The word diazotroph is derived from the words diazo ("di" = two + "a" = without + "zoe" = life) meaning "dinitrogen (N2)" and troph meaning "pertaining to food or nourishment", in summary dinitrogen utilizing. The word azote means nitrogen in French and was named by French chemist and biologist Antoine Lavoisier, who saw it as the part of air which cannot sustain life.
Types of diazotrophs
Diazotrophs are scattered across Bacteria taxonomic groups (as well as a couple of Archaea). Even within a species that can fix nitrogen there may be strains that do not fix nitrogen. Fixation is shut off when other sources of nitrogen are available, and, for many species, when oxygen is at high partial pressure. Bacteria have different ways of dealing with the debilitating effects of oxygen on nitrogenases, listed below.
Anaerobes--these are obligate anaerobes that cannot tolerate oxygen even if they are not fixing nitrogen. They live in habitats low in oxygen, such as soils and decaying vegetable matter. Clostridium is an example. Sulphate-reducing bacteria are important in ocean sediments (e.g. Desulfovibrio), and some Archean methanogens, like Methanococcus, fix nitrogen in muds, animal intestines and anoxic soils.
Aerobes--these species require oxygen to grow, yet their nitrogenase is still debilitated if exposed to oxygen. Azotobacter vinelandii is the most studied of these organisms. It uses very high respiration rates, and protective compounds, to prevent oxygen damage. Many other species also reduce the oxygen levels in this way, but with lower respiration rates and lower oxygen tolerance.
Anoxygenic photosynthetic bacteria do not generate oxygen during photosynthesis, having only a single photosystem which cannot split water. Nitrogenase is expressed under nitrogen limitation. Normally, the expression is regulated via negative feedback from the produced ammonium ion but in the absence of N2, the product is not formed, and the by-product H2 continues unabated [Biohydrogen]. Example species: Rhodobacter sphaeroides, Rhodopseudomonas palustris, Rhodobacter capsulatus.
Rhizobia--these are the species that associate with legumes, plants of the family Fabaceae. Oxygen is bound to leghemoglobin in the root nodules that house the bacterial symbionts, and supplied at a rate that will not harm the nitrogenase.
Frankias--much less is known about these 'actinorhizal' nitrogen fixers. The bacteria also infect the roots leading to the formation of nodules. Actinorhizal nodules consist of several lobes, each lobe has a similar structure as a lateral root. Frankia is able to colonize in the cortical tissue of nodules where it fixes nitrogen. Actinorhizal plants and Frankias also produce haemoglobins, but their role is less well established than for rhizobia. Although at first it appeared that they inhabit sets of unrelated plants (alders, Australian pines, California lilac, bog myrtle, bitterbrush, Dryas), revisions to the phylogeny of angiosperms show a close relatedness of these species and the legumes. These footnotes suggest the ontogeny of these replicates rather than the phylogeny. In other words, an ancient gene (from before the angiosperms and gymnosperms diverged) that is unused in most species was reawakened and reused in these species.
Cyanobacteria--there are also symbiotic cyanobacteria. Some associate with fungi as lichens, with liverworts, with a fern, and with a cycad. These do not form nodules (indeed most of the plants do not have roots). Heterocysts exclude the oxygen, as discussed above. The fern association is important agriculturally: the water fern Azolla harbouring Anabaena is an important green manure for rice culture.
Association with animals--although diazotrophs have been found in many animal guts, there is usually sufficient ammonia present to suppress nitrogen fixation.Termites on a low nitrogen diet allow for some fixation, but the contribution to the termite's nitrogen supply is negligible. Shipworms may be the only species that derive significant benefit from their gut symbionts.
In terms of generating nitrogen available to all organisms, the symbiotic associations greatly exceed the free-living species with the exception of cyanobacteria.
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