Description and Classification of Diazotrophs

Abstract

In comparative biology it is normally observed that organisms possessing a common function also tend to have certain distinguishing physiological, metabolic, structural and genetic characteristics. The organisms which can use N₂ to meet their nitrogen requirement, however, appear to violate this rule: except for the ability to fix N₂, these organisms, the diazotrophs, have little else in common. They comprise an incongruous mixture made up primarily of certain bacteria and algae, but also encompassing, in a unique participation, certain higher plants. However, it appears increasingly evident that the enzyme nitrogenase occurs naturally only in prokaryotic cells. Some genetic bridges and barriers do appear to exist among them, but often with little obvious logic. The inclusion of aerobic, facultative, anaerobic and photosynthetic organisms among the diazotrophs attests to their physiological and metabolic diversity. Representatives of major classes of diazotrophs are shown in Fig. 2-1.