N2 fixation in phototrophs:: adaptation to a specialized way of life

Phototrophic diazotrophs include the photosynthetic green and purple bacteria, the heliobacteria, many cyanobacteria and the unusual chlorophyll-containing rhizobia that are found in the stem nodules of Aeschynomene spp. In this review, which concentrates on cyanobacteria, the interrelations between photosynthesis and N-2 fixation are discussed. Photosynthesis can, in theory, directly provide the ATP and reductant needed to support N-2 fixation but the link between these two processes is usually indirect, mediated through accumulated carbon reserves. In cyanobacteria, which possess an oxygenic photosynthesis, this serves to separate the O-2 that is produced by photosynthesis from the O-2-sensitive nitrogenase. However, in certain circumstances, oxygenic photosynthesis and N-2 fixation coexist. Under these conditions, respiratory consumption of photosynthetically generated O-2 may have an important role in minimizing O-2-damage to nitrogenase.