Nitrogen Fixation in Phototrophic Bacteria

Abstract

N₂-fixing phototrophs (photosynthetic bacteria and cyanobacteria) occur in terrestrial, freshwater and marine environments from polar regions to the tropics. Some of these organisms contribute substantially to global N₂ fixation. For example, blooms of the marine cyanobacterium Trichodesmium may cover 90000 km² of the tropical ocean (Dupouy et cd, 1988) and fix as much as 5.4 x 10¹² g of N per year (Carpenter, Capone, 1992), perhaps 4% of the total N₂ fixed on Earth (see Paul, 1978, and papers cited therein). In addition, Nodularia and Aphanizomenon spp form extensive annual blooms in the Baltic Sea (see, for example, Kahru et al., 1994). Cyanobacterial symbionts of marine diatoms such as Hemiaulis spp and Rhizosolenia clevei may also contribute a great deal locally to oceanic N₂ fixation. (Villareal, 1991; 1992; 1994). Finally, the discovery of marine copepods bearing intestinal Chromatium spp and abie to reduce acetylene (Proctor, 1995; personal communication to JRG) implies that marine photosynthetic bacteria may also make a considerable contribution to global N₂ fixation, perhaps up to i% of the total. Photosynthetic diazotrophs therefore make major contributions to N₂ fixation on Earth and, as such, deserve intensive study.