Cyanobacteria

Forms

Cyanobacteria are found in almost every conceivable habitat, from oceans to fresh water to bare rock to soil. Most are found in fresh water, while others are marine, occur in damp soil, or even temporarily moistened rocks in deserts. A few are endosymbionts in lichens, plants, various protists, or sponges and provide energy for the host. Some live in the fur of sloths, providing a form of camouflage while they are safe.   Cyanobacteria include unicellular and colonial species. Colonies may form filaments, sheets or even hollow balls. Some filamentous colonies show the ability to differentiate into several different cell types: vegetative cells, the normal, photosynthetic cells that are formed under favorable growing conditions; akinetes, the climate-resistant spores that may form when environmental conditions become harsh; and thick-walled ammonia (NH₃), nitrites (NO₂⁻) or nitrates (NO₃⁻), which can be absorbed by plants and converted to protein and nucleic acids. The rice paddies of Asia, which produce about 75% of the world's rice[1], could not do so were it not for healthy populations of nitrogen-fixing cyanobacteria in the rice paddy fertilizer too.

Many cyanobacteria also form motile filaments, called hormogonia, that travel away from the main biomass to bud and form new colonies elsewhere. The cells in a hormogonium are often thinner than in the vegetative state, and the cells on either end of the motile chain may be tapered. In order to break away from the parent colony, a hormogonium often must tear apart a weaker cell in a filament, called a necridium.

Each individual cell of a cyanobacterium typically has a thick, gelatinous cell wall. They differ from other gram-negative bacteria in that the quorum sensing molecules autoinducer-2^[2] and acyl-homoserine lactones^[3] are absent. They lack flagella, but hormogonia and some unicellular species may move about by gliding along surfaces. In water columns some cyanobacteria float by forming gas vesicles, like in archaea.

Photosynthesis

Cyanobacteria have an elaborate and highly organized system of internal membranes which function in lichens), corals, pteridophytes (Azolla), angiosperms (Gunnera) etc.

Cyanobacteria are the only group of organisms that are able to reduce nitrogen and carbon in aerobic conditions, a fact that may be responsible for their evolutionary and ecological success. The water-oxidizing photosynthesis is accomplished by coupling the activity of thylakoid membrane hosts both respiratory and photosynthetic electron transport.

Attached to thylakoid membrane, carotenoids and phycoerythrins which give the cells the red-brownish coloration. In some cyanobacteria, the color of light influences the composition of phycobilisomes. In green light, the cells accumulate more phycoerythrin, whereas in red light they produce more phycocyanin. Thus the bacteria appear green in red light and red in green light. This process is known as complementary chromatic adaptation and is a way for the cells to maximize the use of available light for photosynthesis.

A few genera, however, lack phycobilisomes and have chlorophyll b instead (Prochloron, Prochlorococcus, Prochlorothrix). These were originally grouped together as the prochlorophytes or chloroxybacteria, but appear to have developed in several different lines of cyanobacteria. For this reason they are now considered as part of cyanobacterial group.

Relationship to chloroplasts

Chloroplasts found in eukaryotes (algae and plants) likely evolved from an endosymbiotic relation with cyanobacteria. This endosymbiotic theory is supported by various structural and genetic similarities. Primary chloroplasts are found among the green plants, where they contain chlorophyll b, and among the red algae and glaucophytes, where they contain phycobilins. It now appears that these chloroplasts probably had a single origin, in an ancestor of the clade called Primoplantae. Other algae likely took their chloroplasts from these forms by secondary endosymbiosis or ingestion.

It was once thought that the mitochondria in eukaryotes also developed from an endosymbiotic relationship with cyanobacteria; however, we now suspect that this evolutionary event occurred when aerobic bacteria were engulfed by anaerobic host cells. Mitochondria are believed to have originated not from cyanobacteria but from an ancestor of Rickettsia.

