Algology - Kostikov I.Yu. - 2009-2013

Chapter 13. Golden Algae – Chrysophyta

Golden Algae (Chrysophyta) are considered a relatively young and small phylum within the yellow pigment group. The oldest fossil remains date back approximately 240 million years (to the Triassic period of the Mesozoic era). At the beginning of the Mesozoic, the majority of species within this phylum became extinct. The modern flora of Chrysophyta is represented by about 1,200 species that emerged during the Paleogene and Neogene periods. Golden algae inhabit primarily freshwater bodies and are predominantly monad (flagellate) organisms.

Class="center">Taxonomic CHARACTERISTICS OF THE Phylum

Pigments and Reserve Nutrients

The Biochemical characteristics of the phylum include the presence of chlorophylls $a$ and $c$, $eta$-carotene, and two groups of xanthophylls: firstly, those characteristic of the yellow pigment group as a whole (fucoxanthin, diatoxanthin, diadinoxanthin, dinoxanthin), and secondly, xanthophylls of the lutein series (lutein, neoxanthin, zeaxanthin,

antheraxanthin, violaxanthin). Green chlorophylls and yellow xanthophylls (especially fucoxanthin) impart a bright golden hue to the METABOLISM/14.html">Chloroplasts of Chrysophyta.

The primary assimilation product is chrysolaminarin, with oil and volutin serving as secondary products.

Cytological Characteristics

Cell coverings in Chrysophyta occur in two types: monad and amoeboid representatives are mostly naked, whereas algae with hemimonad, coccoid, and filamentous structures are covered by a cellulosic or Cellulose-pectic Cell wall. Naked Cells may be bounded solely by the Plasmalemma; in such cases, The Cell is often capable of forming pseudopodia. A more rigid covering consists of the plasmalemma with siliceous scales deposited on its surface (see below). Some naked cells reside inside an organic lorica (house).

Occasionally, ejectile structures known as discobolocysts are found beneath the plasmalemma. These represent a capsule formed by the cytoplasmic membrane and filled with fibrillar material, with a ring-rolled ribbon situated at the apex of the capsule. Upon irritation of the plasmalemma, this ribbon uncoils and is discharged outward.

In many colonial algae, small spherical mucous bodies are also located beneath the plasmalemma. When the cell is irritated, they are "shot" outward, forming a mucous capsule around the cell.

Nuclear apparatus. The Nucleus is eukaryotic and enclosed within a double-membrane envelope, the outer membrane of which is directly continuous with the outer membrane of the chloroplast Endoplasmic reticulum (Fig. 13.1).

Mitosis is open. True centrioles are absent. Rhizoplasts serve as the microtubule-organizing centers of the mitotic spindle. In interphase, There is a single rhizoplast, branched near the nuclear envelope, which replicates prior to the onset of division (Fig. 13.2).

Fig. 13.1. Golden algae. A - diagram of Cell Structure; B - formation of tripartite mastigonemes; C - structure of tripartite mastigonemes. 1 - plasmalemma; 2 - mucous body; 3 - vesicle with nascent mastigoneme; 4 - contractile vacuole; 5 - basal body of the longer flagellum; 6 - longer flagellum; 7 - tripartite mastigoneme; 8 - simple mastigoneme; 9 - spiral STRUCTURE OF THE flagellar transition zone; 10 - parabasal body; 11 - shorter flagellum; 12 - pigment globule of the stigma (eyespot); 13 - mitochondrial profile with tubular cristae; 14 - rhizoplast; 15 - membranes of the chloroplast endoplasmic reticulum; 16 - inner membranes of the chloroplast envelope; 17 - nucleus; 18 - nucleolus; 19 - chrysolaminarin vacuole; 20 - thylakoid triad lamella; 21 - girdle lamella; 22 - Golgi apparatus; 23 - intermembrane space of the chloroplast endoplasmic reticulum where the synthesis of tripartite mastigonemes begins; 24 - cisternae of the chloroplast endoplasmic reticulum containing material for future mastigonemes; 25 - Golgi vesicles where the nascent mastigoneme is formed; 26 - vesicle with mastigoneme being transported to the plasmalemma; 27 - release of the mastigoneme onto The surface of the plasmalemma; 28 - Base of the tripartite mastigoneme; 29 - intermediate microtubular region of the mastigoneme; 30 - terminal microfibrillar hairs; 31 - additional microfibrillar hairs On the surface of the intermediate region (after Hibberd, 1970; Bouck, 1971).

