Anabaena

Anabaena is a genus of filamentous, planktonic cyanobacteria. It is notable for its ability to fix nitrogen and for forming symbiotic partnerships with certain plants, including the mosquito fern. Along with three other cyanobacteria genera, it produces neurotoxins that can harm local wildlife, farm animals, and pets. This toxin production is thought to benefit its symbiotic relationships by protecting the host plant from grazing.

Between 1999 and 2005, the U.S. Department of Energy sequenced the complete genome of the model species *Anabaena variabilis* ATCC 29413, which is 7.2 million base pairs long. A paper on this work was published in 2014, focusing on heterocysts—specialized cells that convert nitrogen into ammonia. Some *Anabaena* species have been used as a natural fertilizer in rice paddies.

The genus overlaps significantly with *Aphanizomenon*. Although *An. variabilis* was moved to the new genus *Trichormus* in 1988, it appears phylogenetically to belong within *Anabaena*, sitting between the main planktic group and a cluster formed by *An. augstumalis* and *An. oscillarioides*. The position of *T. variabilis* and two benthic *Anabaena* species may change, as the group around *Trichormus variabilis* is better supported by data than its placement within the main *Anabaena*/*Aphanizomenon* group. The position of *Trichormus azollae* also needs revision. In 2009, a 42-species group within the main planktic lineage, identified by 16S rRNA or gas vesicle morphotype, was transferred to *Dolichospermum*. In 2010, the genus *Sphaerospermopsis* was created, absorbing three species, with three more added later. In 2012, three species were moved to the new genus *Chrysosporum*, and one of these was later moved to *Umezakia*. Many sequenced morphospecies in the *Anabaena*, *Dolichospermum*, and *Aphanizomenon* (ADA) clade are not monophyletic, and further genome sequencing is underway to establish a classification based on genetic branching.

Under nitrogen-limiting conditions, vegetative cells differentiate into heterocysts at semiregular intervals along the filaments. These cells are specialized for nitrogen fixation. Their interior is micro-oxic due to increased respiration, inactivation of oxygen-producing photosystem II, and a thickened cell wall. Nitrogenase, sequestered inside, converts dinitrogen into ammonia using ATP and reductant from carbohydrate metabolism, supplemented in light by photosystem I. Carbohydrate, likely glucose, moves from vegetative cells into heterocysts, while fixed nitrogen, partly as amino acids, moves back.

The fern *Azolla* forms a symbiosis with *Anabaena azollae*, which fixes atmospheric nitrogen. This allows *Azolla* to colonize freshwater quickly and double its biomass in as little as 1.9 days, earning it the nickname “super-plant.” Its growth is typically limited by phosphorus, and chemical runoff can cause blooms. Unlike other known plants, the symbiotic microbe is passed directly from one generation to the next, making *Anabaena azollae* entirely dependent on its host, as several of its genes have been lost or transferred to the *Azolla* nucleus.

*Anabaena* is also a model organism for studying simple vision. Researchers use its sensory rhodopsin, a light-sensitive membrane protein, to investigate how light changes molecular shape in the retina, driving the cellular signals that cause vision in vertebrates.

Double-strand breaks in DNA can be repaired by homologous recombination, a process that includes an early step catalyzed by the RecN protein. Studies of RecN dynamics in *Anabaena* show that this repair pathway is active in vegetative cells but absent in mature heterocysts, which are terminal cells.