N-2 fixation in phototrophs: adaptation to a specialized way of life

Phototrophic diazotrophs include the photosynthetic green and purple bacter ia, the heliobacteria, many cyanobacteria and the unusual chlorophyll-conta ining rhizobia that are found in the stem nodules of Aeschynomene spp. In t his review, which concentrates on cyanobacteria, the interrelations between photosynthesis and N-2 fixation are discussed. Photosynthesis can, in theo ry, directly provide the ATP and reductant needed to support N-2 fixation b ut the link between these two processes is usually indirect, mediated throu gh accumulated carbon reserves. In cyanobacteria, which possess an oxygenic photosynthesis, this serves to separate the O-2 that is produced by photos ynthesis from the O-2-sensitive nitrogenase. However, in certain circumstan ces, oxygenic photosynthesis and N-2 fixation coexist. Under these conditio ns, respiratory consumption of photosynthetically generated O-2 may have an important role in minimizing O-2-damage to nitrogenase.