construction of the phycoplast. The phycoplast is characteristic of chlorophycean and trebouxiophycean algae. It has also been detected in some representatives of Prasinophyceae. Interestingly, in the latter, the fusion of membrane vesicles within the phycoplast produces not a cell plate, but merely portions of the plasmalemma of the daughter cells, since the vesicles lack cellulosic material.

When the microtubules of the cell plate organizer are oriented perpendicular to the future partition, such a microtubule system is called a phragmoplast. Phragmoplast microtubules are synthesized in telophase between the microtubules of the central region of the persistent spindle. The phragmoplast also contains actin microfibrils. The microtubules and microfibrils of the phragmoplast direct and align Golgi vesicles containing the material for the future cell plate toward the central zone of the spindle. The phragmoplast has been found in algae of Conjugatophyceae and Charophyceae, as well as in trentepohrialean algae and higher plants (Table 24.1).

Table 24.1. Certain cytological features of green algal classes

Abbreviations: cell wall - cell wall, cruc. - cruciate, asym. - asymmetric, semi-closed - semi-closed.

Additional Features

Several additional characteristics correlate to some extent with the Main Features of the classes, including peculiarities of pigment composition and assimilation products, The ability to form trichocysts and mucous bodies, types of morphological body structure, ecological characteristics, etc.

The most important cytological features of green algal classes are presented in Table 24.1.

24.1. Evolutionary Lineages: Class Prasinophyceae

This class unites about 200 species of unicellular, predominantly monadoid algae that inhabit marine environments primarily. Prasinophytes are considered the most ancient green algae, having given rise to all Chlorophyta lineages. Unlike other classes, Prasinophyceae are characterized by diverse cell body plans, as well as various types of flagellar root systems, mitosis, and cytokinesis. Fossil remains resembling the cysts of modern prasinophyte algae date back approximately 1.2 billion years.

Features and Characteristics of the Class

Biochemical Features

The chloroplasts of all prasinophyte algae contain chlorophylls $a$ and $b$. In addition, a pigment resembling chlorophyll $c$ has been detected in ten species. According to some researchers, this pigment is true chlorophyll $c$, while others consider it a similar yet non-identical pigment—a complex monomethyl ester of Mg-containing 2,4-divinylpheoporphyrin $a_5$. In some species, prasinoxanthin is found instead of lutein. Representatives possessing siphonoxanthin and siphonein—xanthophylls characteristic of ulvophycean and siphonophycean algae—are also known.

Cytological Peculiarities

Cell Coverings. Prasinophyte algae of the order Pedinomonadales possess cells covered solely by the plasmalemma. However, in most prasinophytes, one or several layers of submicroscopic organic scales are located above the plasmalemma. Sometimes the scales are closely connected to one another, forming a so-called theca, which appears under the Light Microscope as a cell wall (Fig. 24.7).

Fig. 24.7. Types of scales and structure of cell coverings in prasinophyte algae: A-C - cellular scales (A - underlayer, B - intermediate, C - surface); D - arrangement of scale layers on the surface of Pyramimonas cells; E - theca in Tetraselmis; F - flagellar scales in Pyramimonas. 1 - plasmalemma, 2-4 - cellular scales: underlayer (2), intermediate (3), surface (4); 5 - theca formed by the fusion of underlayer scales, 6-8 - flagellar scales: underlayer (6), intermediate (7), scale hairs (8) (schematized after Sym, Pienaar, 1991, 1993; Lewin, Lee, 1985).

Depending on their size and morphology, scales are divided into three main types: underlayer, intermediate, and surface scales. Underlayer scales are very small (up to 50 nm in length), flattened, and predominantly rectangular in shape. Intermediate scales are larger (250-350 nm long), of various outlines, but typically with a raised rim, thus resembling open boxes. Surface scales are even larger (up to 600-700 nm long), generally tall, sometimes bearing spines and bristles, and possess a complex architecture.

The scales are composed of CARBOHYDRATES with a minor (up to 5%) protein admixture. The main carbohydrates are pectin-like polysaccharides based on 2-keto-sugar acids, admixed with neutral sugars such as arabinose, galactose, glucose, and xylose. The presence of 2-keto-sugar acids in the cell coverings is considered a specific feature of prasinophytes; apart from Prasinophyceae, these acids have also been detected in Higher Plants and certain Bacteria.

