Thylakoids are structural components of chloroplasts. Cyanobacteria In cyanobacteria, photosynthesis occurs on invaginations
Cyanobacteria - the inventors of oxygenic photosynthesis and the creators of the Earth's oxygen atmosphere - turned out to be even more versatile “biochemical factories” than previously thought. It turned out that they can combine photosynthesis and atmospheric nitrogen fixation in the same cell - processes previously considered incompatible.
Cyanobacteria, or blue-green algae as they were once called, played a key role in the evolution of the biosphere. It was they who invented the most effective type of photosynthesis - oxygenic photosynthesis, which occurs with the release of oxygen. More ancient anoxygenic photosynthesis, which occurs with the release of sulfur or sulfates, can occur only in the presence of reduced sulfur compounds (such as hydrogen sulfide), substances that are quite scarce. Therefore, anoxygenic photosynthesis could not ensure the production of organic matter in the amount necessary for the development of various heterotrophs (organic matter consumers), including animals.
Cyanobacteria have learned to use ordinary water instead of hydrogen sulfide, which has provided them with widespread distribution and enormous biomass. A byproduct of their activity was the saturation of the atmosphere with oxygen. Without cyanobacteria there would be no plants, because a plant cell is the result of a symbiosis of a non-photosynthetic single-celled organism with cyanobacteria. All plants carry out photosynthesis with the help of special organelles - plastids, which are nothing more than symbiotic cyanobacteria. And it is not yet clear who is in charge in this symbiosis. Some biologists say, using metaphorical language, that plants are just convenient “houses” for cyanobacteria to live.
Cyanobacteria not only created the “modern type” biosphere, but to this day continue to maintain it, producing oxygen and synthesizing organic matter from carbon dioxide. But this does not exhaust the range of their responsibilities in the global biosphere cycle. Cyanobacteria are one of the few living creatures capable of fixing atmospheric nitrogen, converting it into a form accessible to all living things. Nitrogen fixation is absolutely necessary for the existence of earthly life, and only bacteria can carry it out, and not all of them.
The main problem facing nitrogen-fixing cyanobacteria is that the key nitrogen-fixing enzymes, nitrogenases, cannot work in the presence of oxygen, which is released during photosynthesis. Therefore, nitrogen-fixing cyanobacteria have developed a division of functions between cells. These types of cyanobacteria form filamentous colonies in which some cells engage only in photosynthesis and do not fix nitrogen, while others - “heterocysts” covered with a dense shell – do not photosynthesize and are engaged only in nitrogen fixation. These two types of cells naturally exchange their products (organics and nitrogen compounds) with each other.
Until recently, it was believed that it was impossible to combine photosynthesis and nitrogen fixation in the same cell. However, on January 30, Arthur Grossman and his colleagues from (Washington, USA) reported an important discovery showing that scientists had so far greatly underestimated the metabolic abilities of cyanobacteria. It turned out that cyanobacteria of the genus living in hot springs Synechococcus(primitive, ancient, extremely widespread unicellular cyanobacteria belong to this genus) manage to combine both processes in their single cell, separating them in time. During the day they photosynthesize, and at night, when the oxygen concentration in the microbial community (cyanobacterial mat) drops sharply, they switch to nitrogen fixation.
The discovery of American scientists did not come as a complete surprise. In read for last years genomes of several species Synechococcus genes for proteins associated with nitrogen fixation were discovered. All that was missing was experimental evidence that these genes actually worked.
Cyanobacteria include a large group of organisms that combine a prokaryotic cell structure with the ability to carry out photosynthesis, accompanied by the release of O 2, which is characteristic of different groups of algae and higher plants. The combination of features inherent in organisms belonging to different kingdoms or even superkingdoms of living nature made cyanobacteria the object of a struggle for belonging to lower plants (algae) or bacteria (prokaryotes).
The question of the position of cyanobacteria (blue-green algae) in the system of the living world has a long and controversial history. For a long time they were considered as one of the groups of lower plants, and therefore taxonomy was carried out in accordance with the rules of the International Code of Botanical Nomenclature. And only in the 60s. XX century, when a clear distinction was established between the prokaryotic and eukaryotic types of cellular organization and on the basis of this, K. van Niel and R. Steinier formulated the definition of bacteria as organisms with a prokaryotic cell structure, the question arose of revising the position of blue-green algae in the system living organisms.
Studying the cytology of blue-green algae cells using modern methods led to the indisputable conclusion that these organisms are also typical prokaryotes. As a consequence of this, R. Steinier proposed to abandon the name “blue-green algae” and call these organisms “cyanobacteria” - a term reflecting their true biological nature. The reunification of cyanobacteria with other prokaryotes has confronted researchers with the need to revise the existing classification of these organisms and subject it to the rules of the International Code of Nomenclature of Bacteria.
For a long time, algologists have described about 170 genera and more than 1000 species of blue-green algae. Currently, work is underway to create a new taxonomy of cyanobacteria based on the study of pure cultures. More than 300 pure strains of cyanobacteria have already been obtained. For classification, constant morphological characteristics, patterns of culture development, features of cellular ultrastructure, size and nucleotide characteristics of the genome, features of carbon and nitrogen metabolism and a number of others were used.
