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Species of the floating, freshwater fern  Azolla  form a well-characterized symbiotic association with the non-culturable cyanobacterium  Nostoc azollae , which fixes nitrogen for the plant. However, several cyanobacterial strains have over the years been isolated and cultured from  Azolla  from all over the world. The genomes of 10 of these strains were sequenced and compared with each other, with other symbiotic cyanobacterial strains, and with similar strains that were not isolated from a symbiotic association. The 10 strains fell into three distinct groups: six strains were nearly identical to the non-symbiotic strain,  Nostoc ( Anabaena )  variabilis  ATCC 29413; three were similar to the symbiotic strain,  Nostoc punctiforme , and one,  Nostoc sp. 2RC, was most similar to non-symbiotic strains of  Nostoc linckia . However,  Nostoc sp. 2RC was unusual because it has three sets of nitrogenase genes; it has complete gene clusters for two distinct Mo-nitrogenases and an alternative V-nitrogenase. Genes for Mo-nitrogenase, sugar transport, chemotaxis and pili characterized all the symbiotic strains. Several of the strains infected the liverwort  Blasia , including  N. variabilis  ATCC 29413, which did not originate from  Azolla  but rather from a sewage pond. However, only  Nostoc sp. 2RC, which produced highly motile hormogonia, was capable of high-frequency infection of  Blasia . Thus, some of these strains, which grow readily in the laboratory, may be useful in establishing novel symbiotic associations with other plants.

Comparative genomic insights into culturable symbiotic cyanobacteria from the water fern Azolla