Phylogenetic diversity of nitrogen fixation genes in the symbiotic microbial community in the gut of diverse termites

Nitrogen fixation by the microorganisms in the gut of termites is one of th e crucial aspects of symbiosis, since termites usually thrive on a nitrogen -poor diet, The phylogenetic diversity of the nitrogen-fixing organisms wit hin the symbiotic community in the guts of various termite species was inve stigated without culturing the resident microorganisms. A portion of the di nitrogenase reductase gene (nifH) was directly amplified from DNA extracted from the mixed population in the termite gut. Analysis of deduced amino ac id sequences of the products of the clonally isolated nif genes revealed th e presence of diverse nifH sequences in most of the individual termite spec ies, and their constituents were considerably different among termite speci es. A majority of the nifH sequences from six lower termites, which showed significant levels of nitrogen fixation activity, could be assigned to eith er the anaerobic nif group (consisting of clostridia and sulfur reducers) o r the alternative nifH ethanogen group among the nifH phylogenetic groups. In the case of three higher termites, which showed only low levels of nitro gen fixation activity, a large number of the sequences were assigned to the most divergent nif group, probably functioning in some process other than nitrogen fixation and being derived from methanogenic archaea. The nifH gro ups detected were similar within each termite family but different among th e termite families, suggesting an evolutionary trend reflecting the diazotr ophic habitats in the symbiotic community. Within these phylogenetic groups , the sequences from the termites formed lineages distinct from those previ ously recognized in studies using classical microbiological techniques, and several sequence clusters unique to termites were found, The results indic ate the presence of diverse potentially nitrogen-fixing microbial assemblag es in the guts of termites, and the majority of them are as yet uncharacter ized.