Cyanobacteria are a morphologically diverse group of Gram-negative eubacteria, including unicellular, colonial and multicellular forms. In the latter, the unit of structure is a thread (trichome, or filament). Threads can be simple or branching. Simple filaments consist of one row of cells (single-row trichomes) having the same size, shape and structure, or cells that differ in these parameters. Branching trichomes arise as a result of various reasons, and therefore a distinction is made between false and true branching. True branching is caused by the ability of trichome cells to divide in different planes, resulting in the formation of multirow trichomes or single row filaments with single row lateral branches. False branching of trichomes is not associated with the peculiarities of cell division within the filament, but is the result of attachment or connection of different filaments at an angle to each other.
During the life cycle, some cyanobacteria form differentiated single cells or short filaments that serve for reproduction (baeocytes, hormogonies), survival in unfavorable conditions (spores, or akinetes) or nitrogen fixation under aerobic conditions (heterocysts). More detailed characteristics differentiated forms of cyanobacteria are given below when describing their taxonomy and the process of nitrogen fixation. a brief description of akinet is presented in ch. 5. For different representatives This group is characterized by the ability to slide. It is characteristic of both filamentous forms (trichomes and/or hormogonies) and unicellular forms (baeocytes).
Known different ways reproduction of cyanobacteria. Cell division occurs by equal binary fission, accompanied by the formation of a transverse septum or constriction; unequal binary fission (budding); multiple fission (see Fig. 20, A–G). Binary fission can occur only in one plane, which in unicellular forms leads to the formation of a chain of cells, and in filamentous forms - to the elongation of a single-row trichome. Division in several planes leads in unicellular cyanobacteria to the formation of clusters of regular or irregular shape, and in filamentous ones - to the emergence of a multirow trichome (if almost all vegetative cells of the filament are capable of such division) or a single trichome with lateral single row branches (if the ability to divide in different planes reveal only individual cells of the filament). Reproduction of filamentous forms is also carried out with the help of trichome fragments, consisting of one or several cells, in some - also by hormogonies, which differ in a number of characteristics from trichomes, and as a result of the germination of akinetes in favorable conditions.
The work begun on the classification of cyanobacteria in accordance with the rules of the International Code of Nomenclature of Bacteria led to the identification of 5 main taxonomic groups in the rank of orders, differing morphological characteristics(Table 27). To characterize the identified genera, data obtained from the study of cellular ultrastructure, genetic material, and physiological and biochemical properties were also used.
The order Chroococcales includes unicellular cyanobacteria that exist in the form of single cells or form colonies (Fig. 80). Most representatives of this group are characterized by the formation of sheaths that surround each cell and, in addition, hold groups of cells together, i.e., participating in the formation of colonies. Cyanobacteria, whose cells do not form sheaths, easily disintegrate into single cells. Reproduction is carried out by binary fission in one or more planes, as well as by budding.
Table 27. Main taxonomic groups of cyanobacteria
Chloroplasts are membrane structures in which photosynthesis occurs. This process in higher plants and cyanobacteria allowed the planet to maintain the ability to support life by recycling carbon dioxide and replenishing oxygen concentrations. Photosynthesis itself occurs in structures such as thylakoids. These are membrane “modules” of chloroplasts in which proton transfer, photolysis of water, and synthesis of glucose and ATP occur.
The structure of plant chloroplasts
Chloroplasts are double-membrane structures that are located in the cytoplasm of plant cells and chlamydomonas. In contrast, cyanobacterial cells carry out photosynthesis in thylakoids rather than chloroplasts. This is an example of an underdeveloped organism that is able to provide its nutrition through photosynthetic enzymes located on the invaginations of the cytoplasm.
In its structure, the chloroplast is a double-membrane organelle in the form of a vesicle. They are located in large quantities in the cells of photosynthetic plants and develop only in case of contact with ultraviolet radiation. Inside the chloroplast is its liquid stroma. In its composition, it resembles hyaloplasm and consists of 85% water, in which electrolytes are dissolved and proteins are suspended. The stroma of chloroplasts contains thylakoids, structures in which the light and dark phases of photosynthesis directly occur.
Hereditary apparatus of the chloroplast
Next to the thylakoids there are granules with starch, which is a product of the polymerization of glucose obtained as a result of photosynthesis. Plastid DNA along with scattered ribosomes are also free in the stroma. There can be several DNA molecules. They, together with the biosynthetic apparatus, are responsible for restoring the structure of chloroplasts. This occurs without using the hereditary information of the cell nucleus. This phenomenon also allows us to judge the possibility of independent growth and reproduction of chloroplasts in the event of cell division. Therefore, chloroplasts in some respects do not depend on the cell nucleus and represent, as it were, a symbiont, underdeveloped organism.
Structure of thylakoids
Thylakoids are disc-shaped membrane structures located in the stroma of chloroplasts. In cyanobacteria they are located on invaginations of the cytoplasmic membrane, since they do not have independent chloroplasts. There are two types of thylakoids: the first is the lumen thylakoid and the second is the lamellar thylakoid. The thylakoid with lumen is smaller in diameter and is a disc. Several thylakoids arranged vertically form a grana.
Lamellar thylakoids are wide plates that do not have a lumen. But they are a platform to which multiple facets are attached. Photosynthesis practically does not occur in them, since they are needed to form a strong structure that is resistant to mechanical damage to the cell. In total, chloroplasts can contain from 10 to 100 thylakoids with a lumen, capable of photosynthesis. The thylakoids themselves are the elementary structures responsible for photosynthesis.
