Phycology - Kostikov, I.Yu. - 2009-2013

Chapter 9. Blue-green algae – Cyanophyta

Blue-green Algae (Cyanophyta), also known as cyanoprokaryotes (Cyanoprocaryota) or cyanobacteria (Cyanobacteria), are the only phylum of prokaryotic organisms capable of oxygenic Photosynthesis. The group comprises about 2,000 species. Blue-green algae can be unicellular or multicellular and are predominantly microscopic; only a few colonial species reach significant sizes (e.g., Nostoc commune or Gloeotrichia species).

Blue-green algae represent one of the oldest lineages on the planet. It is believed that Cyanophyta emerged approximately 3.5–3.8 billion years ago (for comparison, the age of the Earth, based on meteorite lead analysis, is 4.6 billion years, and the first eukaryotes appeared about 1.5 billion years ago).

The oldest fossil remains of blue-green algae (so-called stromatolites) date back approximately 3.2 billion years. From that time and for nearly 2 billion years, Cyanophyta dominated the globe. Thanks to the ability of blue-green algae to perform oxygenic photosynthesis, an oxygen-rich atmosphere formed on the planet.

Class="center">Taxonomic CHARACTERISTICS OF THE phylum

Pigments and reserve nutrients

Blue-green algae are predominantly blue-green in color, though under extreme conditions they often exhibit various shades of red. Several species with strictly green Cells are also known.

Until recently, the pigment composition in Cyanophyta was considered fairly uniform: chlorophyll "a" imparts the green hue to the cells. The red or blue coloration is due to the presence of significant amounts of phycobilin pigments—phycocyanin, allophycocyanin (blue pigments), and phycoerythrin (red pigment). Carotenoids are represented solely by β-carotene, xanthophylls of the lutein cycle (lutein and zeaxanthin), and specific xanthophylls characteristic of blue-green algae—primarily oscilloxanthin, myxoxanthophyll, aphanin, and aphanizophyll.

Cyanophyta also includes a small yet fascinating group of green prokaryotic algae discovered in 1976, known as prochlorophytes. In addition to chlorophyll "a", these algae possess chlorophyll "b" or a chlorophyll-like pigment (divinyl-chlorophyll a-like Mg-porphyrin), and occasionally α-carotene as well.

In almost all Cyanophyta, the primary assimilation product is a Glycogen-like polysaccharide—cyanophycean starch, and rarely a polysaccharide resembling true plant starch (in "prochlorophytes"). In addition to CARBOHYDRATES, most blue-green algae store cyanophycin and volutin. Cyanophycin is a polymer of the Amino Acids Arginine and asparagine, serving as a reserve source of readily available nitrogen for The Cell. Volutin is a storage substance composed of phosphorus compounds, forming so-called polyphosphate granules.

Cytological characteristics

The cells of Cyanophyta have a typical prokaryotic Structure (Fig. 9.1): they lack a morphologically defined nucleus, Mitochondria, METABOLISM/14.html">Chloroplasts, Endoplasmic reticulum, Golgi apparatus, and Lysosomes. Cyanophyta also lack any microtubule-based structures—flagella, basal bodies, cytoskeletal microtubular elements, centrioles, and the mitotic spindle. Blue-green algae are incapable of mitosis, Meiosis, and the typical eukaryotic sexual process.

Fig. 9.1. Structure of a blue-green algal cell: 1 - Plasmalemma; 2, 3, 3a, 3b - Cell wall: murein layer (2); periplasmic space with hemicellulose-pectin fibrils (3); outer membrane (3a); outer serrated layer with oscillin fibrils (3b); connecting pore complex (3c); 4 - mucilaginous sheath; 5 - transverse septum; 6 - pore with plasmodesma; 7 - thylakoid with phycobilisomes; 8 - cyanophycin granule; 9 - cyanophycean starch granules; 10 - lipid globule; 11 - nascent transverse septum; 12 - nucleoid; 13 - polyhedral body; 14 - polysome; 15 - gas vacuole; 16 - polyphosphate body (schematized after Kondratieva, 1989; Anagnostidis, Komárek, 1986; 1988; 1990; Hoiczyk, Baumeister, 1998).

Cell coverings in Cyanophyta are represented by The cell wall. It lies outside the plasmalemma and consists of several layers: a) a murein layer; b) a periplasmic space filled primarily with pectin and hemicellulose; c) an outer membrane; and d) a serrated layer with surface Glycoproteins known as oscillin fibrils. Passing through the murein layer and the periplasmic space near the transverse septum at an angle of 30–40° to the longitudinal axis of the cell are the so-called connecting pore complexes, through which locomotor slime is secreted. The secretion of this slime enables many blue-green algae to perform active gliding or rotational movement. Overall, the cell walls of Cyanophyta resemble those of Gram-negative Bacteria, although the murein content relative to the total wall mass is higher than in bacteria (22–52% compared to 5–10%).

External to the cell wall, most species secrete pectic substances that form colonial slime or mucilaginous sheaths.

Nuclear apparatus. Blue-green algae lack a true nucleus; its Functions are performed by the nucleoid. The nucleoid consists of one or more circular DNA molecules located directly in the Cytoplasm. The nucleoid DNA is not associated with histone Proteins and lacks chromosomal Organization, although it is considered the functional analog of a single chromosome. The nucleoid attaches directly to the plasmalemma. Typically, the bulk of the nucleoid is concentrated in the center of the cell, which is why this region of the cytoplasm is referred to as the nucleoplasm or centroplasm.

Prior to division, the nucleoid DNA replicates; subsequently, the attachment points of the parent and daughter DNA molecules to the plasmalemma begin to separate, and an invagination of the plasmalemma forms between them, ultimately partitioning the nuclear apparatuses of the two new cells (Fig. 9.2).

