Biochemistry and physiology of nitrogen fixation with particular emphasis on nitrogen-fixing phototrophs

Summary This paper presents an overview of aspects of N₂-fixation in phototrophic N₂-fixers. Nitrogenase is little different in phototrophs from other organisms. Evidence suggests that fixed carbon dissimilation rather than direct photoreduction from oxidised inorganic compounds or exogenous photosynthetic electron donors is the major route of reductant supply to nitrogenase in phototrophs; inRhodospirillum rubrum pyruvate is a possible electron donor to nitrogenase; in cyanobacteria the oxidative pentose phosphate pathway is important, although some recent evidence implicates glycolysis and the tricarboxylic acid cycle in reductant supply in heterocystous cyanobacteria. In photosynthetic organisms light modulation of various enzymes occurs-some Calvin cycle enzymes are light activated, some oxidative pentose phosphate pathway and glycolytic enzymes are deactivated and some tricarboxylic acid cycle enzymes are activated. Reduced levels of thioredoxin in heterocysts may contribute to the sustained functioning of the oxidative pentose phosphate pathway in heterocysts in the light and dark. In photosynthetic bacteria such asRhodospirillum rubrum an activating enzyme which removes a modifying group from inactive Fe protein can activate nitrogenase. O₂ and NH ₄ ⁺ both inhibit N₂-fixation and there is some evidence in cyanobacteria that O₂ stability of whole cell nitrogenase can be achieved by prolonged incubation of cultures at high O₂.