The role of thylakoids in photosynthesis
The most important reactions of photosynthesis take place in thylakoids. The first is the photolysis splitting of a water molecule and the synthesis of oxygen. The second is the transit of a proton through the membrane via cytochrome molecular complex b6f and electric transport chain. The synthesis of the high-energy molecule ATP also occurs in the thylakoids. This process occurs using a proton gradient that develops between the thylakoid membrane and the chloroplast stroma. This means that the functions of thylakoids allow the entire light phase of photosynthesis to occur.
Light phase of photosynthesis
A necessary condition for the existence of photosynthesis is the ability to create a membrane potential. It is achieved through the transfer of electrons and protons, which creates an H+ gradient that is 1000 times greater than in mitochondrial membranes. It is more advantageous to take electrons and protons from water molecules to create the electrochemical potential in the cell. Under the influence of an ultraviolet photon on the thylakoid membranes, this becomes available. An electron is knocked out from one water molecule, which acquires a positive charge, and therefore one proton must be removed to neutralize it. As a result, 4 water molecules break down into electrons, protons and form oxygen.
Chain of photosynthesis processes
After photolysis of water, the membrane is recharged. Thylakoids are structures that can have an acidic pH during proton transport. At this time, the pH in the stroma of the chloroplast is slightly alkaline. This generates an electrochemical potential, which makes ATP synthesis possible. Adenosine triphosphate molecules will later be used for energy needs and the dark phase of photosynthesis. In particular, ATP is used by the cell to utilize carbon dioxide, which is achieved by condensing it and synthesizing a glucose molecule based on it.
Along the way, NADP-H+ is reduced to NADP in the dark phase. In total, the synthesis of one molecule of glucose requires 18 molecules of ATP, 6 molecules of carbon dioxide and 24 hydrogen protons. This requires the photolysis of 24 water molecules to utilize 6 carbon dioxide molecules. This process allows the release of 6 molecules of oxygen, which will later be used by other organisms for their energy needs. At the same time, thylakoids are (in biology) an example of a membrane structure that allows the use of solar energy and transmembrane potential with a pH gradient to convert them into the energy of chemical bonds.
In the name of the department (from Greek. cyanos – blue) reflects a characteristic feature of these algae – the color of the thallus, associated with a relatively high content of the blue pigment phycocyanin. Cyanophytes usually have a specific blue-green color. However, their color can vary greatly depending on the combination of pigments - be almost green, olive, yellowish-green, red, etc. In recent years, another name is increasingly being used for blue-green algae - “cyanobacteria”. This name better reflects the two most important characteristic features of these organisms - the prokaryotic nature of the cells and the close relationship with eubacteria. On the other hand, the traditional name refers to traits such as the ability for oxygenic photosynthesis and the similarity between the structure of blue-green algae and the structure of eukaryotic chloroplasts.
About 2 thousand species of cyanophytes are known, widely distributed in marine and fresh waters and in terrestrial habitats.
Cell blue green algae prokaryotic. It consists of cell covers (cell wall) and internal contents - protoplast, which includes plasmalemma and cytoplasm with various structures: photosynthetic apparatus, nuclear equivalent, ribosomes, granules, etc. (Fig. 12).
Blue-green algae lack organelles surrounded by membranes: nucleus, chloroplasts, etc., as well as non-membrane structures: microtubules, centrioles, microfilaments.
The most characteristic features of the cell structure of blue-green algae are:
Absence of typical nuclei surrounded by nuclear membranes; The DNA lies loose in the center of the cell.
Localization of photosynthetic pigments in thylakoids in the absence of chloroplasts; thylakoids contain chlorophyll A.
Masking of green chlorophylls with red - phycoerythrin and blue pigments - phycocyanin and allophycocyanin.
DNA is located in the fibrillar granular nucleoplasmic region and is not surrounded by a membrane.
Rice. 12. Cell structure of blue-green algae (according to: C.Hoek van den et al., 1995): A – Synechocystis; B – Prochloron; IN - Pseudoanabena; 1 – cell wall; 2 – plasmalemma; 3 – thylakoid; 4 – phycobilisome; 5 – gas vesicles; 6 – carboxysome; 7 – DNA fibrils; 8 – cyanophycin granule; 9 – ribosomes; 10 – polysaccharide cover; 11 – stack of thylakoids; 12 – swollen thylakoid; 13 – pores; 14 – cyanophycin starch granules; 15 – lipid drop; 16 – transverse partition; 17 – young transverse septum; 18 – invagination of the plasmalemma
The presence of rigid (inflexible) layered cell membranes.
Formation in most cases of mucous membranes.
The presence of various inclusions: gas vacuoles (providing buoyancy), cyanophycin granules (nitrogen fixation), polyphosphate bodies (phosphorus fixation).
general characteristics
Unicellular blue-green algae are characterized by coccoid body type . In multicellular individuals, filamentous (trichomal) is found , less commonly, heterotrichous (heterotrichal) form of thallus structure . Very rarely there is a definite tendency towards a lamellar or volumetric arrangement of cells. In filamentous colonies, there is no plasmatic interconnection between cells.
They can be attached or unattached to the substrate, immobile or capable of gliding movement. However, flagella and cilia are never formed. The movement of cyanophytes is affected in various ways by lighting. Firstly, light determines the directions of movement. Movement towards the light source is called “positive phototaxis”, in the opposite direction – “negative phototaxis”. Secondly, the intensity of light changes the speed of movement - "photokinesis". Thirdly, a sharp increase or decrease in light intensity quickly changes the direction of movement - “photophobia”.