Fig. 9.2. Blue-green algae. A - diagram of nucleoid division (1 - cell prior to division; 2 - nucleoid Replication; 3 - onset of Separation of nucleoid attachment points on the plasmalemma; 4 - daughter cells). B - structure of a phycobilisome (5 - thylakoid membrane; 6 - allophycocyanin; 7 - phycocyanin; 8 - phycoerythrin) (B - after Gantt, 1981).

In addition to the nucleoid, many blue-green algae possess Plasmids in their cytoplasm—small, circular DNA molecules that, unlike the nucleoid, lie freely in the cytoplasm.

Genetic information is exchanged between different blue-green algal cells via parasexual processes—Transformation and Conjugation. During transformation, DNA from a donor cell enters the external environment in a dissolved state and is subsequently taken up by a recipient cell, "integrating" into its nucleoid. During conjugation, two cells connect directly via a narrow cytoplasmic channel through which DNA molecules, DNA fragments, or plasmids are exchanged.

Photosynthetic apparatus is represented by thylakoids, which are derivatives of plasmalemma invaginations. The thylakoids are not separated from the cytoplasm by a double-membrane envelope, meaning the photosynthetic apparatus is not organized into discrete Plastids. Thylakoids typically occur singly, though rarely (in certain prochlorophytes) they are stacked in structures somewhat resembling grana. They are usually localized in the peripheral layer of the cytoplasm. Under a Light Microscope, the zone containing thylakoids is intensely pigmented, and is therefore termed the chromatoplasm. Chlorophyll molecules and accessory pigments—carotenes and xanthophylls—are "embedded" in the thylakoid membranes.

On the surface of the thylakoids in all representatives except prochlorophytes, specialized "antenna" structures known as phycobilisomes are located. Each phycobilisome consists of three allophycocyanin globules anchored directly into the thylakoid membrane. Extending from the allophycocyanin globules are six "rods" composed of phycocyanin globules at the base and phycoerythrin globules at the apex (Fig. 9.2). The number of phycoerythrin globules can vary depending on the spectral characteristics of light: under prevailing short-wave blue and violet illumination, their number increases and the cells acquire a red coloration; conversely, during photosynthesis in the middle to long-wave PARTS OF THE spectrum, The amount of phycoerythrin decreases, and the cells turn blue-green. This phenomenon is known as chromatic adaptation.

Polyhedral bodies are also indirectly considered to be elements of the photosynthetic apparatus in Cyanophyta. They are typically located within the chromatoplasm and appear as angular inclusions. Polyhedral bodies are formed by the enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCo), which is involved in carbon dioxide assimilation during the Dark Phase of photosynthesis. Polyhedral bodies serve as precursors to the pyrenoid in eukaryotic algae.

Other cytoplasmic structures. All Cyanophyta possess Ribosomes diffusely distributed throughout the cytoplasm. These ribosomes are smaller than Eukaryotic cytoplasmic ribosomes (measuring 21 × 29 nm with a sedimentation coefficient of about 70S, compared to 22 × 32 nm and 80S in the cytoplasm of Eukaryotic cells). Ribosomes similar to those of Cyanophyta have been found in eukaryotic chloroplasts and mitochondria.

Inclusions include granules of reserve nutrients as well as gas vacuoles. Granules of Reserve Polysaccharides (blue-green algal starch, starch-like polysaccharide of "prochlorophytes") are small; in typical vegetative cells, they are visible only under an Electron microscope and are localized among thylakoids. In resting cells—akinetes—complexes formed by polysaccharide granules can also be observed at the light-optical level.

Cyanophycin and polyphosphate granules are large, reaching up to 2 µm in diameter. The former tend to concentrate near the transverse cell walls in many filamentous blue-green algae, while the latter are more or less diffusely scattered throughout the cytoplasm.

In some (predominantly planktonic) species, gas vacuoles are present in the cytoplasm. Under a light microscope, they appear as small, highly light-refractive areas of the cytoplasm. Electron Microscopy has revealed that such areas consist of numerous hexagonal, tightly packed chambers, each separated from the others by its own membrane, which is proteinaceous rather than phospholipid. Under favorable conditions, the chambers fill with gas, reducing the specific gravity of the cell. Under unfavorable conditions, the gas diffuses out, the gas vacuoles decrease in volume and collapse, the specific gravity of the cell increases, and the alga sinks to the bottom, where it "awaits" improved conditions for photosynthesis.

Physiological and Biochemical characteristics of the Division

Among prokaryotes, Cyanophyta is the only division whose representatives possess both the First and Second Photosystems, thereby being capable of oxygenic photosynthesis. This process is described by the universal plant equation:

2H₂O + CO₂ + hγ = [CH₂O] + H₂O + O₂

At the same time, they possess at least two important PHYSIOLOGICAL AND BIOCHEMICAL features that can be considered physiological atavisms: the capacity for anoxygenic photosynthesis and atmospheric Nitrogen Fixation.

Anoxygenic photosynthesis in Cyanophyta typically occurs under anaerobic conditions in the presence of a sufficient amount of hydrogen sulfide in the environment. In this case, blue-green algae use hydrogen sulfide as a proton and electron donor for photosynthesis:

2H₂S + CO₂ + hγ = [CH₂O] + H₂O+ 2S

Most often, the end product of hydrogen sulfide oxidation is molecular sulfur, which can accumulate in the cytoplasm. However, some marine Cyanophyta oxidize hydrogen sulfide to sulfides. A similar metabolic pathway is fundamental for many gracilicutes photosynthetic bacteria, whereas in Cyanophyta, this pathway is secondary.