In its chemical composition, the theca differs somewhat from the scales: it contains not only 2-keto-sugar acids, but also a significant amount of galacturonic acid, Calcium Ions, and sulfate residues. It is galacturonic acid that links the small submicroscopic scales together into a single, integral cell covering.

Scale formation takes place within the vesicles of the Golgi apparatus, which subsequently migrate to a specialized reservoir. The scale reservoir opens via a pore at the base of the flagella; through this pore, the scales emerge to the exterior and are deposited on the plasmalemma. In taxa possessing a theca, the scales do not accumulate in the reservoir, but instead directly assemble into theca fragments within the Golgi vesicles.

Under certain conditions, some prasinophytes secrete a layer of mucus and enter a palmelloid state. In the overwhelming majority of cases, such mucus is amorphous, whereas in species of the genus Prasinocladus, it forms mucous tubes.

Under adverse conditions and depending on the Phases of the lunar cycle, some prasinophyte algae form cysts enclosed in a distinct two-layered wall. The outer layer is composed of sporopollenin with a fairly high content of lipoids; it may be smooth or ornamented with submicroscopic scales, and sometimes forms ridges or outgrowths. The inner layer is pectic, containing remnants of sulfated galacturonic acid. During cyst germination, the inner layer typically undergoes complete mucilaginous degeneration, causing the daughter cells to emerge inside a mucous vesicle.

Nuclear apparatus and mitosis features. True centrioles are absent in prasinophyte algae, and their functions are indirectly performed by the basal bodies of the flagella. In most species, a structural connection exists between the basal bodies and the nucleus, mediated by rhizoplasts (Fig. 24.8).

Fig. 24.8. Diagram of the cell structure of a prasinophyte alga (using Pyramimonas as an example): 1 - flagellum, 2 - flagellar scales, 3 - apical pit, 4 - flagellar transition zone with a stellate structure, 5 - trichocyst, 6 - Golgi apparatus, 7 - scale reservoir, 8 - mitochondrial profile, 9 - scale in the reservoir, 10 - contractile vacuole, 11 - Microbodies, 12 - rhizoplast, 13 - chloroplast, 14 - pyrenoid, 15 - starch Sheath of the pyrenoid, 16 - cellular scales, 17 - plasmalemma, 18 - thylakoid, 19 - stromal starch granules, 20 - oil droplet, 21 - eyespot (stigma), 22 - nucleus, 23 - nucleolus, 24 - microtubular root, 25 - basal body, 26 - sinistosome (diagrammed after Ettl, 1983; Inouye, Hori, Chihara, 1983).

At the onset of mitosis, the flagella either remain intact or are cast off, but they are never withdrawn. Consequently, during the Initial Stages of karyokinesis, the cell practically lacks a pool of free tubulin. The basal bodies replicate, but do not migrate deep into the cell. The rhizoplasts partially break down in the zone immediately adjacent to the nuclear envelope. A concentration of granular material is observed here, from which the mitotic spindle begins to form. This process is preceded by the partial disintegration of microtubular roots, providing the cytoplasm with the amount of tubulin necessary to construct the spindle.

During mitosis, the nuclear envelope may remain intact (e.g., in Tetraselmis) or disappear (Pyramimonas). The mitotic spindle is either persistent (Nephroselmis) or rapidly breaks down in telophase (Tetraselmis).

Upon completion of karyokinesis, Golgi vesicles accumulate between the daughter nuclei, and their fusion drives the centripetal growth of a cleavage furrow. In some species, this process occurs with the participation of the phycoplast (Fig. 24.9).

Fig. 24.9. Mitosis in Pyramimonas: 1 - early prophase, 2 - metaphase, 3 - late telophase, 4 - early interphase (diagrammed after Woods, Triemer, 1981).

Thus, prasinophyte algae are characterized by closed and open mitoses with either a persistent or disappearing spindle, rhizoplasts acting as the microtubule-organizing center for the spindle, and cytokinesis proceeding via a cleavage furrow mediated by the Golgi apparatus, with or without the formation of a phycoplast.

The flagellar apparatus of Prasinophyceae is highly diverse. In the vast majority of species, the flagella emerge from an apical or lateral depression—the so-called "flagellar pit," which can be viewed as a reduced gullet. Some taxa possess flagella originating from the convex surface of the cell. The number of flagella varies from one to eight, and they may be either isokont or heterokont.

