Anabaena

Anabaena is a genus of filamentous, photosynthetic cyanobacteria notable for forming long, beadlike trichomes composed of intercalated vegetative cells and specialized nitrogen-fixing heterocysts. These trichomes are often enveloped in a gelatinous sheath and may form colonies in freshwater environments. Anabaena species are commonly found in lakes, rivers, ponds, and wetlands and can participate in seasonal blooms under nutrient-rich or stratified conditions. Some species are associated with symbiotic relationships, most famously Anabaena azollae, which inhabits the leaf cavities of the water fern Azolla and provides fixed nitrogen to its host.

Morphology and physiology: Vegetative cells perform photosynthesis, while heterocysts carry out nitrogen fixation. Heterocysts are adapted to an anaerobic microenvironment to protect the oxygen-sensitive nitrogenase enzyme. The spacing of heterocysts along the filament and the production of storage compounds such as glycogen help balance energy and reducing power. Some Anabaena also form akinetes, resting cells that survive unfavorable conditions. Gas vesicles may provide buoyancy, enabling cells to adjust their position in the water column and optimize light exposure.

Ecology and significance: Through nitrogen fixation, Anabaena contributes to the nitrogen input of aquatic ecosystems, particularly in nitrogen-poor waters. They can form blooms that impact water quality and clarity; some strains can produce toxins such as anatoxins or cylindrospermopsin, raising safety concerns for drinking water and recreation.

Applications and relevance: Anabaena is studied as a model for heterocyst development and nitrogen fixation and has been explored for use as a biofertilizer in sustainable agriculture. In nature, Anabaena azollae exemplifies a classic plant–microbe symbiosis.