Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen(they contain heterocyst) e.g. Anabaena, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteria serve as an important biofertiliser and increases yield by $50 \%$. Blue green algae also add organic matter to the soil and increase its fertility.