The cells of blue-green algae are most often spherical, barrel-shaped or ellipsoid in shape, less often elongated to cylindrical and spindle-shaped, straight or bent. Sometimes the cells are pear-shaped. In attached unicellular individuals, and sometimes in unicellular cyanoids, cell heteropolarity is often observed. In this case, mucous legs and discs are formed, with which they are attached to the substrate.
Individuals very often form various compounds - colonies of individuals, sometimes occupying large spaces, and produce a significant amount of mucus, which often noticeably affects the shape and general appearance of the colonies.
Cyanophyta individuals are usually microscopic, but colonial individuals in a number of species can measure centimeters.
The main pigments of blue-green algae are chlorophyll A, carotenoids (carotene, xanthophyll) and phycobiliproteins (allophycocyanin, phycocyanin, phycoerythrin). The latter are found in the form of special structures - phycobilisome, which are located on the surface of the thylakoids.
Blue-green algae are capable of various types of photosynthesis: oxygenic and anoxygenic. Oxygenic Photosynthesis is the process of fixing carbon dioxide using water as an electron donor, accompanied by the release of oxygen. Occurs under aerobic conditions. Anoxygenic photosynthesis is the process of fixing carbon dioxide using hydrogen sulfide or sulfide as an electron donor, accompanied by the release of sulfur. Occurs under anaerobic conditions. In the hyperhaline lakes of Israel, where highly anaerobic conditions are created in winter, the use of a combination of oxygenic and anoxygenic photosynthesis allows algae of the genus Oscillatorium dominate the lake year-round. In oxygen-free conditions, photosynthesis occurs in the sands of the tidal zone of the seas with the release of sulfur or thiosulfate. Many cyanophytes in the light under anaerobic conditions can fix carbon dioxide using hydrogen, but this the process is underway at low speed and stops quickly.
Blue-green algae have several types of nutrition:
Obligate photoautotrophic. They can only grow in light on an inorganic carbon source.
Facultative chemoheterotrophic. Capable of heterotrophic growth in the dark using organic matter, and phototrophic growth in the light.
Photoheterotrophic. Organic compounds are used in light as a source of carbon.
Mixotrophic. Organic compounds are used as an additional source of carbon. They are also capable of autotrophic carbon dioxide fixation.
The product of photosynthesis of cyanobacteria is cyanophycin starch. It is deposited in small granules located between the thylakoids. Cyanobacteria are able to quickly absorb and accumulate nitrogen in the form of cyanophycin granules, usually located near the transverse partitions of the cells. Phosphates in blue-green algae are stored in polyphosphate granules, and lipids are stored in the form of droplets in the cytoplasm at the periphery of the cell.
Reproduction. All living cells of blue-green algae are capable of division. Cell division of metazoans and colonial representatives usually results in growth. Cell division is possible in one, two, three or many planes. In multicellular forms, during longitudinal division, filamentous forms appear in one plane, lamellar forms in two planes, and cubic forms in three planes. When single-celled individuals divide, reproduction occurs at the same time. Single-celled cyanophytes reproduce by equal, or less often unequal, division. In this case, the inner layers of the cell membrane grow inside the cell. In some cases, multiple content divisions are observed. Mitosis and meiosis are absent. Reproduction of individuals is vegetative, less often asexual. A number of representatives of cyanobacteria form resting spores (akinetes) . There is no typical sexual process.
Vegetative propagation in coccoid forms, it is carried out by simple division of the cell in two in all possible directions, depending on the random influences of the environment. As a result, two equal, but not equivalent, parts are formed, giving rise to two new organisms. Cell division in two occurs in one or more planes. In the latter case, colonies are most often formed.
Multiple cell division occurs when the division of the cell and its nuclear region is inconsistent. As a result of increased division of the “nucleus,” the cell becomes multinucleated, then the areas of protoplasm around the “nuclei” are isolated and many isolated embryonic cells are formed. The main factors leading to repeated and multiple cell division of cyanobacteria are excess nutrition, causing its hypertrophied growth, as well as changes in the physicochemical conditions of existence. Hypertrophied growth causes a delay in cell maturation, and then repeated or multiple divisions.
One of the ways of vegetative propagation of cyanophytes is fragmentation (disintegration) of their thalli. The cause of fragmentation may be mechanical factors, the death of some cells, or disruption of the close connections that exist between them. In hormogonium blue-green algae, fragmentation occurs by disintegration of the thread on the hormogonium due to the death of some trichome cells - necroids. Each hormogonium consists of 2-3 or more cells, which, with the help of the mucus they secrete, slip out of the vaginal mucosa and, making oscillatory movements, move in water or along the substrate. Each hormogonium can give rise to a new individual. If a group of cells similar to hormogonium is covered with a thick membrane, it is called hormocyst. It performs the functions of reproduction and tolerating unfavorable conditions. In some species, single-celled fragments called gonidia, cocci or planococci separate from the thallus. Gonidia retain a mucous membrane; cocci and planococci lack distinct membranes. Like hormogonians, they are capable of active movement.
Asexual reproduction carried out using special cells that do not have thickened membranes: “exospores” and “endospores”. Exospores are formed by unequal cell division, when a smaller one buds from the mother cell.
When unfavorable conditions occur (drying, cold, nutrient deficiency), cyanobacteria form akinetes. These large, thick-walled, resting spores, filled with reserve products, serve to survive these unfavorable conditions. Akinetes can remain viable for decades, for example, in lake sediments in the absence of oxygen.