Heterokonty is observed in species that do not shed their flagella at the onset of division, but instead evenly distribute them between the daughter cells. In this case, each daughter cell receives half of its flagella from the mother cell and constructs the rest de novo. Unlike algae from other classes, the assembly of a new flagellum in prasinophytes proceeds slowly because there is no reserve of free tubulin in the cytoplasm. Therefore, the flagella inherited from the mother cell are longer than those synthesized anew. The former typically cease growing during interphase, whereas the length of the latter increases continuously in proportion to the amount of tubulin synthesized by the cell.

If the mother cell casts off its flagella prior to division—that is, through autotomy—the daughter cells synthesize all their flagella de novo. In this case, the monad cells are isokont.

The flagella are typically covered with one to three layers of submicroscopic scales produced in the Golgi apparatus and accumulated in the same reservoir as the cellular scales. The inner layer consists of small (40–70 nm wide) flattened rectangular scales, followed by an intermediate layer of larger (200–400 nm) spiked scales; the outer layer is composed of scales bearing bristles up to 1 µm long. The number of layers, morphology, and Chemical composition of the flagellar scales are used as taxonomic characters at the ordinal level.

In the flagellar transition zone, prasinophyte algae exhibit a stellate structure typical of green algae. Many species also possess additional transition zone elements—one or two plates, a helical structure, or a cylinder (Fig. 24.10).

Fig. 24.10. Longitudinal section of the flagellar transition zone in prasinophyte algae: A - Mesostigma, B - Mantoniella, C - Pterosperma, D - Pyramimonas (1 - stellate structure, 2 - transverse plate, 3 - helical structure, 4 - cylinder) (after Sym, Pienaar, 1993).

Unlike those in other algal classes, the basal bodies of the flagella in prasinophytes are quite elongated. They are typically connected to the plasmalemma and sometimes to the nucleus by means of two cross-banded rhizoplasts constructed from centrin protein. Rhizoplasts are capable of contraction, allowing the cells to change shape. Contraction occurs as a result of the spiralization of centrin fibrils. This process happens spontaneously and exclusively in the presence of calcium ions. Conversely, the opposite process—despiralization of centrin—requires Energy Expenditure and proceeds with the involvement of ATP.

The basal bodies are interconnected by various microfibrillar linking structures. For instance, in mamieallan algae, the basal bodies are joined by a single smooth centrin fiber. In pseudoscurfieldialean algae, There are two such fibers: a cross-banded one located immediately beneath the flagellar transition zone (the so-called distal fiber), and a smooth one situated slightly lower (the proximal fiber). In pyramimonadalean algae, both the distal and proximal fibers are smooth, and in addition, a massive cross-banded structure known as the sinistosome is positioned between the basal bodies. In thehecate algae, alongside the distal and proximal fibers, an additional microfibrillar linking fiber runs near each basal body, connecting the basal bodies and the flagellar root system to the theca. This system is termed the rhizanchor, or root anchor.

The microtubular root systems are similarly diverse. For example, in the pedinomadalean alga Scourfieldia, microtubular roots are entirely absent. The genera Pedinomonas and Mantoniella possess only two microtubular roots, Nephroselmis has three, and Tetraselmis and Pterosperma have four. In the latter genus, one of the roots consists of numerous microtubules and is associated with a multilayered structure; in Representatives of the genus Mesostigma, two of the four existing microtubular roots consist of numerous microtubules and are linked to a multilayered structure (Fig. 24.11).

Fig. 24.11. Flagellar root systems in prasinophyte algae: A - Mantoniella, B - Nephroselmis, C - Tetraselmis, D - Pterosperma, E - Mesostigma. 1 - basal bodies of flagella, 2 - distal fiber, 3 - microtubular roots, 4 - multilayered structure, 5 - microfibrillar root (after Sym, Pienaar, 1993).

Other organelles. Contractile vacuoles, trichocysts, and mucous bodies have been found in Prasinophyceae. Trichocysts (extrusomes) are membrane-bound capsules adjacent to the plasmalemma that contain cylinders of rolled-up ribbons. Trichocysts have been discovered in the genera Mantoniella, Mamiella, Pyramimonas, Pterosperma, and Monomastix.