Atmospheric nitrogen fixation (nitrogen fixation) is considered one of the most ancient physiological processes, having emerged as early as in archaebacteria. Meanwhile, no eukaryotic Organism is capable of nitrogen fixation. During nitrogen fixation, molecular nitrogen is reduced to ammonium compounds and, in this form, incorporated into the cell's core metabolic pathways. The nitrogen-fixation process is catalyzed by The Nitrogenase Enzyme complex, which is completely inhibited by molecular oxygen. Therefore, nitrogen fixation occurs exclusively in an anaerobic environment.

Today, the capacity for nitrogen fixation under anaerobic conditions has been discovered in a fairly wide range of unicellular and multicellular blue-green algae. However, among multicellular Cyanophyta, there are numerous species that also fix atmospheric nitrogen in the presence of oxygen. In this group, the nitrogenase complex is localized in specialized cells called heterocysts, which feature several adaptations that prevent free oxygen from entering the cell, thereby creating anaerobic conditions within the heterocyst cytoplasm against the Background of the aerobic external environment.

Thallus Structure

Blue-green algae can be either unicellular or multicellular. Both unicellular and multicellular representatives may be solitary or form various colonies in which individuals are held together mostly by colonial mucilage.

The fundamental and mandatory structural element of multicellular algae is the trichome. A trichome is an aggregate of physiologically connected cells. This connection is mediated through plasmodesmata passing through Pores in the transverse cell walls. Structured mucous formations, known as sheaths, may lie exterior to the trichome and typically serve a protective function. A trichome together with its sheath is called a filament. In species lacking sheaths, the trichome and the filament are synonymous concepts.

Trichomes can be unbranched or branched. Cells of unbranched trichomes divide in only one plane. If cells are capable of division in multiple planes, the trichomes branch; this type of branching is termed true branching. Conversely, when the filaments branch while the trichomes themselves remain unbranched, the branching is considered false (Fig. 9.3).

Main Cell Types

Three main cell types are distinguished in blue-green algae: a) vegetative cells, which carry out photosynthesis and are capable of division; b) heterocysts, specialized cells that perform the function of atmospheric nitrogen fixation; and c) akinetes, resting cells through which algae endure unfavorable conditions (Fig. 9.3). Vegetative cells are characteristic of all Cyanophyta, whereas heterocysts and akinetes occur only in some multicellular representatives. Trichomes consisting solely of vegetative cells are termed homocytic, while those comprising vegetative cells, heterocysts, and akinetes are termed heterocytic.

Heterocysts contain the nitrogenase enzyme complex and perform the functions of atmospheric nitrogen fixation. This function—specifically the need to protect nitrogenase from the destructive action of oxygen—determines the morphological features of heterocysts: they are colorless, incapable of photosynthesis, and possess a thickened, typically triple-layered wall. Heterocysts obtain the necessary organic nutrients from adjacent vegetative cells. Connection with the latter is achieved via plasmodesmata passing through pores in the transverse cell walls. On the heterocyst side, the pores are sealed by a plug—readily visible under a light microscope—that blocks oxygen from entering the heterocyst while remaining permeable to organic substances and Water.

Akinetes develop from vegetative cells and also feature a thickened wall. However, they retain chlorophyll and accumulate large quantities of reserve nutrients, typically a glycogen-like polysaccharide.

Fig. 9.3. Filament branching (A–C) and cell types (D) in multicellular Cyanophyta. A – unbranched filaments, B – filaments with false branching, C – true branching; D – filaments with heterocytic trichomes: 1 – vegetative cells, 2 – heterocysts, 3 – akinetes (after Anagnostidis, Komárek, 1988; Kondrateva, 1968)

Cell Division and Reproduction

Reproduction in all blue-green algae is fundamentally based on cell division. During this process, the plasmalemma first grows centripetally into the protoplast, followed by the murein layer of the cell wall, dividing the cell into two (rarely more) equal or unequal parts. Cell division in blue-green algae can be either complete or incomplete.

In complete division, the plasmalemma and the cell wall completely separate the daughter cells from one another. Pores and plasmodesmata are not formed in the process, and each daughter cell represents a physiologically independent individual. Complete division is characteristic of unicellular Cyanophyta.

Complete division is divided into three types: normal (binary), equal multiple, and unequal (Fig. 9.4).

Fig. 9.4. Types of division in blue-green algae: A - normal (binary); B - equal multiple with nanocyte formation; C - unequal with exospore formation; D - incomplete division. 1 - cell wall, 2 - mucilaginous sheath, 3 - nanocyte, 4 - exocyte, 5 - pore (after Komárek, Anagnostidis, 1986; Anagnostidis, Komárek, 1988)

During normal division, the mother cell divides into two daughter cells of equal size, which usually become more or less separated from each other. This type of division is typical for almost all unicellular blue-green algae.

In equal multiple division, several rapid successive binary divisions occur, and the daughter cells are held together for some time by the mucilaginous Sheath of the mother cell. They do not grow during division and become significantly smaller in size than the mother cell. Therefore, they are called nanocytes¹.

Unequal division somewhat resembles budding: the mother cell divides into two cells of unequal size. Occasionally, these cells remain connected for some time by the mother cell's mucilaginous sheath, but more often, upon completion of division, the sheath breaks down on the side of the smaller cell, and the latter separates from the larger one. In such cases, the smaller cell is called an exocyte².

Incomplete binary division is essentially similar to normal binary division; however, the transverse septum does not completely divide the daughter cells, and they remain connected by a plasmodesma passing through a pore in the transverse cell wall. This type of division results in The formation of multicellular filamentous trichomes.

Short, motile fragments of trichomes consisting of 2–50 cells are called hormogonia, while non-motile ones are called hormocytes. Hormogonia and hormocytes are specialized reproductive structures of multicellular Cyanophyta.

Colonial unicellular and multicellular blue-green algae are also capable of reproducing through colony fragmentation.

