Non-Symbiotic Nitrogen Fixation
Microorganisms which pass independent life and fix atmospheric nitrogen are known as free living diazotrophs. There are two groups of such micro-organisms : bacteria and cyanobacteria (blue-green algae). Based on the mode of nutrition (carbon, nitrogen and oxygen and requirement of reducing groups) bacteria are divided into (i) aerobic bacteria (Azonomas, Azotobacter, Beijerinckia, Mycobacterium, Methylomonas), (ii) facultative anaerobic bacteria (Bacillus, Enterobacter, Klebsiella, etc.), (iii) anaerobic bacteria (Clostridium, Desulfovibrio, etc.) and (iv) photosynthetic bacteria (Rhodomicrobium, Rhodopseudomonas, Rhodospirillum, Chromatium, Chlorobium, etc.)
Among cyanobacteria, both heterocystous and nonheterocystous forms fix atmospheric nitrogen, for example, Anabaena, Anabaenopsis, Aulosira, Calothrix, Cylindrospermum, Gloeocapsa, Lyngbya, Nostoc, Oscillatoria, Plectonema, Scytonema, Stegonema, Tolypothrix, Trichodesmium, etc. (Alexander, 1977).