In Pyramimonas and Halosphaera, mucous bodies are located beneath the plasmalemma, somewhat resembling analogous structures in Raphidophyte algae. In some species, mucous bodies appear only during cyst formation. They participate in the Formation of the outer layer of the cyst wall.

Types of morphological body structure

The overwhelming majority of prasinophytes are small monadoid forms. However, hemimonadoid (Prasinocladus) and coccoid representatives (Bathycoccus) are also known. Cells are typically solitary and do not form colonies or coenobia. Multicellular forms within Prasinophyceae are absent, although cysts of certain genera (Halosphaera) may consist of many cells enclosed by a common wall, forming a so-called rosette-like phycoma.

Reproduction and life cycles

Both Selection/8.html">Asexual and sexual reproduction are known in prasinophyte algae. Life cycles are represented predominantly by cyclomorphoses. A haphasic life cycle with zygotic reduction and without alternation of generations is known in a single species.

Asexual reproduction in unicellular monadoid forms occurs via longitudinal binary fission in the motile state. In coccoid representatives (genus Bathycoccus), division proceeds through schizotomy facilitated by a circular furrow. Reproduction by zoospores occurs in thalloid/thecate representatives (e.g., Tetraselmis). Prior to zoospore formation, the parent cell sheds its flagella, the protoplast divides, and daughter cells develop their own theca. Thus, zoospore reproduction involves schizogony.

Sexual reproduction has been discovered only in Nephroselmis olivacea. The Sexual process in this species is isogamous; the zygote develops a thick wall and enters a resting phase. Upon completion of the resting period, Meiosis occurs within the zygote, and it germinates into haploid monadoid cells.

Special genetic studies of 66 strains belonging to the genus Tetraselmis have demonstrated that some of these strains are heterozygous. Since, according to Mendelian laws, only eugamic diploid organisms can be heterozygous, it was concluded that a sexual process exists in Tetraselmis (even though it has not yet been observed directly).

Cyclomorphoses in prasinophyte algae. The vast majority of prasinophyte algae are agamic, and their life cycles are represented by cyclomorphoses.

In some prasinophyte algae, monadoid and coccoid stages alternate during cyclomorphosis. For instance, cells of the genus Pycnococcus lack flagella and remain in a coccoid state for the major part of their life cycle. However, under certain conditions, flagella are regenerated, and the alga briefly transitions to a monadoid state. In the genus Mantoniella, cyclomorphosis is associated with the alternation of monadoid and palmelloid states. Cells in both states are capable of schizotomy.

Cyclomorphosis in Pterosperma involves the alternation of vegetative monadoid stages and giant cysts known specifically as phycomas. During phycoma formation, the cell loses its flagella, and numerous mucous bodies appear beneath the plasmalemma; these rapidly increase in size and fuse to form the outer layer of the cyst wall. Chloroplasts divide repeatedly, yet the phycoma remains uninucleate. It rapidly increases in size, reaching 200–800 µm in diameter, which is 25 to 100 times larger than the vegetative monadoid cells. Within the phycoma, the inner layer of the wall becomes distinct, while the outer layer forms two large wing-like outgrowths. The entire process takes from two to fourteen weeks. Subsequently, a series of nuclear and cytoplasmic divisions takes place, yielding several dozen monadoid cells. The outer cyst wall ruptures, and the inner wall gelatinizes, transforming into a fragile mucous vesicle from which the young cells are released.

The life cycle of Tetraselmis alternates among monadoid, hemimonadoid, palmelloid states, and cysts. In the hemimonadoid state, cells externally mirror the morphology of monads yet lack flagella. Such a cell is capable of either transforming into a zoosporangium or transitioning into a palmelloid or cyst state. Palmelloid cells typically secrete mucus and form colonies resembling simple or branched tubes; similarly to hemimonadoid cells, they are capable of producing zoospores. Unlike the phycomas of Pterosperma, the cysts of Tetraselmis do not increase in size. Upon germination, 1–4 monadoid cells emerge from a single cyst.

Systematics of the class

The class is divided into five orders. This classification is primarily based on the characteristics of the cell coverings, Cytoskeleton, mitosis, and overall cellular architecture (Table 24.2).

Table 24.2. Main taxonomic characters of the various orders of Prasinophyceae

Legend: "+" - present; "-" - absent; 1, 2, 3 - inner, intermediate, and outer scales, respectively; spec. - specific; MLS - multilayered structure; structures enclosed in parentheses are not found in all representatives of the given order.