Systematics of the Division

Today, there are several Classification systems for Cyanophyta, which can broadly be divided into three groups: classical morphological, morphocytological, and

molecular-biological. These three groups of systems differ significantly from one another, with each having its own Advantages and disadvantages.

Classical Systems

Systems in this group are primarily based on the morphological Features of the thallus and colony structure, as well as on types of reproduction. The morphological approach to the systematics of Cyanophyta was developed in the early 20th century by several phycologists simultaneously, among whom the works of L. Geitler and A.A. Elenkin played a particularly significant role. Classical systems are user-friendly, allow for relatively easy species identification, and help place newly discovered, previously unknown taxa within the system. The main drawback of classical systems is that they largely fail to reflect the actual Phylogeny of the division.

According to classical systems, the division was most commonly divided into three classes: Chroococcophyceae, Chamaesiphonophyceae, and Hormogoniophyceae. The main diagnostic features of these classes were the structural plan of the vegetative body (unicellular or multicellular), the mode of reproduction (in particular, The ability to form nanocytes, exocytes, hormogonia, and hormocytes), and, to some extent, the lifestyle (attached or free-floating).

Morphocytological System

This system was developed by J. Komárek and K. Anagnostidis in the mid-1980s to synthesize extensive data on the Cytology of individual representatives alongside comparative physiological and biochemical research on Cyanophyta. Taxa at the rank of class and certain orders aligned well with molecular phylogenetic trees. However, at the intermediate taxonomic levels—families and genera—the system remained largely artificial. Furthermore, The Use of various ultrastructural features as diagnostic criteria for families and genera made it more complex for Practical Application compared to the morphological system. Despite this, the morphocytological system has become the leading framework in global phycological research today.

According to the morphocytological system, the division Cyanophyta includes only a single class, Cyanophyceae, which is divided into four orders: Chroococcales, Oscillatoriales, Nostocales, and Stigonematales (Table 9.1).

Table 9.1. Diagnostic features of the orders of the class Cyanophyceae

Chroococcales includes all unicellular blue-green algae. Cell division is complete; the primary type is binary division, while supplementary types include equal multiple and unequal division resulting in the formation of nanocytes and exocytes. Chroococcal algae may occur as solitary cells or form various types of colonies.

Gloeobacter, which inhabits hot springs, is considered the most primitive blue-green alga. The cells of this alga are solitary and more or less cylindrical. Division occurs exclusively in a plane perpendicular to the longitudinal axis of the cell. The cells of Gloeobacter lack thylakoids and true phycobilisomes: chlorophyll and phycobilin pigments are located directly on the plasmalemma. Certain biochemical features indicative of primitiveness have also been discovered: in particular, the alga does not synthesize certain Lipids (from the diacylglycerol group) that are characteristic of all other oxygenic photosynthetic organisms, including eukaryotes. The assumption regarding the primitiveness of Gloeobacter is in full agreement with molecular-biological data.

The morphological "double" of Gloeobacter is the genus Synechococcus. However, the cells of this alga already possess thylakoids and typical phycobilisomes, and they synthesize diacylglycerols. Representatives of this genus are found in continental water bodies (including thermal biotopes) as well as in marine environments. In the freshwater alga Tubiella, the cells resemble those of Gloeobacter and Synechococcus, but they are enclosed within tubular mucilaginous colonies (Fig. 9.5).

Fig. 9.5. Chroococcales. 1 – Synechococcus; 2 – Tubiella; 3 – successive stages of cell division in Synechocystis; 4 – Merismopedia; 5, 6 – Microcystis (5 – successive stages of cell division, 6 – colonies); 7 – Gloeocapsa; 8, 9 – Chlorogloea (8 – colony of cubic aggregates, 9 – gloeocapsoid cubic aggregate); 10 – fragment of a Siphononema colony (after Kondratieva, Kovalenko, Prykhodkova, 1984; Kovacik, 1988).

Species of the genus Synechocystis possess more or less spherical cells that typically do not form colonies. The cells divide alternately in two planes, with the plane of each successive division perpendicular to that of the previous one. This phenomenon is clearly observable when the algae are grown on an agarized nutrient medium. Similar cells and division patterns are characteristic of species of the genus Merismopedia. However, the cells of Merismopedia are always held together by colonial mucilage, forming rectangular single-layered plates in which the cells are arranged in regular, mutually perpendicular rows. Both genera inhabit the plankton of freshwater bodies and coastal saline lakes.

Representatives of the genus Microcystis also feature spherical cells and form mucous colonies. However, cell division occurs in three planes, and the plane of each successive division is not always perpendicular to the preceding one. As a result, the colonies acquire an irregular shape. In most species of this genus, the cells contain gas vacuoles. Microcystis inhabits the plankton of freshwater and brackish water bodies; certain species (M. aeruginosa, M. wesenbergii) are capable of massive development and represent some of the most dangerous agents of water "blooming". Individual species are also found in marine environments, soils, and on damp aerophytic substrates.

The cells of Gloeocapsa are more or less spherical, divide in three mutually perpendicular planes, are capable of rapid multiple division with the formation of nanocytes, and form characteristic mucous colonies consisting of a system of nested mucous envelopes. Species of this genus are found in all types of biotopes (marine and continental waters, soils), yet they most frequently settle on spray-drenched rocks, waterfall faces, and damp stones.

Massive, typically macroscopic mucous colonies, in which the cells are arranged in indistinct rows, are characteristic of the genera Chlorogloea and Siphononema. In both representatives, cells divide in three directions and are capable of producing nanocytes. The differences lie in the fact that the cells of Chlorogloea generally lack distinct individual mucous sheaths, making the colonial mucilage appear structureless. One species of this genus, Chlorogloea sarcinoides, develops massively in saline lakes and participates in the formation of therapeutic mud (peloids), whereas other species occur sporadically, primarily in the periphyton of freshwater bodies. In Siphononema, individual cell envelopes are distinct, and the colonies resemble filamentous complexes composed of numerous gloeocapsas. This alga is occasionally found in the periphyton of clean mountain streams and rivers.