Fig. 13.2. Scheme of mitosis in golden algae. A - prophase, B - metaphase, C - anaphase, D - telophase. 1 - rhizoplast, 2 - Golgi apparatus, 3 - nucleus, 4 - endoplasmic reticulum, 5 - spindle microtubules, 6 - chloroplast, 7 - daughter nuclei (after Gorbunova, 1991).

Photosynthetic apparatus. Chloroplasts in Chrysophyta are usually single and located at the periphery of the cell. The chloroplast envelope is four-layered. The two inner layers are formed by the membranes of the chloroplast itself, while the two outer layers are the membranes of the chloroplast endoplasmic reticulum. A periplastidal space lies between the second and third membranes. The outer membrane of the chloroplast endoplasmic reticulum bears 80S Ribosomes on its surface and plays a crucial role, at least in The formation of flagellar tripartite mastigonemes and surface siliceous scales.

Thylakoids in the chloroplast stroma are grouped in threes, and a peripheral girdle thylakoid lamella surrounds the chloroplast. Chloroplast DNA is closed into a ring and occupies a specific region within the chloroplast, meaning the genophore is concentrated. It comprises approximately 120,000 nucleotide pairs. Occasionally, naked pyrenoids and an eyespot (stigma) are also present in the chloroplast stroma. The stigma in Chrysophyta consists of a single row of pigment globules, and its position is always coordinated with the basal body of the shorter flagellum.

Some golden algae lack chloroplasts and feed heterotrophically. Heterotrophs either absorb Organic compounds dissolved in Water (osmotrophic Nutrition) or capture food particles via pseudopodia (phagotrophic/holozoic nutrition).

Mitochondrial apparatus is represented by a branched mitochondrion. In cross-sections, mitochondrial profiles exhibit tubular cristae. Mitochondrial DNA has a linear rather than circular Organization and consists of approximately 40,000 nucleotide pairs.

Flagellar stages are represented by vegetative cells, zoospores, and Gametes. Monad cells possess two flagella of unequal length. The long flagellum is locomotor, covered with numerous mastigonemes of typical tripartite structure; the short flagellum is smooth or covered with simple mastigonemes. In many Chrysophyta, the short flagellum is reduced, leaving only the basal body.

Mastigonemes of two types—tripartite and simple—are arranged pinnately on the surface of the long flagellum. Tripartite (stramenopile) mastigonemes consist of a basal region attached to the flagellar membrane, an intermediate region formed by microtubules, and one to three apical hairs. In addition, several fine auxiliary filaments are located on the surface of the intermediate region (Fig. 13.1).

The synthesis of tripartite mastigonemes begins in the space between the outer and inner membranes of the nuclear envelope. Enclosed in vesicles formed by the outer nuclear membrane, the nascent mastigoneme enters the cisternae of the Golgi apparatus, where its formation is completed. Subsequently, within Golgi vesicles, the mastigoneme is transported to the plasmalemma at the base of the flagellum; the vesicle membrane fuses with the plasmalemma, leaving the mastigoneme on the outer surface of the latter, where it gradually shifts onto the surface of the flagellum.

Mastigonemes of the second type, so-called simple ones, cover both the short and long flagella. They have a fibrillar structure and are synthesized directly on the surface of the flagellar membrane.

The transition zone of each flagellum contains a helical structure. At the base of the basal body of the short flagellum lies a thickening, the parabasal body, which Functions as a photoreceptor and whose position is coordinated with the stigma.

The flagellar roots of golden algae consist of a single striated ROOT, the rhizoplast, and four microtubular roots. The rhizoplast connects the basal bodies to the nucleus and is composed of the contractile protein centrin.

Two microtubular roots are connected to the basal body of each flagellum. The first root consists of three microtubules and forms a loop beneath the plasmalemma at the anterior end of the cell. Numerous single microtubules depart from this root, directed toward the posterior end of the cell to form the Cytoskeleton framework. The second root consists of two microtubules and is also located directly beneath the plasmalemma. The third and fourth roots originate from the basal body of the shorter flagellum and encircle it. The number of microtubules in these two roots varies among different representatives (Fig. 13.3).

Fig. 13.3. System of microtubular flagellar roots in golden algae: 1, 2 - First and Second roots originating from the basal body of the long flagellum; 3, 4 - third and fourth roots originating from the basal body of the short flagellum; 5 - microtubules forming the cytoskeletal framework; 6 - long flagellum; 7 - short flagellum (after Andersen, 1990).

One or two contractile vacuoles are also located near the base of the flagella.

