British distribution (genus): Widespread and common, 23 freshwater species accepted by Whitton (2011).
In Anabaena the cells are ovoid or barrel-shaped, often giving the filaments (trichomes) the appearance of a string of beads (though this is not so evident in the species photographed). Anabaena possesses heterocysts [heterocytes]* and can also develop akinetes (thick walled resting cells that can survive in sediments for many years), though ability to do this may be lost in culture. Sometimes the trichomes are set in mucilage, but Anabaena does not form the clearly defined mucilaginous colonies seen in its close relative, Nostoc.
Anabaena has the ability to fix nitrogen and is widespread in freshwater and also in damp soil. Certain species are symbionts in higher plants, e.g. A. azollae in species of Water Fern (Azolla). Some species have been used successfully to provide nitrogen to rice crops in flooded paddy fields, adding up to 40 kg bound nitrogen per hectare per year. Use of Azolla gives even higher levels of nitrogen fixation, reported as 120 - 310 kg per hectare per year (Fay, 1983, quoted in van den Hoek, et al., 1995).
Like certain other cyanobacteria, Anabaena may also be cause of toxic blooms in freshwaters. Strains of A. flos-aquae, a common bloom-forming species in Britain, produce neuromuscular poisons, anatoxins, and constitute a serious, sometimes fatal, danger to farm animals drinking infected water. Potentially they they also represent a hazard to human health, though since they also add a taste and smell to such water, their consumption is normally less likely. The hazard remains for swimmers in inland, eutrophic waters.
*The term 'heterocyte' is claimed to be etymologically preferable to 'heterocyst' and is gaining acceptance, at least by a number of authors, though it is untruthful to suggest that 'heterocyst' has been universally dropped. In fact the claimed etymology is most likely incorrect and 'heterocyst' (differently-shaped 'box', not, in this case, 'bladder') remains a long-established and acceptable term.
A number of photographs of Anabaena species can be seen at the excellent Cyanosite resource (Purdue University).