Distinctly filamentous colonies are formed by Pascherinema (=Endonema), Hyella, and Pleurocapsa. Although all cells are capable of dividing in multiple directions, division typically occurs in a plane parallel to that of the previous division. The cells within the filaments are held together by thin, tough mucous sheaths. Nanocytes are formed quite frequently. In Pascherinema, the colonies are usually unbranched, whereas in Hyella they are sparingly branched. Pleurocapsa colonies branch abundantly, acquiring a parenchymatous appearance in places (Fig. 9.6).

Fig. 9.6. Chroococcales. 1 – Pascherinema: filamentous colony and formation of nanocytes; 2 – successive developmental stages and nanocyte formation in the boring alga Hyella; 3 – parenchymatous-filamentous colony and nanocyte formation in Pleurocapsa; 4 – Chamaesiphon: mature cells, formation of exocytes, and their germination (after Kondratieva, Kovalenko, Prykhodkova, 1984; Komárek, Anagnostidis, 1986).

Pascherinema is occasionally found in freshwater bodies as epiphyton on higher aquatic plants as well as on Sphagnum moss leaves. Hyella belongs to the group of so-called boring algae: it settles on calcareous substrates and penetrates them, causing their destruction. Pleurocapsa has a broad ecological amplitude—freshwater, marine, soil, and aerophytic species are known within this genus.

An example of an alga capable of forming exocytes is Chamaesiphon. The cells of Chamaesiphon do not form colonies; they are surrounded by a thin, tough mucous sheath that ruptures at the apex upon completion of division, releasing the exocyte. The alga inhabits freshwater bodies, predominantly growing on filamentous green algae.

Oscillatoriales unites multicellular homocytes having unbranched trichomes. Cell division is incomplete and occurs in a plane perpendicular to the longitudinal axis of the trichome. Reproduction is carried out via hormogonia or hormocytes.

Examples of primitive oscillatorialeans are Pseudanabaena and Leptolyngbya (Fig. 9.7). Both genera possess thin trichomes (0.5–2 µm wide) and more or less barrel-shaped cells, the length of which is approximately twice their width. Thylakoids are located at the cell periphery in several concentric circles (Fig. 9.8). Any cell of the trichome is capable of division, and before each new division, the daughter cell grows to the size of the mother cell. Reproduction is accomplished through motile hormogonia formed by trichome fragmentation. Species of these genera are capable of chromatic adaptation.

Fig. 9.7. Oscillatoriales. 1, 2 – Pseudanabaena (1 – P. galeata, 2 – P. catenata); 3 – Leptolyngbya boryana; 4, 5 – Trichodesmium erythraeum (4 – colony, 5 – trichome); 6 – Planktothrix agardhii; 7 – Phormidium autumnale; 8, 9 – Microcoleus vaginatus (8 – central part of the filament with multiple trichomes, 9 – trichomes in the apical part of the filament); 10 – Arthrospira platensis; 11 – Spirulina major; 12 – Oscillatoria limosa (after Kondratieva, 1969; Anagnostidis, Komárek, 1988; Komárek, Lund, 1990; Garbacki et al., 1999; orig.).

Fig. 9.8. Diagram of thylakoid arrangement in various representatives of multicellular cyanoprokaryotes/cyanobacteria. 1 – Pseudanabaena, Leptolyngbya; 2 – Trichodesmium, Planktothrix, Phormidium, Microcoleus; 3 – Oscillatoria, Scytonema, Anabaena, Stigonema (after Anagnostidis, Komárek, 1988, 1990; Komárek, Anagnostidis, 1989).

Pseudanabaena has deeply constricted cells containing one or two gas vacuoles. The trichomes are relatively short (up to 50–80 cells) and lack mucous sheaths. Representatives are widespread in marine and freshwater habitats.

Leptolyngbya unites thin-filamentous species that, in classical Taxonomy, were predominantly included in the genera Phormidium, Lyngbya, and Plectonema. Leptolyngbyas typically possess mucous sheaths, quite frequently exhibit false branching (especially in culture), and lack gas vacuoles. Interestingly, in some cases, cells exhibit uneven division resembling that of Chamaesiphon. Species of this genus are widely distributed, primarily in benthic and periphytic communities of freshwater bodies, as well as in soils. Several representatives have been found in marine biotopes.

A numerous group is constituted by algae possessing medium-sized trichomes (2–8 [15] µm wide), cylindrical cells not constricted at the transverse cross-walls, whose length in the mature state is approximately equal to their width. Thylakoids are located radially at the cell periphery. Any cell of the trichome is capable of division; prior to each new division, the daughter cell grows to the size of the mother cell. Following division, relatively large pores (about 15–20 nm) remain between the cells. The ability for chromatic adaptation within this group has not been detected. Characteristic representatives include the genera Trichodesmium, Planktothrix, Phormidium, Microcoleus, and Arthrospira.

Species of the genus Trichodesmium inhabit predominantly marine plankton (specifically, T. erythraeum causes water blooms in the Red Sea, from which the latter derived its name). The trichomes of algae of this genus are usually gathered into colonies held together by amorphous mucilage, and the cells are filled with numerous gas vacuoles. Species of the genus Planktothrix are morphologically similar to Trichodesmium, yet they inhabit the plankton of freshwater bodies and do not form mucous colonies. These algae also cause water blooms: for example, P. agardhii in lakes and ponds of lowland areas, and P. rubescens in mountain lakes (in particular, blooms caused by this species in Swiss lakes were recorded as early as 1825).

