The blue-green algae (Cyanobacteria)

In this article, I briefly describe the blue-green algae, also known as cyanobacteria.

The cyanobacteria- oxygenic phototrophic bacteria

The blue-green algae (cyanobacteria) are one of the largest and most important groups of bacteria on earth. They belong to the diverse phylum of photosynthetic prokaryotes. They are chlorophyll-containing oxygenic phototrophic bacteria. These bacteria use various photosynthetic pigments, i.e., carotenoids, phycobilins, and different forms of chlorophyll to absorb energy from light. They use light as an energy source and evolve oxygen during photosynthesis. Cyanobacteria are aquatic and photosynthetic gram-positive prokaryotes and can produce their food. Photosynthesis is performed on the internal membranes, which are flattened sacs, also known as thylakoids. Photosynthesis in green plants is performed in plastids, which are thought to have their ancestry in cyanobacteria acquired long ago through endosymbiosis. In eukaryotes, these endosymbiotic cyanobacteria then evolved and differentiated into specialized organelles, i.e., chloroplasts, chromoplasts, etioplasts, and leucoplasts, collectively known as plastids.

Cyanobacteria are the first organisms known to have produced oxygen, and they have the distinction of being the oldest known fossils. They release oxygen as a byproduct of photosynthesis and are believed to have converted the early oxygen-poor reducing atmosphere to an oxidizing one. Thus, they dramatically changed the composition of life forms on Earth. They are among the oldest organisms on Earth, having fossil records dating back at least 2.1 billion years.

Habitat

Cyanobacteria usually live in colonial aggregates, which are large enough to visualize. They can be found in almost every terrestrial and aquatic habitat, such as oceans, freshwater, and even in bare rock and soil. They can occur as planktonic cells or form phototrophic biofilms in freshwater and marine environments. These bacteria occur in damp soil or even on temporarily moistened rocks in deserts.

The upper layers of microbial mats found in extreme environments like hot springs, hypersaline water, deserts, and the polar regions are dominated by filamentous species. Aquatic cyanobacteria are probably best known for the extensive and highly visible blooms in both freshwater and marine environment. They can have the appearance of blue-green paint or scum.

Cyanobacteria are ubiquitous in marine environments and play vital roles as primary producers. Among the marine cyanobacteria, the smallest of all is the Prochlorococcus, which is just 0.5 mm to 0.8 millimeters in size. The organism dominates the oligotrophic regions of the oceans. Some cyanobacteria grow in symbiosis with other organisms, i.e., they may occur as algal symbionts of lichens. Some live within the plant bodies of certain liverworts, water ferns, cycads, and angiosperms. The cyanobacteria associated with certain protozoa are called cyanellae.

Morphology

Cyanobacteria have diverse morphology, i.e., ranging from unicellular and filamentous (figure 1) to colonial forms. Unicellular cyanobacteria are non-motile. Many cyanobacteria form motile filaments of cells called hormogonia that travel away from the main biomass to bud and form new colonies somewhere else. The cells in a hormogonium are often thinner than in the vegetative state. Some filamentous species can be differentiated into various cell types, i.e., vegetative cells, akinetes, and thick-walled heterocysts. Vegetative cells are normal photosynthetic cells formed under favorable growing conditions. The akinetes are climate-resistant spores, which are formed in unfavorable conditions, whereas thick-walled heterocysts contain the nitrogenase enzyme essential for nitrogen fixation.

Each single cyanobacterium has a thick gelatinous cell wall around it. Cyanobacteria lack flagella and hormogonia, and some species move by gliding along surfaces. The genus Oscillatoria (figure 2) includes a filamentous cyanobacterium, often found in freshwater environments and appears blue-green. It can oscillate its filaments and is capable of a waving motion; the filaments oscillate back and forth. Each filament of oscillatoria consists of a trichome, which is made up of rows of cells and the tip of the trichome oscillates like a pendulum.

Spirulina (figure 2) is a biomass of filamentous cyanobacteria, used as an important staple diet in humans, and has been used as a source of protein and vitamin supplements in humans. In water columns, some cyanobacteria float by forming gas vesicles. These vesicles are not bounded by lipid membranes but by a protein sheath. Cyanobacteria add organic matter to the soil and thus prevent incipient soil erosion.

Photosynthesis by cyanobacteria

Cyanobacteria with the help of sunlight, drive the process of photosynthesis, where light energy is used to synthesize organic compounds from carbon dioxide. They are aquatic organisms and apply a CO₂-concentrating mechanism to help in the acquisition of inorganic carbon. Carboxysomes (figure 3) are the bacterial microcompartments cooperate with active transporters of CO₂ and bicarbonate to accumulate bicarbonate into the cytoplasm of the cell.

Carboxysomes are icosahedral structures composed of hexameric cell proteins. It is believed that they tether the CO₂-fixing enzyme RuBisCO to the interior of the cell. Cyanobacteria have a bluish pigment, phycocyanin, which captures light for photosynthesis. Blue phycocyanin absorbs light at wavelengths between 500 and 650nm.

