Phylum Cyanobacteria (Blue-green algae)

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Taxonomy

Because of the large number of genera, only examples are given.

  • Phylum Cyanobacteria
    • Class Cyanobacteria
      • Family Chroococcales (e.g. Microcystis, Synechococcus)
      • Family Pleurocapsales (e.g. Dermocarpa, Pleurocapsa)
      • Family Oscillatoriales (e.g. Lyngbya, Oscillatoria, Spirulina)
      • Family Nostocales (e.g. Anabaena, Nostoc, Calothrix)
      • Family Stigonemateles (e.g. Fischerella, Stigonema)
      • Family Prochlorales (Prochloron, Prochlorothrix)

General characteristics of the Cyanobacteria

Diversity

The members of this phylum are incredibly diverse phenotypically, but represent a relatively small phylogenetic range. Traditionally mistakenly classified with the eukaryotic “algae”, the taxonomy of this group remains problematic. Most species do not have formal names, and are instead referred to by their genus name and number in the Pasteur Culture Collection (PCC), e.g. Anabaena PCC 6309 rather than Anabaena variabilis.

The prochlorophytes (Prochloron and Prochlorothrix) are often consider separately from the remainder of the cyanobacteria, although they are phylogenetically members of the Chroococcales. Photosynthesis in prochlorophytes is different in some than in other cyanobacteria, resembling in many ways that of the plastids of eukaryotic phototrophs, which are also members of this group.

Metabolism

Despite the morphological diversity of cyanobacteria, they are physiologically much alike. They carry out oxygenic photosynthesis, using two photosystems in the traditional “Z-scheme” to obtain both energy and reducing power for fixing CO2 via the Calvin cycle. Water is the electron donor for CO2 reduction, generating oxygen. Cyanobacteria use chlorophyl a as their only chlorophyl, and use phycobilins as accessory photopigments. One form of phycobilin, phycocyanin, in combination with chlorophyl a produces the blue-green color from which these organisms get their name, but most familiar members of this group produce phycoerythrin instead, resulting in a rust-red or brown color.

Most cyanobacteria can fix atmospheric nitrogen, but this creates is a dilemma for them. Nitrogenases, the enzymes that reduce N2 to NH4+, are strongly inhibited by O2, the product of oxygenic photosynthesis. As a result, unicellular species generally fix nitrogen at night and photosynthesize by day, separating these mutually-exclusive processes in time. Some filamentous species separate the process physically, by producing specialized cells called heterocysts for fixing nitrogen. Heterocysts are terminally differentiated cells (they cannot reproduce nor revert to vegetative cells) that do not carry out oxygenic photosynthesis; they fix nitrogen and carry out cyclic photophosphorylation. Heterocysts provide fixed nitrogen (in the form of glutamine) to the nearby cells in the filament in exchange for energy (in the form of glutamic acid or sugar). This represents a differentiation between somatic cell lines and germ cell lines; in other words, these filaments are true multicellular organisms.

Morphology

Cyanobacteria very diverse morphologically, coming in all shapes and sizes. Unicellular forms are typically rods or cocci and are found in distinctive organized clusters (see below, the Chroococcales). Filamentous forms are generally motile by gliding, and may have sheaths. Filamentous species can have complex life cycles with a variety of specialized cell types, including nitrogen-fixing heterocycsts and resting akinetes. Only the Stigonematales contain filaments with branches. Cyanobacteria range widely in size, from typical bacterial sizes (about 1μm) to macroscopic; many types of cyanobacterial filaments can be seen distinctly without magnification.

Cells typically contain thylakoids flattened against this cytoplasmic membrane. These can be discrete disks like the chlorosomes of Chlorobi, or concentric layers of thylakoid membrane around the periphery of the cell. Many planktonic species produce gas vacuoles to regulate buoyancy and position in the water column.

Habitat

Cyanobacteria are common in any almost environment there’s sunlight and water. They are found in freshwater and marine environments, soils, on and in the surface of rocks, &c, &c. They are the predominant primary producers in environments that aren’t hospitable for eukaryotic algae. Cyanobacteria are found in acid springs and soda lakes, hot springs and permanently frozen rocks, the open ocean and crusty desert soils.

