azollae

Azollae is the name used for the diazotrophic bacterial endosymbionts of the aquatic ferns of the genus Azolla. In older literature, these endosymbionts were described as Nostoc azollae, reflecting their taxonomic placement within the cyanobacteria (family Nostocaceae) and their close association with Azolla species. Azollae are filamentous cyanobacteria that reside inside specialized leaf cavities of Azolla species, forming a tight, intracellular symbiosis.

Within the Azolla leaf pockets, Azollae fix atmospheric nitrogen to ammonia, supplying essential nitrogen compounds to the host plant. In return, the bacteria receive carbon from photosynthates produced by the Azolla plant, and a protected microenvironment with low oxygen tension supports the activity of nitrogenase.

The symbiosis is typically transmitted with the host plant; Azolla propagates vegetatively, carrying the endosymbionts to new growth, enabling persistent association across generations. The genomes of Nostoc azollae strains associated with Azolla show signs of reductive evolution, including gene losses consistent with a long-term endosymbiotic lifestyle, while retaining the machinery for nitrogen fixation and plant interaction.

Azollae and Azolla together play a role in nutrient cycling in freshwater ecosystems and have been explored as a natural biofertilizer in sustainable agriculture, particularly in rice paddies where Azolla applications reduce the need for synthetic nitrogen inputs. Ongoing research investigates the molecular basis of the symbiosis, host control of nitrogen fixation, and the evolution of this intimate partnership.