Anabaena

1.0Anabaena Introduction

Anabaena, a filamentous cyanobacteria genus in planktonic form, is known for its nitrogen-fixing abilities and symbiotic relationships with plants like the mosquito fern. As a neurotoxin-producing cyanobacterium, Anabaena poses a threat to wildlife, farm animals, and pets, with neurotoxin production believed to serve a defensive role against grazing pressure. A DNA sequencing project unveiled Anabaena's 7.2 million base pairs genome, focusing on heterocysts that convert nitrogen into ammonia. Certain Anabaena species act as effective natural fertilizers in rice paddy fields, highlighting their agricultural potential.

2.0Structure of Anabaena 

Anabaena, a filamentous cyanobacterium, exhibits a distinctive structural organization. Its cells arrange themselves into elongated filaments, each comprising vegetative cells capable of diverse functions, including photosynthesis. Notably, in response to nitrogen scarcity, approximately one in every ten cells differentiates into specialized cells called heterocysts. Heterocysts play a pivotal role in nitrogen fixation, a process essential for the organism's survival. To create an optimal environment for nitrogenase, the enzyme responsible for nitrogen fixation, heterocysts form additional layers outside the cell wall, giving rise to a protective envelope. This envelope, featuring a polysaccharide layer, facilitates nitrogen fixation while minimizing oxygen interference. Crucially, heterocysts lack Photosystem II, reducing oxygen production, and exhibit heightened rates of respiration to consume any remaining oxygen. This intricate structural adaptation ensures Anabaena's efficiency in nitrogen fixation, contributing to its ecological success.

3.0Classification of Anabaena 

4.0Anabaena Nitrogen Fixation 

Under conditions of nitrogen fixation, the differentiation of vegetative cells into heterocysts occurs at semi-regular intervals along filaments in cyanobacteria. Heterocyst cells are specialized for nitrogen fixation and exhibit a micro-oxic environment due to increased respiration, inactivation of oxygen-producing photosystem II (PS II), and the formation of a thickened envelope outside the cell wall. Within these heterocysts, nitrogenase, responsible for transforming dinitrogen into ammonia, is sequestered. This process consumes ATP and reductant, generated through carbohydrate metabolism, which is further aided by the activity of photosystem I (PS I) in the light. Carbohydrates, likely in the form of glucose, are synthesized in vegetative cells and transported into heterocysts. In exchange, nitrogen fixed in heterocysts is transferred into vegetative cells, partly in the form of amino acids.

The fern Azolla establishes a symbiotic relationship with the cyanobacterium Anabaena azollae, known for fixing atmospheric nitrogen and providing the plant with this essential nutrient. Azolla, referred to as a "super-plant," exhibits rapid growth in freshwater environments, doubling its biomass in as little as 1.9 days. Phosphorus is the primary limiting factor for Azolla's growth, often abundant due to chemical runoff, leading to Azolla blooms. Notably, the symbiotic microorganism is directly transferred from one generation to the next, rendering Anabaena azollae entirely dependent on its host. Several of its genes have either been lost or transferred to the nucleus in Azolla's cells.

5.0Anabaena Variabilis 

Anabaena variabilis, a filamentous cyanobacterium belonging to the Eubacteria domain, engages in photosynthesis as a crucial energy-producing process. Interestingly, in the presence of fructose, this species exhibits heterotrophic behavior, thriving without the need for light. Additionally, it demonstrates the capacity for nitrogen fixation, converting atmospheric dinitrogen into ammonia. Anabaena variabilis shares a phylogenetic connection with Nostoc spirillum, a more widely recognized genus, and both are known to establish symbiotic relationships with plants, alongside several other Anabaena cyanobacteria. Unlike some cyanobacteria observed to form symbiotic bonds with diatoms, Anabaena variabilis does not exhibit this behavior. Due to its filamentous structure and the ability for cellular differentiation, Anabaena variabilis serves as a model organism in research exploring the origins of multicellular life.