Many species form symbioses with fungi, animals, plants, various protists, and other bacteria. Lichens are composite organisms, composed to a fungus and an algae, and the phototrophic component are very often cyanobacteria. The Cyanobacteria form endosymbiotic relationships with a wide variety of unicellular eukaryotes, for example diatoms (this is in addition to their usual chloroplasts).The water fern Azolla remains associated with its cyanobacterial symbiont (an Anabaena) throughout its life cycle; specialized lobes on the leaves house the symbionts, which provide their host with fixed nitrogen. Cyanobacteria are commonly associated with freshwater and marine sponges, in the gill arches crustaceans, and in tropical reef clams and corals. The most extreme example of symbiosis of cyanobacteria with eukaryotes are the plastids (chloroplasts) of photosynthetic eukaryotes, which are specialized cyanobacteria.

Family Chroococcales

These cyanobacteria are unicellular, but usually grow as aggregates imbedded in sheaths, capsules or slime. The form of these aggregates depends on which type of covering (if any) the cells have, and how the cells divide. Division is by budding, or binary fission in one, two or three dimensions, or irregularly. Fission in one plane only produces strings of cells, fission in two dimensions (cleavage planes alternate) produce sheets of cells in rowas and columns, and division in three dimensions produces three-dimensional arrays of cells. Irregular cleavage produces irregular cell masses.

Example: Microcystis

Microcystis
Microcystis : Jason Oyadomari, http://www.keweenawalgae.mtu.edu/index.htm

Microcystis is a common freshwater and estuarine specie. Cells are coccoid, 3-5μm in diameter, clustered irregularly in gelatinous masses. It flourishes in the summer months, and in polluted waters can form blooms that looks like bright green latex paint floating on and in the water. May produce toxins (microcystins) that cause skin irritation or gastrointestinal discomfort (in the short run) or liver damage (in the long run) if ingested.

Family Pleurocapsales

Members of this family reproduce by multiple fission, a single large cell producing a number of small spore-like cells known as baeocytes. The simplest forms are unicellular, a single baeocyte growing into a large vegetative cell which divides into many baeocytes. If the cell divisions are in alternating planes, the baeocytes are arranged in orderly three-dimensional cubical arrays. In some species, an early single asymmetric binary fission of the baeocyte produces a large cell that continues to grow, and then divides by multiple fission. The smaller “mother” cell then grows and divides again in an asymmetric binary fission, regenerating both the mother cell and a cell destined for multiple fission. In the genus Pleurocapsa and its relatives, the growing baeocyte also divides early into two cells; one of these cells expands and ultimately undergoes multiple fission. The other cell goes through a series of asymmetric divisions, creating branched pseudohyphae of vegetative cells. These can be simple masses of cells, or complex threee-dimensional structures. Eventually, cells within these pseudohyphae can undergo multiple fission, releasing baeocytes.

Example: Dermocarpa

Dermocarpa
Dermocarpa : unattributed from www.ibvf.csic.es/ Cultivos/Seccion_II.htm

Dermocarpa has a relatively simple life cycle, in which the baeocytes (which are transiently motile, by gliding) grow into large vegetative cells, then divide by multiple fission into many baeocytes. These baocytes are contained in the spherical husk of the mother cells, which splits open to release the baeocytes into the environment.

Family Oscillatorales

These very common and conspicuous cyanobacteria are filamentous and divide only by binary fission. The only cellular differentiation in these filaments are the terminal cells in some cases, which can be rounded, tapered, or pointed. The form of the ends of filaments is important in the identification of these species. Filaments can be straight, loosely coiled, or tightly helical. The individual cells of the filament can be obvious, or the septa can be difficult to see. Sheaths are common, and most are motile by gliding.

Example: Oscillatoria

Oscillatoria
Oscillatoria : Jason Oyadomari, http://www.keweenawalgae.mtu.edu/index.htm

Oscillatoria is a common member of this group, ubiquitous in freshwater environments. They are often large enough to be mistaken for filamentous eukaryotic green algae. Filaments typically have a light sheath, if any, and the individual cells are cylindrical disks. Filaments are rigid, and rotate during gliding; this can make them appear to writhe as they rotate around slight curves and bends in the filament.