Pedinomonadales comprises uniflagellate and biflagellate monadoid algae whose cell surfaces lack submicroscopic scales, although the latter are present on the flagella. Unlike members of other orders, the flagellar scales of pedinomonadalean algae do not contain 2-keto sugar acids. Characteristic representatives include the genera Scourfieldia, Pedinomonas, Resultor, and Monomastix.

Species of Scourfieldia are biflagellate and lack microtubular roots. Pedinomonas cells possess a single flagellum, from whose basal body two microtubular roots extend. The uniflagellate alga Resultor has four microtubular roots corresponding to the formula 3-1-2-2. The genus Monomastix is characterized by the presence of a single flagellum and basal trichocysts (Fig. 24.12).

Fig. 24.12. Selected representatives of Pedinomonadales: 1 - Scourfieldia, 2 - Pedinomonas, 3 - Resultor, 4 - Monomastix (after Dedusenko-Shchegoleva et al., 1959; Moestrup, 1991).

According to some researchers, pedinomonadalean algae diverged early from other prasinophytes and may be regarded as an independent class of green algae—Loxophyceae or Pedinophyceae. Molecular phylogenetic reconstructions partially support this hypothesis.

Mamiellales unites algae in which the monadoid stages possess two heterokont flagella; in some representatives, the shorter flagellum is reduced, with only its basal body persisting. The root system consists of two microtubular roots. Both the cells and the flagella are covered by one to two layers of scales, whereas the layer of inner small scales is absent. Cysts have not been observed.

Cells of the genus Mamiella bear two lateral flagella. Mantoniella has a single flagellum emerging from a small protrusion. Cells of Bathycoccus completely lack flagella (Fig. 24.13).

Based on cytological features, mamiellalean algae are considered the most primitive group of green algae, which gave rise to other prasinophyte lineages.

Pseudoscourfieldiales comprises algae whose cells and flagella are covered with inner and intermediate (and occasionally also outer) scales. The monad stages possess two heterokont flagella associated with basal bodies connected to three microtubular roots. One of the species exhibits an isogamous sexual process. In its structure and The chemical composition of its wall, the zygote resembles a cyst, although true cysts have not been found in representatives of this order.

In Pseudoscourfieldia, the flagella emerge from an apical pit, and the flagellar roots follow a 4-3-1 formula. In species of the genus Nephroselmis, the flagella are positioned laterally, the root formula is 3-4-8, and a multilayered structure is associated with the third root (Fig. 24.13).

Fig. 24.13. Selected representatives of Mamiellales (1-3), Pseudoscourfieldiales (4, 5), and Pyramimonadales (6-10): 1 - Mantoniella, 2 - Mamiella, 3 - Bathycoccus, 4 - Nephroselmis, 5 - Pseudoscourfieldia, 6 - Mesostigma, 7 - Pyramimonas, 8 - Halosphaera, 9, 10 - Pterosperma (10 - phycoma) (after Sym, Pienaar, 1993).

It is believed that representatives of Pseudoscourfieldiales could have given rise to green algae with an asymmetrical flagellar root system, thus serving as ancestors of the class Charophyceae.

Pyramimonadales groups algae with two, four, rarely eight or even sixteen flagella emerging from a flagellar pit. In mature cells, the flagella are typically isokont, whereas in young cells, they are heterokont. The root system includes four microtubular roots, sometimes featuring a multilayered structure and a cross-striated microfibrillar root. The cells of pyramimonadalean algae are always covered with inner and outer (and frequently also intermediate) scales, but do not form a theca. An inner layer of scales is present on the flagella, while other layers may be present or absent. Cysts have been discovered in many representatives, and phycomas in several genera.

Species of the genus Mesostigma have two flagella emerging from a lateral pit; the flagellar root system is cruciform and corresponds to the 6-4-6-4 formula, with a multilayered structure associated with one of the roots.

Cells of Pyramimonas bear four, eight, or even sixteen flagella originating in an apical pit; the root system is also cruciform, but lacks a multilayered structure.

Mature monad cells of the genus Halosphaera have four apical flagella and resemble species of the genus Pyramimonas. When vegetative cells divide slowly, the daughter cells are four-flagellated, similar to the parent cells. However, under rapid division, the daughter cells decrease in size and possess only two or even one flagellum. The root system is cruciform, and one of the roots forms a multilayered structure. The genus is characterized by phycomas of a distinct rosette-like architecture.

