A nitrogen fixing microbe associated with Azolla in rice fields is

Anabaena azollae is a free-living nitrogen fixing blue-green alga or cyanobacterium but it may also live symbiotically in the leaf cavities of Azolla, an aquatic, free floating, fresh water pteridophyte (fern). This cyanobacterium has nitrogenase enzyme, therefore can fix nitrogen. If this fern is grown in paddy (rice) fields, a remarkable $50 \%$ increase in yield can be noticed.