Taxonomy
All modern forms of the division Cyanophuta can be grouped into one, two or three classes. If we accept the idea of 3 main paths of evolutionary development of blue-green algae from the original coccoid unicellular forms, then we can agree with the identification of three classes within Cyanophyta: Chroococcophyceae - chroococcal algae, Chamaesiphonophyceae - chamesiphon algae and Hormogoniophyceae - hormogonian algae.
Class Hormogoniaceae–Hormogoniophyceae
(Orders Oscillatoryaceae, Nostocaceae, Stigonemus -
Oscillatoriales, Nostocales, Stigonematales)
The species are characterized by a trichal form of the body structure of individuals, as well as the ability to form hormogonies, i.e. special fragments of threads capable of active voluntary movement and germination into new individuals. Individuals are multicellular, "simple" or colonial (with multicellular cyanoids). Threads can be branched or unbranched, branching can be real or false. In true branching, trichome branching occurs. With false branching, only the vaginas branch. Trichomes can be single-rowed or multirowed, unbranched or branched, homocytic or heterocytic. Homocyte trichomes consist of similar cells that are not differentiated in shape and function. Heterocyte trichomes consist of cells that are unequal in shape, function and location. Cells that are similar in appearance to homocytic trichome cells are called vegetative; sharply different from them - special. The latter include heterocysts and akinetes.
Development cycles are often complex, during which a number of morphologically different stages are observed. In addition, hormogonium algae are characterized by multivariate development.
Genus Oscillatorium(Fig. 13, A). There is no differentiation of cells according to shape, function and localization. The threads are unbranched, uniseriate, homocytic. Vaginas are absent or present.
Rice. 13. Morphological diversity of blue-green algae (according to:): A – Oscillatorium; B – Nostok; IN - Anabena; G - Lingbia; D – Rivularia; E – Gleocapsa; AND - Chroococcus: 1 – general form, 2 – view at low magnification, 4 – heterocyst
Genus Nostok(Fig. 13, B). Cells are differentiated by form and function. Exclusively colonial organisms, with well-developed mucus that affects the shape of the colonies. Trichomes are heterocytic, uniseriate, unbranched, with or without sheaths.
Genus Rivularia(Fig. 13, D). Thallus in the form of unbranched or branched filaments, with or without sheaths. Individuals are solitary or form compounds. trichomes are heterocytic, asymmetrical in maturity, tapering from the base to the apex, often ending in a hair consisting of vacuolated cells.
Genus Stigonema(Fig. 14, A). Cells are differentiated by form and function. Species of the genus are characterized by true lateral branching. Trichomes are heterocytic, uniseriate or multiseriate, forming plexuses and bundles. Threads with sheaths or, less commonly, without them. There is no clear dimorphism of the branches. The main threads are usually creeping. In the old parts of the filaments, the cells are often in a gleocapsoid state: they are united in groups and surrounded by developed mucous membranes.
Rice. 14. Stygonema blue-green algae (according to: R.E. Lee, 1999; M.M. Gollerbach et al., 1953): A – Stigonema; B – Mastigocladus: 1 – heterocyst, 2 – sheath
Genus Mastigocladus(Fig. 14, B). The thallus has complex branching and is heterocytic. Branching true and false. The cells of the main filaments are more or less spherical, the cells of the branches are elongated and cylindrical. The sheaths of the threads are narrow, strong or slimy. Heterocysts are intercalary, no spores are known. Species of the genus are widely distributed in thermal springs.
Class Chroococcus –Chroococcophyceae
Order– Chroococcales
They occur as single-celled “simple” individuals or more often form mucous colonies. When cells divide in two planes, single-layer lamellar colonies appear. Division in three planes leads to the formation of cube-shaped colonies. When cells divide in many planes, the cells are randomly located throughout the entire thickness of the mucus, and the shape of the colonies is varied. Colonial mucus combines simple and complex cyanoids of colonies. Mucus can be homogeneous or differentiated, in the form of mucous blisters sequentially inserted into one another (genus Gleocapsa) or tubes and cords (birth Voronikhinia, Gomphospheria). Mucus can be colorless or colored in blue-green, grayish, olive, brown, reddish, purple, and black tones.
The cells are mostly spherical or ellipsoidal, less often elongated, sometimes variously bent, cylindrical or spindle-shaped, in some species ovoid, pear-shaped or heart-shaped. Chroococcal algae are characterized by vegetative reproduction. Single-celled individuals divide into two in one, two, three or many planes. Colonial individuals reproduce by dividing colonies and forming endogenous colonies. Most often, reproduction occurs by dividing colonies. Within this method, there is fragmentation of colonies, or breaking them into several parts, or re-lacing of the mother colony; and budding of colonies, that is, the formation of protrusions on the mother colony that eventually separate from it. Colonial individuals also reproduce using regular vegetative cells and spores.
Genus Gleocapsa forms “simple” or complex colonies (Fig. 13, E). Cells are spherical, ellipsoidal, cylindrical. Each cell is covered with a mucous sheath. During division, the walls of the mother cells are preserved. Colonies are round or cubic, consisting of mucous bubbles sequentially included one within the other.
Genus Microcystis - colonies are spherical or irregular in shape, spherical cells are immersed in mucus and can divide in any direction (Fig. 15). Cells of many species contain gas vacuoles. The genus is widespread in freshwater plankton. Developing in masses, it can cause algae in water. Some species are toxic.