Species of the genus Pterosperma also have four flagella, which, however, are arranged laterally. The flagellar root system consists of four cruciform roots and a fifth lateral multi-microtubular root associated with a multilayered structure. This genus is characterized by the ability to form phycomas with wing-like appendages. Fossil remains resembling such phycomas have been found in sedimentary rocks dated to 1.2 billion years old (Fig. 24.13).

Pyramimonadales is considered an order that likely gave rise to the classes Ulvophyceae and Siphonophyceae. This assumption is supported, in particular, by the presence of the xanthophylls siphonaxanthin and siphonein in pyramimonadalean algae, and the fact that certain Developmental Stages of phycomas share features with specific ontogenetic stages of dasycladalean algae from the class Siphonophyceae.

Chlorodendrales. This order includes monad and hemimonad algae in which the inner and intermediate scales partially fuse to form a theca. Monad cells possess four isokont flagella emerging from an apical pit. The flagellar root system is cruciform, corresponding to the 4-2-4-2 formula, and lacks a multilayered structure. Cytokinesis involves a phycoplast. Unlike other orders, reproduction occurs via zoospores, and exclusively in the non-motile state. Cysts are known, but phycomas have not been observed.

The monad structural type is characteristic of species belonging to the genera Scherffelia and Tetraselmis. The distinctions between these genera are evident in the structure of their microfibrillar roots and the presence of pyrenoids—absent in the former genus and present in the latter (Fig. 24.14).

Fig. 24.14. Selected representatives of Chlorodendrales: 1-3 - Tetraselmis (1 - monad cell, 2 - hemimonad stage prior to zoosporogenesis, 3 - zoosporangium), 4 - Scherffelia, 5 - Prasinocladus (1-3, 5 - orig., 4 - after Sym, Pienaar, 1993).

Representatives of the genus Prasinocladus exhibit a hemimonad structural type. The cells reside inside mucilaginous tubes and, although devoid of flagella, retain basal bodies and an apical pit. Reproduction proceeds via zoospores, which in appearance and ultrastructure are practically indistinguishable from the monad cells of Tetraselmis.

It is believed that chlorodendralean algae could have given rise to the green algal classes characterized by a cruciform root system and phycoplast-mediated cytokinesis, namely Chlorophyceae and Trebouxiophyceae. This hypothesis is partially corroborated by molecular phylogenetic analyses based on the nuclear Gene encoding the 18S subunit of cytoplasmic Ribosomes.

Distribution and Ecology

Prasinophyte algae inhabit marine environments predominantly, with only a few representatives occurring in freshwater, hypersaline water bodies, or saline soils. Marine forms can be found in supralittoral tide pools along oceanic coasts. Local green "blooms" driven by the mass proliferation of Pyramimonas and Tetraselmis species occasionally occur here. Instances are known where prasinophytes triggered water blooms over quite large areas—for example, in bays of the Gulf of Finland (Tetraselmis) or the Sea of Japan (Pyramimonas).

Small monad and coccoid algae from the orders Mamiellales, Pseudoscourfieldiales, and Pyramimonadales are typical constituents of open-ocean phytoplankton and are thought to play a notable role in the planetary oxygen and carbon balance. Certain genera (Halosphaera, Pterosperma) inhabit the plankton during their monad phase, yet transition to a neustonic lifestyle upon reaching the phycoma stage.

Marine colonial forms are Components of the benthos and periphyton. For instance, representatives of the genus Prasinocladus can rather frequently be observed in fouling communities on the brown alga Cystoseira along the Black Sea coast.

In hypersaline water bodies and saline soils, species of Pedinomonas occasionally proliferate massively, causing green "blooms" of brine and participating in the formation of therapeutic mud.

Freshwater prasinophytes from the genera Pyramimonas, Nephroselmis, and Tetraselmis can be found in ephemeral, organically polluted water bodies, where they—alongside other flagellates—cause water blooms.

In addition, prasinophytes are quite frequently found as photosynthetic endosymbionts of turbellarians (Tetraselmis convoluta), radiolarians (some mamielean algae), and colorless dinophytes, in particular Noctiluca.

¹ A more detailed Overview of various cytological systems can be found in the book Vodorosli. Spravochnik (Wasser, Kondratieva, Masiuk et al., 1989).