Cyanobacteria and Earth History

The biochemical capacity to use water as the source for electrons in algae joined blue-greens as major primary producers on continental shelves near the end of the Proterozoic, but only with the Mesozoic (251-65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did primary production in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae.^[4]

Cyanobacterial Evolution from Comparative Genomics

Recent high-throughput nitrogen cycles. By integrating phylogenetic data inferred for hundreds to nearly 1000 protein coding genes common to all or most cyanobacteria, we are able to reconstruct an evolutionary history of the entire phylum, establishing a framework for resolving how their metabolic and phenotypic diversity came about.^[4]

Classification

The cyanobacteria were traditionally classified by morphology into five sections, referred to by the numerals I-V. The first three - Chroococcales, Pleurocapsales, and Oscillatoriales - are not supported by phylogenetic studies. However, the latter two - Nostocales and Stigonematales - are monophyletic, and make up the heterocystous cyanobacteria. The members of Chroococales are unicellular and usually aggregated in colonies. The classic taxonomic criterion has been the cell morphology and the plane of cell division. In Pleurocapsales, the cells have the ability to form internal spores (baeocytes). The rest of the sections include filamentous species. In Oscillatorialles, the cells are uniseriately arranged and do not form specialized cells (akinets and heterocysts). In Nostocalles and Stigonematalles the cells have the ability to develop heterocysts in certain conditions. Stigonematales, unlike Nostocalles include species with truly branched trichome. Most taxa included in the phylum or division Cyanobacteria have not yet been validly published under the Bacteriological Code. Except:

• The classes Chroobacteria, Hormogoneae and Gloeobacteria
• The orders Chroococcales, Gloeobacterales, Nostocales, Oscillatoriales, Pleurocapsales and Stigonematales
• The families Prochloraceae and Prochlorotrichaceae
• The genera Halospirulina, Planktothricoides, Prochlorococcus, Prochloron, Prochlorothrix.

Biotechnology and applications

Certain cyanobacteria produce cyanotoxins like anatoxin-a, anatoxin-as, aplysiatoxin, cylindrospermopsin, domoic acid, microcystin LR, nodularin R (from Nodularia), or saxitoxin. Sometimes a mass-reproduction of cyanobacteria results in algal blooms.

The unicellular cyanobacterium Synechocystis sp. PCC6803 was the third prokaryote and first photosynthetic organism whose genome was completely sequenced.^[5] It continues to be an important model organism.^{[citation needed]} The smallest genomes have been found in Prochlorococcus spp. (1.7 Mb)^[6][7] and the largest in Nostoc punctiforme (9 Mb)^[8]. Those of Calothrix spp. are estimated at 12-15 Mb,^[9] as large as yeast.

At least one secondary metabolite, cyanovirin, has shown to possess anti-HIV activity.

See hypolith for an example of cyanobacteria living in extreme conditions.

Some cyanobacteria are sold as food, notably Aphanizomenon flos-aquae (E3live) and Arthrospira platensis (Spirulina). It has been suggested that they could be a much more substantial part of human food supplies, as a kind of superfood.

Along with algae, some hydrogen producing cyanobacteria are being considered as an alternative energy source, notably at Oregon State University, in research supported by the U.S. Department of Energy, Princeton University, Colorado School of Mines as well as at Uppsala University, Sweden

Health Risks

Some species of cyanobacteria produce neurotoxins, hepatotoxins, cytotoxins, and endotoxins, making them dangerous to animals and humans. Several cases of human poisoning have been documented but a lack of knowledge prevents an accurate assessment of the risks.[2][3]

See also

• Oxygen Catastrophe
• Archean
• Proterozoic

References

• Gillian Cribbs (1997) Nature's Superfood, the Blue-Green Algae Revolution. Newleaf. ISBN 0-7522-0569-2
• Marshall Savage, (1992, 1994) The Millennial Project: Colonizing the Galaxy in Eight Easy Steps. Little, Brown. ISBN 0-316-77163-5
• Dyer, Betsey D. A Field Guide to Bacteria. Ithaca: Comstock Publishing Associates, 2003.
• Fogg, G.E., Stewart, W.D.P., Fay, P.and Walsby, A.E. 1973. The Blue-green Algae. Academic Press, London and New York. ISBN 0-12-261650-2
• "Architects of the earth's atmosphere." Introduction to the Cyanobacteria. University of California, Berkeley. 03 Feb. 2006.

Further references

• Whitton, B.A. Phylum Cyanophyta (Cyanobacteria). in The Freshwater Algal Flora of the British Isles. Cambridge University Press, Cambridge ISBN 0 521 77051 3