Phormidium is the richest species genus not only of the order, but of the division as a whole (uniting over 300 species). Unlike Trichodesmium and Planktothrix, the cells lack gas vacuoles and inhabit the benthos and plankton of marine and freshwater continental bodies, as well as soils. Trichomes are typically clothed in mucous sheaths or devoid of them.

Species of the genus Microcoleus are characterized by the presence of mucous sheaths enclosing multiple trichomes. In other features, Microcoleus resembles the preceding genus and inhabits primarily soils.

A characteristic feature of the genus Arthrospira is the presence of Phormidium- or Trichodesmium-like trichomes twisted into a regular, wide spiral. Species of the genus are found in freshwater bodies with a high carbonate content (e.g., A. platensis, A. maxima) or polluted with organic matter (A. jenneri). The first two species have been introduced into industrial culture and are cultivated in many countries worldwide as food algae and pharmacological raw material. The genus Spirulina is also considered close to Arthrospira in its morphological and cytological classification system. Trichomes in species of this genus are thin, twisted into very tight spirals. Spirulina species inhabit mainly marine environments and coastal lagoons, and are less frequently encountered in freshwater continental bodies.

An example of another morphological and cytological group is the genus Oscillatoria. The trichomes of the alga are typically wide (6–60 µm wide), occasionally enclosed in mucous sheaths, and consist of very short disk-shaped cells. Thylakoids are convoluted, filling almost the entire cell cavity. Oscillatoria and its allied genera are characterized by a specific type of cell division and the formation of hormogonia and hormospores. Cell division occurs only in specific Regions of the trichome known as meristematic zones. Here, the division of each cell proceeds very rapidly, and new transverse cell walls begin to form even before the previous division is completed. Pores in the transverse walls are very minute compared to other oscillatorialeans (about 5 nm in diameter). Hormogonia and hormocytes are formed following the death of individual trichome cells, which are termed necridia. Species of this genus are most frequently found in coastal lagoons and freshwater continental bodies.

Nostocales unites multicellular heterocystous algae with unbranched trichomes. As in Oscillatoriales, cell division is incomplete and occurs in a plane perpendicular to the longitudinal axis of the trichome, while reproduction is carried out by hormogonia or hormocytes.

Nostocalean algae are widespread in seas, continental water bodies, and soils, and due to their ability to fix atmospheric nitrogen under aerobic conditions, they play a crucially important role in the global nitrogen cycle. Some species of nostocalean algae are dangerous agents of water "blooming".

A primitive Lineage of nostocaleans is represented by the genus Scytonema. This alga possesses long, uniformly wide trichomes surrounded by rather firm mucilaginous sheaths. Cell division is incomplete, somewhat resembling that in Oscillatoria. Intercalary heterocysts are located between the vegetative Cells of the trichome. The filaments are capable of double (so-called scytonemoid) branching, which typically arises as a result of the death of one or several vegetative cells. In this process, the trichomes continue to grow, exert pressure on each other, and eventually rupture the maternal sheath, forming two lateral false branches. Species of this genus are most commonly found in terrestrial habitats, being especially numerous in subtropical and tropical regions as well as tundra biogeocenoses (Fig. 9.9).

Gloeotrichia is more advanced in evolutionary terms. The alga forms massive mucous colonies consisting of unbranched asymmetric filaments. At the Base of the trichome lies a basal heterocyst, followed (in mature individuals) by an elongated akinete, and then by vegetative cells. The latter are wide and short near the heterocyst, narrow and elongate as they approach the apex, and terminate at the very top in a long, colorless Hair. Gloeotrichia inhabits mainly standing freshwater bodies. At the beginning of their development, the colonies are attached to underwater substrates (mostly to stems and leaves of higher aquatic plants), but later detach and float freely on the water surface. In some Asian countries, local residents collect colonies of Gloeotrichia and use them for food.

Fig. 9.9. Nostocalean algae. 1 - Scytonema julianum; 2-5 - Gloeothrichia intermedia (2 - general appearance of the colony, 3 - arrangement of filaments within the colonial mucilage, 4 - fragment of the colony with a group of filaments, 5 - filament with a heterocyst, akinete, and hair); 6-9 - Aphanizomenon flos-aquae (6 - colony, 7 - central part of the colony, 8 - central part of the trichome with a heterocyst and akinete, 9 - apex of the trichome with a colorless cell); 10, 11 - Anabaena flos-aquae; 12 - Nostoc commune (after Kondratieva, 1969; Garbacki & al., 1999).

Algae featuring symmetric unbranched filaments are considered the pinnacle of evolution within the order. Typical examples of this branch include the genera Aphanizomenon, Anabaena, and Nostoc.

The trichomes of Aphanizomenon combine into flaky aggregates floating in the water Column. Each trichome tapers slightly at the apices and terminates in colorless, elongated cells. The central part of the trichome is formed by vegetative cells interspersed with solitary intercalary heterocysts and akinetes. The cells are filled with numerous gas vacuoles. All species of the genus are dangerous agents of toxic water "blooming". For instance, in the reservoirs of the Dnieper cascade, approximately 20% of "blooming" events are caused by A. flos-aquae. Certain forms of this species also trigger water blooms in the Sea of Azov (observed regularly since the mid-1990s).

The trichomes of Anabaena are solitary, do not form colonies, and are quite frequently coiled into a tight spiral. In many species, the cells contain gas vacuoles. Species of this genus inhabit primarily the plankton of freshwater bodies. Some Anabaena species (specifically A. flos-aquae, A. spiroides, and A. scheremetievi) are responsible for toxic water "blooming".

The genus Nostoc comprises algae with symmetric trichomes embedded in massive mucous colonies. Nostoc cells lack gas vacuoles and are usually deeply constricted at the transverse walls. They inhabit freshwaters and soils. Species with large colonies are used for food.