Other Organelles. The main assimilation product, chrysolaminarin, accumulates in special single-membrane vacuoles located at the posterior end of the cell.

Types of morphological body structures

Among Chrysophyta, unicellular algae with monad and amoeboid structures predominate; hemimonad, coccoid, and filamentous representatives are less common. Unicellular algae frequently form colonies and coenobia, while multicellular ones appear as branched or unbranched filaments or parenchymatous plates.

Cysts and their formation

A characteristic feature of chrysophytes is the ability of cells to form endogenous siliceous cysts, known as statospores. Cysts are spherical or ellipsoidal in shape, and their surface may be encrusted with bristles, spines, or ridges. At the apex of the cyst, there is a large opening closed by a polysaccharide plug; it typically lacks silica or contains only trace amounts of it.

Cyst development begins with the formation of a large cisterna in the Cytoplasm, termed the silicalemma. It arises from the fusion of Golgi complex vesicles and ultimately takes the form of a double-membrane sphere with an apical opening. The silicalemma divides the Cell Cytoplasm into two layers: outer (extracycisternal) and inner (intracycisternal). The outer layer contains contractile vacuoles, a mitochondrion, and part of The endoplasmic reticulum channels, while the inner layer contains the nucleus, chloroplast, Golgi complex, and the major part of the endoplasmic reticulum. Silica gradually accumulates between the membranes of the silicalemma, forming the cyst walls. Subsequently, spines, spinules, and bristles may form.

Upon completion of the siliceous wall formation, the extracycisternal cytoplasm is drawn into the cyst, and Golgi vesicles form a polysaccharide plug that seals the cyst opening from the inside. Afterward, the remnants of the extracycisternal cytoplasm, the maternal cell plasmalemma, and the outer cyst membrane disintegrate, while the inner membrane of the silicalemma becomes the plasmalemma of the cyst. Cyst formation is thus completed, and it enters a resting state (Fig. 13.4).

During cyst germination, the plug breaks down, the protoplast emerges through the opening, and develops into a new vegetative cell.

Fig. 13.4. Scheme of successive stages of cyst formation. A - vegetative cell; B - Formation of the silicalemma cisterna; C - accumulation of silica in the silicalemma and the onset of extracycisternal cytoplasm retraction into the cyst; D - mature cyst. 1 - contractile vacuole, 2 - Golgi complex, 3 - nucleus, 4 - chrysolaminarin vacuole, 5 - mitochondrial profile, 6 - chloroplast, 7 - silicalemma cisterna, 8 - Golgi vesicles whose fusion forms the silicalemma cisterna, 9 - siliceous cyst wall inside the silicalemma, 10 - extracycisternal cytoplasm being retracted and partially differentiated into the plug, 11 - plug, 12 - inner silicalemma membrane becoming the cyst plasmalemma (after various authors).

Reproduction and life cycles

Chrysophyta reproduce asexually and, rarely, sexually. Asexual reproduction in monad and amoeboid representatives occurs through Cell Division in the motile state and via zoospores. In addition, multinucleate amoeboid forms (so-called plasmodial algae) can disintegrate into many uninucleate amoeboids, each of which, through growth and mitoses, forms a new plasmodium. Multicellular representatives are capable of fragmentation and asexual reproduction via zoospores. Some hemimonad, coccoid, and filamentous Chrysophyta also reproduce using aplanospores.

The sexual process is rare and is represented predominantly by hologamy, with cells uniting by their posterior ends during copulation. Autogamous, isogamous, and heterogamous sexual processes have also been observed. Eugamic golden algae possess a haplophasic life cycle with zygotic reduction and a specific stage of binucleate zygotic cysts (Fig. 13.5).

Fig. 13.5. Zygotic cyst of the golden alga Dinobryon: 1 - pore, 2 - plug, 3 - oil droplet, 4 - chrysolaminarin vacuole, 5 - cyst wall, 6 - chloroplast, 7 - nucleus, 8 - mitochondrial profile, 9 - contractile vacuole, 10 - Golgi complex (after Gorbunova, 1991).

For example, in Dinobryon, Mallomonas, Ochromonas, following Cell Fusion (plasmogamy), the nuclei do not fuse immediately. Consequently, a binucleate cell is formed, which becomes spherical and transforms into a cyst with a structure characteristic of Chrysophyta. Unlike ordinary cysts, this cyst contains two dissimilar nuclei—female and male—meaning it is dikaryontic. The zygotic cyst enters a resting state. Upon completion of the resting period, the cyst nuclei fuse (karyogamy takes place) to form a true zygote. Subsequently, the diploid zygote nucleus undergoes reduction division, the pore opens, and one to four haploid zoospores emerge through it, giving rise to new vegetative cells.

