Anabaena: Structure, Function, and Significance

Anabaena is a plankton-forming genus of filamentous cyanobacteria. They're noted for their nitrogen-fixing ability and their symbiotic relationships with plants like the mosquito fern. They are one of four cyanobacteria genera that contain neurotoxins, which are toxic to local wildlife, farm animals, and pets. These neurotoxins are thought to be input into the plant's symbiotic relationships, which protect it from grazing strain. In 1999, a DNA sequencing project was launched to map Anabaena's entire genome, which is 7.2 million base pairs long. 

Heterocysts, which convert nitrogen to ammonia, were the subject of the research. Anabaena species have been used as a natural fertilizer in rice paddy fields and have proven to be reliable. Anabaena serves as a model for studying gene differentiation during heterocyst formation. Anabaena algae is a nitrogen-fixing blue-green algae genus with beadlike or barrel-like cells and interspersed extended spores (heterocysts) that can be found in shallow water and damp soil as plankton.

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Genome Structure

The Anabaena PCC 7120 strain's circular chromosome genome has been sequenced. It has one chromosome with 6,413,771 base pairs and 5,368 predicted protein-coding regions and six plasmids with 408,101 to 5,584 base pairs. The Anabaena genome will be sequenced to help researchers better understand the genetics and physiology of cellular differentiation, pattern formation (as seen in Anabaena heterocysts), and nitrogen fixation. 

Cyanobase provided distribution maps for chromosomes and six plasmids. Anabaena variabilis ATCC 29413's genome has also been completed. Anabaena sp. strain ATCC 29413 differentiates between its heterocysts and akinetes varieties, unlike most other Anabaena strains. This provides a model for studying gene differentiation in heterocysts and akinetes formation. Akinetes are an organism's dormant forms.

Structure and Metabolism

Anabaena is photoautotrophic cyanobacteria that form heterocysts and perform oxygenic photosynthesis. Anabaena is a vegetative cell that develops in long filaments. When there is a lack of nitrogen in the system, one out of every ten cells can divide into a heterocyst. Heterocysts then trade fixed nitrogen for photosynthesis products that they can no longer conduct with their neighbours.  Since the nitrogen-fixing enzyme in heterocysts, nitrogenase, is unstable in the presence of oxygen, this separation of functions is important.

Heterocysts have evolved elements to maintain a low level of oxygen within the cell due to the importance of holding nitrogenase separated from oxygen. The forming heterocyst develops three additional layers outside the cell wall to prevent oxygen from entering the cell, giving it its distinctive expanded and rounded appearance. As a result, the rate of oxygen diffusion into heterocysts is 100 times lower than that of vegetative cells. In an oxygen-restricted environment, one layer produces an envelope polysaccharide layer where nitrogen is set. The presence of photosystem II is removed to reduce the amount of oxygen in the cell, and the rate of respiration is increased to use up any remaining oxygen.

Nitrogen Fixation by Anabaena

Vegetative cells differentiate into heterocysts at semi regular intervals along the filaments when nitrogen is scarce. Heterocyst cells are nitrogen-fixing cells that have reached the end of their life cycle. Because of increased respiration, inactivation of O2-producing photosystem (PS) II, and the formation of a thickened envelope outside of the cell wall, the interior of these cells is micro-oxic. Nitrogenase, which is sequestered inside these cells, converts dinitrogen to ammonium at the cost of ATP and reductant, both of which are generated by carbohydrate metabolism, which is aided by PS I activity in the sun. Carbohydrate is synthesized in vegetative cells and moved into heterocysts, most likely in the form of glucose. In exchange, nitrogen fixed in heterocysts passes into vegetative cells in the form of amino acids, at least in part.

The fern Azolla is symbiotic with the cyanobacterium Anabaena azollae, which fixes nitrogen in the atmosphere and supplies it to the plant. The plant has been called a "super-plant" because it can easily colonize freshwater areas and expand at a rapid rate, doubling its biomass in as little as 1.9 days. The most common limiting factor on its growth is phosphorus, which is often abundant due to chemical runoff, resulting in Azolla blooms. In contrast to other known plants, the symbiotic microorganism is passed down from generation to generation. Azolla and Anabaena have become fully reliant on their host, as many of their genes have been lost or moved to the nucleus of Azolla's cells.

The cellular fatty acids of free-living, nitrogen-fixing cyanobacteria belonging to the genera nostoc and Anabaena were examined to distinguish the general. Gas-liquid chromatography-mass spectroscopy was used to determine the fatty acid compositions of 20 Anabaena nostoc strains that were grown for 12 days on BG-11o medium.

Anabaena Variabilis

Anabaena variabilis is a filamentous cyanobacterium genus. Photosynthesis is possible in this Anabaena species from the Eubacteria domain. In the presence of fructose, this species is heterotrophic, meaning it can thrive without light. It may also use nitrogen fixation to convert atmospheric dinitrogen to ammonia. Anabaena variabilis is a phylogenetic relative of Nostoc spirillum, a more well-known genus. Plants are known to establish symbiotic relationships with both of these organisms, and several other Anabaena cyanobacteria. Other cyanobacteria have been observed to form symbiotic relationships with diatoms, but Anabaena variabilis has not been observed to do so. Due to its filamentous characterization and cellular differentiation capacity, Anabaena variabilis is also a model organism for researching the origins of multicellular life.