Nitrogen fixation by symbiotic and free-living spirochetes

Spirochetes from termite hindguts and freshwater sediments possessed homolo gs of a nitrogenase gene (nifH) and exhibited nitrogenase activity, a previ ously unrecognized metabolic capability in spirochetes. Fixation of 15-dini trogen was demonstrated with termite gut Treponema ZAS-9 and free-living Sp irochaeta aurantia. Homologs of nifH were also present in human oral and bo vine ruminal treponemes. Results implicate spirochetes in the nitrogen nutr ition of termites, whose food is typically Low in nitrogen, and in global n itrogen cycling. These results also proffer spirochetes as a Likely origin of certain nifHs observed in termite guts and other environments that were not previously attributable to known microbes.