Cyanobacteria possessing the pigment phycoerythrin have a red or brown color instead of the usual bluish-green color. Instead of bacteriochlorophyll, cyanobacteria contain chlorophyll a, which helps the cells absorb red light of wavelength 680 to 683 nm. The major light-absorbing pigments are carotenoids and phycobilins, which can transmit the energy of absorbed light to the chlorophyll. In photosynthesis, cyanobacteria generally use water as an electron donor and oxygen is released as a by-product, though some may also use hydrogen sulfide as occurs among other photosynthetic bacteria. Carbon dioxide is reduced to form carbohydrates via the Calvin cycle.

Photosynthesis is performed in flattened membranous sacs known as thylakoids, the surfaces of which are studded with granules called phycobilisomes (figure 3). The phycobilisomes act as light-harvesting antennae attached to the thylakoid membrane giving the green pigmentation to cyanobacteria. In cyanobacteria, thylakoid membranes are not continuous with the plasma membrane, unlike in bacteria performing anoxygenic photosynthesis. A few genera lack the pigment phycobilisomes, possess chlorophyll b, and were originally known as the procholrophytes.

Respiration in cyanobacteria

In cyanobacteria, like photosynthesis, respiration occurs in the thylakoid membrane. The respiratory electron transport and the photosynthetic electron transport occur in the same compartment. The process of photosynthesis leads to the storage of energy by making carbohydrates from CO₂, whereas respiration leads to the re-conversion of carbohydrates into CO₂. For the respiration process, cyanobacteria use electrons from succinate dehydrogenase rather than from NADPH.

Photosynthesis and respiration are separated in cyanobacteria with the plasma membrane as it contains only the components of the respiratory chain. The thylakoid membrane contains an interlinked respiratory and photosynthetic electron transport chain. Respiration happens only at night in cyanobacteria.

Nitrogen fixation by cyanobacteria

Under anaerobic conditions some cyanobacteria can fix atmospheric nitrogen with the help of specialized cells, called heterocysts. The vegetative cyanobacterial cells are unable to carry out nitrogen fixation, which is done by specialized cells called heterocysts that occur periodically along or at the ends of the trichome.

Heterocysts lack photosystem II and do not evolve oxygen. They can carry out nitrogen fixation. Some cyanobacteria that form heterocysts also form large, thick-walled cyst-like cells called akinetes, which are resistant to desiccation. The heterocysts forming species fix nitrogen into ammonia, nitrites, or nitrates, which can be absorbed by plants and converted to proteins and nucleic acids. Anabaena is a type of filamentous bead-like nitrogen-fixing cyanobacterium (figure 2), found as plankton in shallow water and on moist soil.

Toxins secreted by cyanobacteria

Some cyanobacteria form harmful algal blooms, thus disrupting aquatic ecosystems. Some cyanobacteria produce toxins called cyanotoxins, such as anatoxin-a, anatoxin-as, aplysiatoxin, cylindrospermopsin, domoic acid, microcystin LR, nodularin R (from Nodularia), or saxitoxin. These toxins cause the intoxication of wildlife and humans. Cyanobacteria reproduce explosively under certain conditions, which results in algal blooms that can become harmful to other species. Cyanobacteria play an important role as primary producers in the marine environment. Nowadays, cyanobacterial blooms are increasing in frequency and magnitude globally, thus posing a serious threat to aquatic environments and public health.

Conclusion

The cyanobacteria belong to the diverse phylum of photosynthetic prokaryotes. They are cholorophyll-containing oxygenic phototrophic bacteria. They are aquatic and photosynthetic gram-positive prokaryotes, and can produce their food. Cyanobacteria are the first organisms known to have produced oxygen, and they have the distinction of being the oldest known fossils.

They can be found in almost every terrestrial and aquatic habitat, such as in oceans, freshwater, and even in bare rock and soil. They can occur as planktonic cells or form phototrophic biofilms in freshwater and marine environments. Cyanobacteria have diverse morphology, i.e., ranging from unicellular and filamentous to colonial forms.

Photosynthesis is performed on the internal membranes, which are flattened sacs, also known as thylakoids. Cyanobacteria have a bluish pigment phycocyanin, which captures light for photosynthesis. The major light-absorbing pigments are carotenoids and phycobilins, which can transmit the energy of absorbed light to the chlorophyll. The process of photosynthesis leads to the storage of energy by making carbohydrates from CO₂, whereas respiration leads to the re-conversion of carbohydrates into CO₂. The heterocysts forming species fix nitrogen into ammonia, nitrites, or nitrates, which can be absorbed by plants and converted to proteins and nucleic acids.

Cyanobacteria reproduce explosively under certain conditions, which results in algal blooms that can become harmful to other species. Some cyanobacteria produce toxins called cyanotoxins, which cause intoxication of wildlife and humans.

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