Rice. 15. Chroococcal blue-green algae Microcystis(after: M. M. Gollerbach et al., 1953)
Class Hamesiphonaceae -Chamaesiphonophyceae
(Order Pleurocaps -Pleurocapsales)
Unicellular, often differentiated into base and apex, and colonial (with unicellular cyanoids), usually attached to the substrate, individuals. The formation of endospores (beocytes) is characteristic. Cells of various shapes, often with well-defined colorless or yellowish or brown mucous membranes. Cell division occurs in one, two or three planes. Cells in colonies are often very compressed and form false parenchyma, sometimes arranged in several layers. Many species are characterized by the formation of relatively clear rows of cells that resemble threads. But there is no plasmatic connection between the cells of such “threads”. The “threads” crawl along the substrate, go deeper into it or rise above it, and the threads often branch.
Endospores (beocytes) arise inside a mother cell (sporangium), similar to or different from ordinary cells in size and shape. Beocytes are released by rupture, licking of the sporangium shell, or by throwing off part of the sporangium wall as a cap; the entire contents of the sporangium or only part of it are used for their formation.
Genus Dermocarpa. Individuals are unicellular, differentiated into base and apex, attached to the substrate. They usually live alone, in small groups. They usually reproduce by beocytes.
Rice. 16. Chamesiphon blue-green algae Dermocarpa(after: M. M. Gollerbach et al., 1953)
Ecology and significance
Blue-green algae are ubiquitous. They can be found both in hot springs and artesian wells, and on the surface of snow and wet rocks, on the surface and in the thickness of soils, in symbiosis with other organisms: protozoa, fungi, sea sponges, echiurids, mosses, ferns, gymnosperms. Species of blue-greens are common in plankton and benthos of standing and slowly flowing fresh waters, in brackish and salt water bodies. They - important components of marine phytoplankton. Blue-green algae play a key role in oceanic ecosystems, where the majority of total photosynthetic production comes from picoplankton. Picoplankton consists mainly of single-celled coccoid cyanophytes. It is estimated that 20% of the oceans' photosynthetic production comes from planktonic blue-green algae. The benthos contains epiphytic, epilithic and endolithic forms. Cyanobacteria usually have special attachment organs in the form of a sole, foot, and mucous cords. Species of blue-green algae, which attach to underwater objects using mucus, are also abundant.
Cyanobacteria are typical inhabitants of hot waters. They vegetate at temperatures of 35–52°C, and in some cases up to 84°C and higher, often with an increased content of mineral salts or organic substances (heavily polluted hot wastewater from factories, factories, power plants or nuclear plants).
The bottom of hyperhaline reservoirs is sometimes completely covered with blue-green algae, among which species of the genera predominate Phormidium, Oscillatoria, Spirulina etc. Blue-green algae live on the bark of trees (species of genera Synechococcus, Afanotheke, Nostoc). They often epiphyte on mosses, where, for example, one can observe blackish-blue tufts of species of the genus Schizotrix.
Representatives of Cyanophyta are the most common among algae living on the surface of exposed rocks. Cyanophytes and accompanying bacteria form “mountain tan” (rock films and crusts) on crystalline rocks of various mountain ranges. Algae growths are especially abundant on the surface of wet rocks. They form films and growths of various colors. As a rule, species equipped with thick mucous membranes live here. The growths come in different colors: bright green, golden, brown, ocher, purple or dark blue-green, brown, almost black, depending on the species that form them. The types of genera that are especially characteristic of irrigated rocks are: Gleocapsa, Gleoteke, Hamesiphon, Calothrix, Tolipothrix, Scytonema.
Representatives of Cyanophyta make up the vast majority of soil algae. They live in deep and surface layers of the soil and are resistant to ultraviolet and radioactive radiation. In the soils of the steppe zone Nostoc vulgare forms thick films of dark green or, in the dry season, slate-black crusts on the surface. The massive development of microalgae causes greening of the slopes of ravines, roadsides, and arable soils.
Blue-green algae are components of the thallus of many lichens and coexist with higher plants, for example, Azolla aquatic fern and others. How symbionts they protect their partner from high light intensity, supply him with organic substances, and provide nitrogen compounds. At the same time, they receive protection from unfavorable external factors from the host, as well as the organic substances necessary for growth. Only a few symbiotic associations of cyanophytes with various organisms are obligate. Most cyanophytes are able to grow independently, although worse than in symbiosis. They form two types of associations with other organisms - extracellular: with fungi and intracellular: with sponges, diatoms, etc.
Blue-green algae are among the most ancient organisms; their fossil remains and waste products were found in rocks formed 3–3.5 billion years ago, in the Archean era. It is believed that the first ecosystems on Earth (Precambrian) consisted only of prokaryotic organisms, including cyanobacteria. The intensive development of cyanophytes was of enormous importance for the development of life on Earth, and not only because of their accumulation of organic matter, but also due to the enrichment of the primary atmosphere with oxygen. Blue-green algae also played a significant role in creating limestone rocks.
