Abstract
This chapter explores nitrogen-fixing microorganisms and discusses the discovery of biological nitrogen fixation in 1886. The chapter reviews studies done on biological nitrogen fixation in the 1900s and the technical advances since World War II. Nitrogen gas can be fixed only by a limited and odd assortment of procaryotes, either alone or in association with higher plants or animals. The free-living fixers are to be found amongst aerobic soil and water microorganisms such as Azotobacter and anaerobic sulfate-reducing bacteria—for example, Desulfovibrio and the photosynthetic bacteria. The microsymbiont Rhizobium that fixes the nitrogen, resides in specialized nodular outgrowths on the roots. Work on the biochemistry of BNF was hampered by the failure to obtain an active enzyme preparation from any source. This was largely because of the enzyme's extreme sensitivity to inactivation by oxygen. At the same time the nitrogen-fixing enzyme, nitrogenase, has an absolute requirement for oxygen, later shown to be met, in the nodule, by a specific hemoglobin and leghemoglobin that allows the transfer of oxygen at low partial pressure.
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