Family Nostocales

These cyanobacteria are morphologically similar to the Oscillatoriales, growing in linear filaments, but have complex life cycles and cellular differentiation. In nitrogen-limiting conditions, some cells differentiate into nitrogen-fixing heterocysts. This is a terminal differentiation; heterocysts can neither divide nor develop back into vegetative cells. Heterocysts have a heavy cell wall, and are much lighter in color than are vegetative cells, and so are readily identified. These heterocycsts provide fixed nitrogen to the nearby cells. As the distance between heterocycsts increases as the vegetative cells between continue to grow and divide, those cells midway between heterocysts become starved for fixed nitrogen, and one will therefore develop into a new heterocyst.

Most members of this group also produce “resting” stage cells called akinetes when filaments are nutritionally limited. Akinetes are usually larger than vegetative cells, contain good reserves of storage granules, and are depleted for photopigments. It is common, however, for akinetes to accumulate other pigments, making them appear dark or brown. Akinetes are resistant to a variety of environmental insults, but not heat. Filaments are prone to breakage at the junctions between akinetes or heterocycts and the adjacent vegetative cells, and so it is common to see filaments with these specialized cells at one or both ends. When provided a fresh supply of nutrients, akinetes germinate to produce a new filament.

Some members of this group, especially the plant symboints, also produce specialized filaments called hormogonia. Hormogonia are short filaments composed of small cells that glide rapidly. Hormogonia are produced in response to the detection by the vegetative filament of a soluble factor produced by the plant. The hormogonia glide rapidly toward the source of this “hormone”, develop heterocysts, and then the remainder of the cells of the hormogonia develop into vegetative cells

Example: Anabaena

Anabaena
Anabaena : Jason Oyadomari, http://www.keweenawalgae.mtu.edu/index.htm

Anabaena is a common freshwater specie. Cells are in the shape of flattened beads to short barrel-like cylinders. Filaments are not covered in slime or sheaths, and akinetes are not only produced adjacent to heterocycts. Does not produce hormongonia. Filaments are not tapered.

Family Stigonematales

These cyanobacteria divide in more than one plane, producing branched filaments or filaments composed of clusters of cells (multiseriate). Side branches are often morphologically distinct from the main filament. In some species, filaments are easily disrupted, producing clusters of cells that are easily confused with Chroococcales, but unlike these non-filamantous forms will often contain heterocysts or akinetes. Hormonogonia are common.

Example: Fischerella

Fischerella
Fischerella : Peter A. Siver and Hannah A. Shayler, http://silicasecchidisk.conncoll.edu/LucidKeys/Carolina_Key/html/Fischerella_Main.html

Fischerella is composed of multiceriate main filaments and uniseriate side branches. Grow to become large, dense mats of filaments than can glide along the substrate. Heterocysts form in both the main filament and side branches, whereas akinets form only on the main filaments. The main filament, but not the side branches, and usually covered in a dense sheath.

Family Prochlorales

Members of this family resemble the chloroplasts of eukaryotic algae and plants more than any other cyanobacteria. Chloroplasts are, in fact, specific relatives of the Prochlorales (thus the name), and so are formally members of this Family as well.

Prochlorales utilize both chlorophyl a and chlorophyll b as primary photopigments for oxygenic photosynthesis. Cells contain distinct thylakoids, but these lack phycobilins or phycobilisomes. Thylakoids are generally stacked, reminiscent of those of chloroplasts.

These organisms are sometimes considered separately from the other cyanobacteria, and in this context are referred to as the Oxychlorobacteria. Nevertheless, they are phylogenetic members of the cyanobacteria. Species of the genus Prochlorococcus, which also contain chlorophyl b instead of phycobilin, were originally thought to be members of this group, but seem to have evolved this trait independently (or by horizontal transfer), and are in reality members of the Chroococcales.

Example: Prochloron

Prochloron
Prochloron : Euichi Hirose, Tadashi Maruyama, Lanna Cheng, and Ralph A. Lewin, Nihon University, Japan

Prochloron is a symbiont of ascidians (sea squirts), in which it resides on the surface and imbedded in the surface test of the animals, and especially in the communal cloacal cavity (these are colonial species), where they form visible green patches. Although usually considered an exosymbiont, it is also found in vacuoles of the host cells living as an endosymbiont. They are especially common in sea squirts living in shady zones of the coral reefs. Prochloron provides up to half of the organic carbon requirements of the host animal. The cells are individual spheres or ovals 10-30μm in diameter.