Nitrogen fixation. The Earth's atmosphere consists of 78% nitrogen, but the ability to fix it is found only in prokaryotes, and among algae exclusively in cyanophytes. Blue-green algae are unique organisms that are capable of fixing both carbon dioxide and atmospheric nitrogen. When nitrogen is fixed, ammonia and hydrogen are released. This process usually occurs in special thick-walled cells with mucous sheaths - heterocysts. Conditions with a low oxygen content are created inside the heterocysts. Nitrogen fixation occurs faster during the day than at night, since during photosynthesis the ATP necessary for this process is formed - adenosine triphosphoric acid. By fixing atmospheric nitrogen, blue-green algae obtain the nitrogen they need to synthesize their proteins and continue to grow. Other algae depend entirely on nitrates and ammonium dissolved in water.
Biological fixation of atmospheric nitrogen is one of the important factors increasing soil fertility. The leading role in this process belongs to cyanophytes, which do not require ready-made organic matter to assimilate molecular nitrogen, but themselves bring it to the soil. For example, for temperate zone soils, the annual production of nitrogen-fixing blue-green algae is estimated at 20-577 kg/ha (dry weight). Only heterocyst forms of cyanophytes (species of the genera Nostok, Anabena, Calothrix, Tolipothrix And Cylindrospermum).
Some representatives of blue-green algae are edible (Nostok, Spirulina). In special biological ponds, communities of blue-green algae and bacteria used to decompose and detoxify herbicides. Some cyanobacteria decompose phenylcarbamate herbicides into aniline and chlorine derivatives. Wastewater, purified using the most advanced methods, still remains toxic to aquatic organisms. Only algobacterial communities, which are used for wastewater tertiary treatment, make it possible to obtain water that complies with GOST "Drinking Water".
"Blooming" of water. By “blooming” of water we mean the intensive development of algae in the water column, as a result of which it acquires different colors, depending on the color and number of organisms causing the “bloom”. The massive development of algae up to the point of “blooming” of water is facilitated by the increase in eutrophication of water bodies, which occurs both under the influence of natural factors (over thousands and tens of thousands of years), and to a much greater extent under the influence of anthropogenic factors (over years, tens of years). "Blooming" of water is observed both in continental reservoirs (fresh, brackish and saline), and in the seas and oceans (mainly in coastal areas). The Red Sea got its name due to the abundant development of blue-green algae in it. Oscillatoria erythraea. The puddle-shaped freshwater bodies of Central Europe are often colored red Haematococcus pluvialis. Of the freshwater bodies of water, large lowland rivers and their reservoirs, as well as ponds for various purposes, lakes, and cooling ponds are primarily susceptible to blooming.
Moderate vegetation of cyanophytes has a positive effect on the ecosystem of the reservoir. With a significant increase in algae biomass (up to 500 g/m3 and above), biological pollution begins to appear, as a result of which the quality of water significantly deteriorates. In particular, its color, pH, viscosity changes, transparency decreases, and the spectral composition of the substance penetrating into the water column changes. solar radiation as a result of the scattering and absorption of light rays by algae. Toxic compounds and large amounts of organic substances appear in water, serving as a breeding ground for bacteria, including pathogenic ones. The water usually acquires a musty, unpleasant odor. Hypoxia, or deficiency of dissolved oxygen, occurs; it is spent on the respiration of algae and the decomposition of dead organic matter. Hypoxia leads to summer death of aquatic organisms and slows down the processes of self-purification and mineralization of organic matter.
Among cyanophytes there are pathogenic species (about 30), causing diseases and death of reef corals; during “blooming” waters may be diseases of domestic animals and humans, mass death of aquatic organisms, waterfowl and domestic animals, especially in the hot summer months. Poisoning of people is much less common. Children and people with liver and kidney disease are most at risk. Based on their mode of action, cyanobacterial toxins are divided into 4 groups: hepatotoxins, neurotoxins, cytotoxins and dermatotoxins. They cause food intoxication, allergies, conjunctivitis, damage to the central nervous system, etc. In their action, cyanotoxins are several times superior to such poisons as curare and botulin. Preventing the cleanliness of water bodies involves preventing the accumulation of algae near water intakes and resting places or watering places for domestic animals.
"Solar reactors" and algae. IN Lately Humanity is faced with an acute problem of rational use of natural energy resources and the search for unconventional energy sources. Such sources include solar energy conserved in plant biomass (biopreservation of solar energy). Unlike nuclear energy, this energy source is absolutely safe; its use does not disrupt the ecological balance and does not lead to radioactive or thermal pollution of the environment.
The most promising is the use of blue-green algae to produce biofuels by methanization of algae biomass grown in wastewater. Installations for producing methane from algae have been created in the USA and Japan. Their productivity is respectively 50 and 80 t/ha (dry mass) per year, and 50-60 t of dry algae biomass can provide 74 thousand kW/h of electricity.
Control questions
Name character traits structure of a cyanobacterial cell.
What pigments and nutritional types are known in cyanophytes?
How do blue-green algae reproduce? What are hormogoniums, exospores, akinetes?
What groups of organisms are blue-green algae most similar to and when did they arise?
Name characteristics and typical representatives of blue-green algae of the Chroococcal class.
Name the characteristic features and typical representatives of blue-green algae of the Hormogonium class.
Name the characteristic features and typical representatives of blue-green algae of the Chamesiphonaceae class.
In what habitats are blue-green algae found? Their meaning in nature.
The role of cyanophytes in the biological fixation of atmospheric nitrogen.
Economic importance of cyanophyte. Water quality assessment.
What are algal blooms and cyanotoxins?
Blue-green algae as non-traditional energy sources.
The only energy-transforming membrane of Gloeobacter is the cytoplasmic one, where the processes of photosynthesis and respiration are localized.