Stigonematales unites heterocystous algae with branched trichomes. Representatives of the order morphologically indistinguishable from modern ones have been known since the Cambrian (570–500 million years ago). Cells of stigonematalean algae are capable of dividing in multiple planes, as a result of which the trichomes form true branches and sometimes even parenchymatous structures resembling true Tissues. Stigonemataleans inhabit mainly tropical and subtropical regions. Within Ukraine, only the genus Stigonema is quite frequently encountered from this order (Fig. 9.10).

Fig. 9.10. Stigonema intermedia: general appearance of a mature branched filament (Kondratieva, 1969).

Molecular-Biological System

The first attempts to apply molecular-biological Methods to build a system of blue-green algae as close to natural as possible were undertaken in the mid-1980s. In the 1990s, these studies expanded on a wide scale and made significant adjustments to views on the main evolutionary trends of Cyanophyta.

The foundation of molecular-biological research in systematics became the method of constructing molecular phylogenetic trees based on the analysis of nucleotide sequences of the Gene encoding the small subunit (SSU) of ribosomal RNA. This gene was not chosen by chance: it is present in All living organisms (except Viruses), is responsible for a universal cellular process—Protein Biosynthesis—and contains both highly conserved and highly variable regions. Due to the latter, SSU performs the same universal function in all organisms while possessing specific variations in The nucleotide sequences of its variable regions, which can be used to calculate phylogenetic distances between different representatives. In addition to SSU, molecular phylogenetic reconstructions have been carried out using other genes and Amino acid sequences of various proteins (around 20 in total).

The main advantage of the molecular-biological system of Cyanophyta is that today it is the closest to The Natural System of the division and allows for the Determination of the main evolutionary trends of blue-green algae. The major drawbacks of this system are, firstly, that a relatively small number of species have been studied using molecular characters (around 400 by the year 2000, i.e., only 20% of the known number), and secondly, that when constructing molecular phylogenetic trees, biologists do not strictly follow the International Code of Nomenclature for algae, Fungi, and plants, operating with clades and groups rather than formal taxa. Consequently, molecular systems seem rather chaotic at first glance, and working with them requires prior detailed familiarity with specific genera, species, and even individual strains—furthermore, based on systems grounded not in genotypic traits, but in classical phenotypic features.

A somewhat simplified version of the molecular Phylogenetic Tree of Cyanophyta is shown in Fig. 9.11.

Fig. 9.11. Molecular phylogenetic tree of Cyanophyta constructed using the 16S rRNA-coding gene, and its alignment with the morphological-cytological system. Question marks indicate groups whose placement in the given order is questionable (based on data from Wilmotte, 1994; Turner, 1997).

According to the molecular phylogenetic tree, the vast majority of Cyanophyta fall into the fifth, sixth, and seventh groups, which generally correspond to the orders distinguished in the morphological-cytological system.

Analysis of the molecular phylogenetic tree also reveals several evolutionary trends within the division. Firstly, primitive Cyanophyta groups (the first and second) unite unicellular thermophilic species, whereas more advanced groups possess a rather broad ecological amplitude. Secondly, multicellularity in blue-green algae arose multiple times, notably in the third, fifth, eighth, and ninth groups; in some cases, a secondary transition from a multicellular structure to a unicellular one occurred (tenth group). Thirdly, additional chlorophylls (specifically chlorophyll b) and chlorophyll-like pigments (divinyl pheophytin a₅) repeatedly appeared in various Cyanophyta groups. For instance, chlorophyll b is present in algae of the genera Prochloron (group 5) and Prochlorothrix (group 9), and a chlorophyll-like pigment is found in Prochlorococcus (group 10). Fourthly, it was precisely the blue-green algae that gave rise to the plastids of eukaryotic organisms (group 4).

Prochlorophytes and THE ORIGIN OF Plastids

Until the mid-1980s, it was believed that all prokaryotic oxygenic photoautotrophic organisms possessed only a single chlorophyll a along with an obligatory complex of phycobilin pigments, and belonged to the division Cyanophyta. In 1975, a green-colored unicellular prokaryotic alga was discovered in subtropical marine coastal waters and described as a new species to science, Synechocystis didemni. The alga lived in Symbiosis with ascidians, developing in their cloacal cavities, integuments, and on The surface of colonies. Biochemical studies of its pigment composition revealed that the cells of S. didemni possess two chlorophylls—a and b—and lack phycobilins. The assimilation product turned out to be a polysaccharide resembling true starch. Since the chlorophyll composition was considered the most essential feature at the division level at that time, this alga was transferred to a new genus, Prochloron, with the single species P. didemni, and segregated into a new independent division of so-called prokaryotic green algae—Prochlorophyta (Fig. 9.12).

Fig. 9.12. Ultrastructure of Prochloron based on electron microscopy data. 1 - vacuole-like structure, 2 - polyhedral body, 3 - double-layered murein-containing wall, 4 - single thylakoid, 5 - group of closely appressed thylakoids resembling a lamella. The whole cell is shown on the left, and a high-magnification view of its portion on the right (schematized according to Lewin, 1987, 1994).

Approximately ten years later, a multicellular free-living prokaryotic alga containing chlorophylls a and b and lacking phycobilins was discovered in the plankton of lakes in the Netherlands. This organism, named Prochlorothrix hollandica, was also assigned to the Prochlorophyta.

Two more years later, marine plankton samples yielded a new group of prochlorophytes represented by tiny (0.6-0.8 µm in diameter) prokaryotic algae which, although lacking chlorophyll b, contained a chlorophyll a-like pigment, divinyl pheophytin a₅, alongside chlorophyll a. Phycobilin pigments were also absent. Species exhibiting these traits were included in the genus Prochlorococcus.

