Cyanobacterial biofertilizers in rice agriculture

Floodwater and the surface of soil provide the sites for aerobic phototrophic nitrogen (N) fixation by free-living cyanobacteria and the Azolla-Anabaena symbiotic N-2-fixing Complex. Free-living cyanobacteria, the majority of which are heterocystous and nitrogen fixing, contribute an average of 20-30 kg N ha(-1), whereas the value is up to 600 kg ha(-1) for the Azolla-Anabaena system (the most beneficial cyanobacterial symbiosis from an agronomic point of view). Synthesis and excretion of organic/growth-promoting substances by the cyanobacteria are also on record. During the last two or three decades a large number of studies have been published on the various important fundamental and applied aspects of both kinds of cyanobacterial biofertilizers (the free-living cyanobacteria and the cyanobacterium Anabaena azollae in symbiotic association with the water fern Azolla), which include strain identification, isolation, purification, and culture; laboratory analyses of their N-2-fixing activity and related physiology, biochemistry, and energetics; and identification of the structure and regulation of nitrogen-fixing (nif) genes and nitrogenase enzyme. The symbiotic biology of the Azolla-Anabaena mutualistic N-2-fixing complex has been clarified. In free-living cyanobacterial strains, improvement through mutagenesis with respect to constitutive N-2 fixation and resistance to the noncongenial agronomic factors has been achieved. By preliminary meristem mutagenesis in Azolla, reduced phosphate dependence was achieved, as were temperature tolerance and significant sporulation/spore germination under controlled conditions. Mass-production biofertilizer technology of free-living and symbiotic (Azolla-Anabaena) cyanobacteria was studied, as were the interacting and agronomic effects of both kinds of cyanobacterial biofertilizer with rice, improving the economics of rice cultivation with the cyanobacterial biofertilizers. Recent results indicate a strong potential for cyanobacterial biofertilizer technology in rice-growing countries, which opens up a vast area of more concerted basic, applied, and extension work in the future to make these self-renewable natural nitrogen resources even more promising at the field level in order to help reduce the requirement for inorganic N to the bare minimum, if not to zero.