Ans: Anabaena is a plankton-forming genus of filamentous cyanobacteria. They're noted for their nitrogen-fixing ability and their symbiotic relationships with plants like the mosquito fern. The Azolla Anabaena blue-green algae and Anabaena azollae have a symbiotic relationship in which the alga supplies nitrogen to the fern and the fern provides the alga with a shelter. These photosynthetic bacteria can convert nitrogen from the atmosphere into nitrate fertilizer.