Microbiology of Nitrogen Fixation by Anoxygenic Photosynthetic Bacteria

Abstract

SummaryAnoxygenic phototrophic bacteria conduct many specialized metabolic processes but one of the most important is nitrogen fixation, the reduction of N2 to NH3. Nitrogen fixation is catalyzed by the enzyme nitrogenase which is widely distributed among anoxygenic phototrophs. The most comprehensive understanding of N2 fixation in photosynthetic bacteria is among the nonsulfur purple bacteria, where the capacity to fix N2 is nearly universal. In this group N2 fixation occurs both in the light and in darkness and nitrogenase expression and activity is highly regulated. Available evidence indicates that N2 fixation is fairly widespread among purple and green sulfur bacteria and probably universal among heliobacteria. By contrast, the thermophilic phototroph Chloroflexus aurantiacus, a very few purple nonsulfur bacteria and green sulfur bacteria, and the ‘aerobic phototrophs’ are not diazotrophic. The recently discovered bacteriochlorophyll-containing rhizobia are also nitrogen fixers but their status as anoxygenic phototrophs is at present unclear. Although the ecological significance of N2 fixation by anoxygenic phototrophs is unknown, the widespread distribution of these organisms in nature and the fact that most species are diazotrophic, suggests that they may well be significant, particularly in specialized environments such as microbial mats and paddy soils.