Phylum systematics

The phylum Chrysophyta is divided into two classes: Chrysophyceae and Synurophyceae. In Chrysophyceae, unlike Synurophyceae, cells are not covered with siliceous scales. This phenotypic feature is in complete agreement with the molecular Phylogenetic Tree derived from The nucleotide sequence Analysis of the nuclear 18S ribosomal RNA subunit Gene.

Until recently, the class Prymnesiophyceae was also included in the Chrysophyta. However, both cytological and molecular studies have demonstrated that prymnesiophytes do not belong to the stramenopiles at all and should be segregated into an independent phylum within another kingdom, the Platicornea (Platicrysta).

Class Chrysophyceae

This class unites unicellular and multicellular algae featuring all morphological body types typical of the division: monadal (including amoeboid and plasmodial), hemimonadal, coccoid, and filamentous. A distinctive feature of the class is the absence of silica scales on the surface of the plasmalemma. All members of the class are freshwater algae. Based on their morphological structure, chrysophytes are divided into four orders (Table 13.1).

Table 13.1. Main characteristics of the orders of chrysophyte algae

The Ochromonadales are the most widely distributed and species-rich order. It includes unicellular, colonial, and coenobial species with a typical monadal structure. They occur predominantly in the plankton of clean freshwater bodies. The order is subdivided into families and genera According to the number of flagella, body organization plans (unicellular or coenobial), and outer coverings (such as the presence of loricae). The leading genera of the order are Ochromonas, Chromulina, and Dinobryon (Fig. 13.6).

Fig. 13.6. Ochromonadales (1-5) and Chrysamoebales (6-8) algae. 1 – Ochromonas; 2, 3 – Chromulina: vegetative cell (2) and cyst (3); 4, 5 – Dinobryon: general view of the colony (4) and vegetative cells after division (5) (after Matvienko, 1965; Starmach, 1968).

The genus Ochromonas is characterized by naked, slightly metabolic, solitary monadal cells bearing two unequal flagella and a single yellow chloroplast. Ochromonas is most frequently encountered in winter in the plankton of water bodies situated among sphagnum bogs.

Ochromonas is considered the most primitive golden alga. Interestingly, some species of this genus are capable of forming short pseudopodia, allowing them to feed not only photoautotrophically but also holozoically.

Chromulina resembles the previous genus, except that the short flagellum is reduced, leaving only the basal body. Species of this genus are found primarily during the colder seasons in sphagnum bogs.

The cells of Dinobryon species are also structurally similar to Ochromonas, but they reside inside loricae. These loricae are funnel-shaped with a blindly closed, tapered base and a flared, open apex. Each cell is attached to the base of the lorica by a contractile basal stalk.

The organic substances of the lorica—cellulose microfibrils mixed with Amino Acids—are synthesized in the Golgi apparatus, subsequently transported to the surface of the plasmalemma, and polymerized there.

During reproduction, the protoplast of the mother cell divides to form two naked daughter cells. They emerge from the lorica, settle on its rim near the aperture, and subsequently build their own loricae. As a result, typically treelike, dichotomously branched colonies are formed.

Species of this genus are typical inhabitants of freshwater plankton (most commonly lakes) and can be found at any time of the year.

The Chrysamoebales comprise algae with an amoeboid structure. The cells of chrysamoebalean algae are capable of forming slender, unbranched or branched rhizopodia, and more rarely, short axopodia.

A characteristic representative is Chrysamoeba (Fig. 13.6). Its cells are solitary and possess numerous long rhizopodia. The cytoplasm clearly reveals a golden chloroplast, a chrysolaminarin vacuole, and contractile vacuoles. The amoeboid cells are capable of transitioning into a short-lived motile state, during which the rhizopodia are retracted and a single long flagellum is formed. Under unfavorable conditions, the alga enters a palmelloid state, becoming enclosed in a mucilaginous capsule. Species of this genus inhabit primarily the plankton of standing water bodies.

The Chrysocapsales constitute a small order of algae with hemimonadal, and less frequently, coccoid structural types. As a rule, the cells of chrysocapsalean algae are embedded in colonial mucilage. Almost all chrysocapsaleans are rare algae.