Cyanobacteria are interesting because they contain a variety of physiological capabilities. In the depths of this group, photosynthesis probably formed and took shape as a whole, based on the functioning of two photosystems, characterized by the use of H2O as an exogenous electron donor and accompanied by the release of O2.
Cyanobacteria have been found to have the ability for oxygen-free photosynthesis, which is associated with the shutdown of photosystem II while maintaining the activity of photosystem I (Fig. 75, B). Under these conditions, they have a need for exogenous electron donors other than H2O. As the latter, cyanobacteria can use some reduced sulfur compounds (H2S, Na2S2O3), H2, and a number of organic compounds (sugars, acids). Since the flow of electrons between the two photosystems is interrupted, ATP synthesis is associated only with cyclic electron transport associated with photosystem I. The ability for oxygen-free photosynthesis has been found in many cyanobacteria from different groups, but the activity of CO2 fixation due to this process is low, usually amounting to several percent of the rate of CO2 assimilation under the operating conditions of both photosystems. Only some cyanobacteria can grow by anoxic photosynthesis, such as Oscillatoria limnetica, isolated from a lake with high hydrogen sulfide content. The ability of cyanobacteria to switch from one type of photosynthesis to another when conditions change illustrates the flexibility of their light metabolism, which has important ecological significance.
Although the vast majority of cyanobacteria are obligate phototrophs, in nature they often live in dark conditions for long periods of time. In the dark, cyanobacteria have discovered active endogenous metabolism, the energy substrate of which is glycogen stored in the light, catabolized through the oxidative pentose phosphate cycle, which ensures complete oxidation of the glucose molecule. At two stages of this path, hydrogen enters the respiratory chain with NADP*H2, in which O2 serves as the final electron acceptor.
O. limnetica, which carries out active oxygen-free photosynthesis, also turned out to be capable of transferring electrons to molecular sulfur in the dark under anaerobic conditions in the presence of sulfur in the environment, reducing it to sulfide. Thus, anaerobic respiration can also supply energy to cyanobacteria in the dark. However, how widespread this ability is among cyanobacteria is unknown. It is possible that it is characteristic of crops that carry out oxygen-free photosynthesis.
Another possible way for cyanobacteria to obtain energy in the dark is glycolysis. In some species, all the enzymes necessary for the fermentation of glucose to lactic acid are found, but the formation of the latter, as well as the activity of glycolytic enzymes, are low. In addition, the ATP content in the cell under anaerobic conditions drops sharply, so, probably, the vital activity of cyanobacteria cannot be maintained solely through substrate phosphorylation.
In all studied cyanobacteria, the TCA cycle is “not closed” due to the absence of alpha-ketoglutarate dehydrogenase (Fig. 85). In this form, it does not function as a pathway leading to energy production, but only performs biosynthetic functions. The ability, to one degree or another, to use organic compounds for biosynthetic purposes is inherent in all cyanobacteria, but only some sugars can ensure the synthesis of all cellular components, being the only or additional carbon source to CO2.
Cyanobacteria can assimilate some organic acids, primarily acetate and pyruvate, but always only as an additional carbon source. Their metabolization is associated with the functioning of the “broken” TCA cycle and leads to inclusion in a very limited number of cellular components (Fig. 85). In accordance with the peculiarities of constructive metabolism, cyanobacteria are noted for their ability to photoheterotrophy or obligate affinity for photoautotrophy. Under natural conditions, cyanobacteria often carry out constructive metabolism of the mixed (mixotrophic) type.
Some cyanobacteria are capable of chemoheterotrophic growth. The set of organic substances supporting chemoheterotrophic growth is limited to a few sugars. This is associated with the functioning of the oxidative pentose phosphate cycle in cyanobacteria as the main catabolic pathway, the initial substrate of which is glucose. Therefore, only the latter or sugars that are easily converted enzymatically into glucose can be metabolized along this pathway.
One of the mysteries of cyanobacteria metabolism is the inability of most of them to grow in the dark using organic compounds. The impossibility of growth due to substrates metabolized in the TCA cycle is associated with the “brokenness” of this cycle. But the main pathway of glucose catabolism - the oxidative pentose phosphate cycle - functions in all studied cyanobacteria. The reasons cited are the inactivity of the transport systems of exogenous sugars into the cell, as well as the low rate of ATP synthesis associated with respiratory electron transport, as a result of which the amount of energy generated in the dark is only sufficient to maintain cellular vital activity, but not culture growth.
Cyanobacteria, a group of which probably developed oxygen photosynthesis, were for the first time faced with the release of O2 inside the cell. In addition to creating a variety of defense systems against toxic forms of oxygen, manifested in resistance to high concentrations of O2, cyanobacteria have adapted to an aerobic mode of existence by using molecular oxygen to obtain energy.
At the same time, a number of cyanobacteria have been shown to grow in light under strictly anaerobic conditions. This applies to species that carry out oxygen-free photosynthesis, which, in accordance with the accepted classification, should be classified as facultative anaerobes. (Photosynthesis of any type is an anaerobic process by its nature. This is clearly visible in the case of oxygen-free photosynthesis and is less obvious for oxygenic photosynthesis.) For some cyanobacteria, the fundamental possibility of dark anaerobic processes (anaerobic respiration, lactic fermentation) has been shown, but low activity puts their role in the energy metabolism of cyanobacteria is doubtful. O2-dependent and -independent modes of energy production found in the group of cyanobacteria are summarized in