CONCEPT:cyanobacteria or Blue-green algae are widely used as a biofertilizer to improve the soil fertilityThey fix the atmospheric nitrogen required by the plants for proper growth and developmentSome of the important nitrogen-fixing BGA are Nostoc, ANABAENA, Aulosira, Oscillatoria & CylindrospermumExplanation:The most COMMON nitrogen-fixing cyanobacterium of paddy field is AulosiraIn India, Aulosira fertilissima is regarded as the most active cyanobacteria for fixing nitrogen in rice or paddy fields It is a free-living BGA found in a variety of habitats like soil, on moist rocks, at the bottom of lakes and springs, etc.Oscillatoria It is a free-living BGA AnabaenaIt can be both free-living & symbiotic also.It FORMS a symbiotic RELATIONSHIP with Azolla & CycasCylindrospermumIt is a free-living BGANostoc It can be both free-living & symbiotic also.It forms a symbiotic relationship with Cycas