More frequently encountered are the genus Hydrurus, a freshwater benthic alga, and the genus Chrysostephanosphaera, which inhabits the plankton of small freshwater bodies (Fig. 13.7). Interestingly, the cells of Chrysostephanosphaera are capable of transitioning into a monadal or amoeboid state, thereby resembling Chromulina or Chrysamoeba, respectively. Chrysocapsalean algae reproduce via zoospores.

Fig. 13.7. Chrysocapsales (1-4) and Phaeothamniales (5, 6) algae. 1, 2 – Hydrurus: general view of the colony (1) and cell fragment (2); 3, 4 – Chrysostephanosphaera: colony (3) and zoospore (4); 5 – Phaeothamnion; 6 – Phaeodermatium (after Topachevsky, Masyuk, 1984).

The Phaeothamniales comprise a small number of multicellular freshwater species. The thalli of filamentous phaeothamnialeans are uniseriate or multiseriate, branched or simple. Heterotrichous representatives feature thalli in the form of parenchymatous plates. Characteristic genera include Phaeothamnion (possessing a thallus in the form of a uniseriate branched filament) and Phaeodermatium (with a thallus shaped like a parenchymatous plate), which are occasionally found in clean mountain rivers.

Class Synurophyceae

This class unites unicellular, predominantly monadal algae whose plasmalemma is covered with spirally arranged silica scales. These scales are usually clearly visible under a Light Microscope. Each scale consists of a plate and an attached bristle. The plate is perforated with minute pores, features a thickened margin, and occasionally bears small spines and warts. The bristle is attached to the lower edge of the plate and resembles a hollow tube.

The formation of the plates and bristles involves the chloroplast Endoplasmic reticulum and vesicles derived from the Golgi apparatus. Specifically, a silica-containing vesicle first buds off from the outer membrane of the chloroplast endoplasmic reticulum. In the cytoplasm, it fuses with vesicles that have budded off from the Golgi apparatus. This creates a vesicle in which the final shaping of either the plate or the bristle is completed. The vesicle then migrates to the cell surface, its membrane fuses with the plasmalemma, and the structural element of the scale (either the plate or the bristle) is delivered to the cell surface. The bristles are extruded first, followed by the plates. The assembly of the bristles and plates into a complete, normal scale takes place outside the cell.

The class includes a single order, Synurales (= Mallomonadales).

The most species-rich genus of the class is Mallomonas. The algal cells are solitary, free-swimming, and possess only a single flagellum. The plasmalemma is covered with numerous imbricate silica scales bearing long bristles. The cysts also feature an additional layer of scales, typically armed with long spines.

Algae of the genus Synura form spherical colonies consisting of numerous cells joined at their posterior ends. Each cell bears two flagellates. The plasmalemma is covered with characteristic Synurophycean silica scales, each consisting of a plate and a short bristle.

In some species (notably S. petersenii), sexual reproduction has been identified and thoroughly studied. The type of sexual process is hologamy. Vegetative cells functioning as female gametes secrete an attractive hormone. Stimulated by this hormone, cells functioning as male gametes leave the colony, swim toward the female cell, and copulate. As a result, a zygotic dikaryotic cyst is formed. Its subsequent development follows the pattern typical of golden algae.

Fig. 13.8. Synurophycean algae. 1 - Mallomonas; 2-4 - Synura: colony (2), diversity of scales (3); a single scale viewed in three planes (4) (after Matvienko, 1965; Gorbunova, 1991).

Distribution and Significance

Golden algae are primarily distributed in freshwater bodies. Like most Representatives of the yellow-pigmented group, chrysophytes are psychrophilic organisms that undergo mass development either in polar latitudes or during the cold seasons in temperate latitudes.

Siliceous-scaled golden algae have contributed to the formation of sedimentary silicate deposits known as archaeomonadites. Fossil remains of Chrysophyta, along with those of other organisms, are utilized in geology to determine the age of sedimentary rocks.

Freshwater Chrysophyta occur mainly in the plankton of clean water bodies (predominantly species of Chromulina), and much less frequently in the benthos and periphyton (Chrysamoebales, Chrysocapsales, Phaeothamniales). During cold seasons, certain freshwater representatives can cause a golden or yellowish-brown water "bloom" (species of the genera Chromulina, Mallomonas, Dinobryon, Synura); in such cases, the water acquires a characteristic odor reminiscent of stale fish oil. Some golden algae are used as bioindicators for water quality assessment.

Taxonomic Position

Chrysophyta represent a typical division of photoautotrophic tubulicristate stramenopiles. According to molecular data—which correlate well with certain morphological and cytological features—they are most closely related to the division Eustigmatophyta.