In the 1980s and 1990s, prochlorophytic algae became the focus of intensive biochemical and electron microscopic studies, as it was hypothesized that the Prochlorophyta harbored the very organisms that, having entered into an intracellular symbiosis with primitive heterotrophic eukaryotes, gave rise to true chloroplasts³.

Between 1990 and 1996, the hypothesis of the prochlorophytic origin of eukaryotic plant plastids was tested simultaneously by several research groups working in molecular phylogenies. The main approach involved constructing molecular phylogenetic trees based on the analysis of genes encoding the 16S subunit of ribosomal RNA (see Fig. 11.11), one of the Photosystem II reaction center Enzymes (psbA), and light-harvesting complex proteins (LHCs).

As a result of this work, several independent groups reached two main Conclusions. First, the algae united in the division Prochlorophyta are not phylogenetically related and fall within different groups of Cyanophyta (the fifth, ninth, and tenth). Since this violates THE PRINCIPLE OF monophyly for the division, the taxon Prochlorophyta must be discarded.

Second, none of the so-called "prochlorophytic" algae belong to the ancestral group of eukaryotic plastids. Plastids originate from cyanobacteria of an intermediate evolutionary grade that formed an independent molecular phylogenetic lineage within Cyanophyta. Modern species of cyanobacteria from this group are unknown.

Studies on "prochlorophytic" algae also refuted the notion that Different types of chlorophylls could not have arisen multiple times during evolution.

Distribution and Significance in nature and Human Life

Cyanophyta are extremely widespread: in oceans, freshwater and hyperhaline water bodies, soils, on snow and ice, in hot springs, under aerophytic conditions, and elsewhere. In terms of resistance to extreme factors, Cyanophyta rank first on the planet. For instance, in model experiments, blue-green algae retained viability across a Temperature range from -195 to +130 °C, pressures from 0.05 to 300 atm., and withstood radioactive ⁶⁰Co irradiation of 160 kR per hour (Microcoleus vaginatus - up to 1280 kR per hour). The extraordinary endurance of blue-green algae has even attracted the attention of exobiologists: specifically, since the early 1960s, A number of laboratories in the USA (such as the Jet Propulsion Laboratory in California) have been developing methods and selecting strains of blue-green algae capable of multiplying in extraterrestrial environments, notably on Mars.

Today, blue-green algae play a planetary role in the nitrogen balance. This is due to the ability of Cyanophyta representatives to assimilate nitrogen directly from the atmosphere. The activity of nitrogen-fixing blue-green algae is linked to the restoration of soil fertility during fallow periods, the preservation of virgin soil fertility, and the productivity of World Ocean biohydrocenoses.

Two species of the genus Arthrospira (A. platensis and A. maxima) are valuable biotechnological subjects. They have been introduced into industrial cultivation, and their biomass is used to produce dietary foods, vitamin supplements, pharmacological preparations (primarily radioprotectants, metabolic stimulants, and hormonal drugs), food colorants, etc.

Blue-green algae are also used for water quality control and in geological practice for dating Precambrian sedimentary rocks.

Among Cyanophyta, there are extremely harmful species responsible for water "blooms". For example, in the reservoirs of the Dnieper cascade during the summer, Microcystis aeruginosa, Anabaena flos-aquae, and Aphanizomenon flos-aquae multiply massively. This leads to a sharp deterioration in the oxygen regime, the release of decomposition products from dead cells into the water, and The excretion of toxic substances by living individuals. Factors promoting "bloom" development include high water temperature (23-32 °C), high concentrations of biogenic elements (nitrogen, phosphorus, potassium), and the lack of water mass mixing.

Cyanobacterial Toxins

Based on their MECHANISM OF ACTION, cyanobacterial toxins are divided into three groups: hepatotoxins, neurotoxins, and dermatotoxins. The first two types are distinguished by the clinical picture of mouse mortality upon internal administration of the test toxin at lethal doses in standard tests. Hepatotoxins (so-called fast-death factors) cause progressive Liver cirrhosis and lead to the death of the test animal within 45 minutes to several hours. Neurotoxins (very fast-death factors) at acute doses cause death within 2-30 minutes due to respiratory failure. Dermatotoxins cause acute dermatitis upon surface contact.

Today, three main hepatotoxins have been identified in Cyanophyta: microcystin, nodularin (both are low-molecular-weight peptide toxins), and the alkaloid cylindrospermopsin. Microcystin is produced by Microcystis aeruginosa, Anabaena flos-aquae, and Planktothrix agardhii. Nodularin and cylindrospermopsin have been found in several species of nostocalean algae.

Among neurotoxins, the greatest hazard to humans is posed by Alkaloids of the anatoxin and saxitoxin groups. Among water-bloom formers, the main producers of anatoxins are planktonic species of the genus Anabaena (primarily A. flos-aquae) and certain Phormidium species (specifically, Ph. formosum). Cyanobacterial saxitoxins are known as paralytic shellfish poisons. This toxin is harmless to cold-blooded organisms, such as Mollusks, but can accumulate in their tissues. Consumption of such mollusks by warm-blooded animals leads to poisoning. In freshwater and brackish water bodies, the main saxitoxin producer among Cyanophyta is Aphanizomenon flos-aquae, whereas in marine environments, it is species of the genus Trichodesmium.

Dermatotoxins (aplysiatoxin, lyngbyatoxin A) have been found in some marine oscillatorialean cyanobacteria.

¹ Obsolete term for nanocytes - endospores.

² Obsolete term for exocytes - exospores.

³ Regarding The history of Prochlorophyta research, discussions on their taxonomic position, and their role in the origin of plastids during this period, see: N.V. Kondratieva, Prokaryotic green algae - Prochlorophyta (review of literature data). Algologia, 1991, vol. 1, no. 3, pp. 87-101.