ABSTRACT
Cyanobacteria, also called blue-green algae (BGA), have been wellknown since ancient times as important contributors to rice field fertilization. They are distributed worldwide, and their capability as fertilizers derives from their potential to fix atmospheric nitrogen and elaborate nitrogenous compounds, either as free-living organisms or in symbiotic association with the water fern Azolla (Agarwal, 1979; Fay, 1983; Singh and Singh, 1983; Shi and Hall, 1988). The existence of an agronomic potential for cyanobacteria in rice cultures was first recognized with scientific bases by De (1939), who noticed that the natural fertility of several rice fields was due to biological nitrogen fixation performed by these microorganisms (Roger, 1989b).
