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Abstract. Diversity and community composition of nitrogen (N) fixing microbes in the threemain oxygen minimum zones (OMZs) of the world ocean were investigated usingoperational taxonomic unit (OTU) analysis of nifH clone libraries.Representatives of three of the four main clusters of nifH genes were detected.Cluster I sequences were most diverse in the surface waters, and the mostabundant OTUs were affiliated with Alpha- and Gammaproteobacteria. Cluster II, III, and IV assemblages were most diverse at oxygen-depleted depths, and noneof the sequences were closely related to sequences from cultivatedorganisms. The OTUs were biogeographically distinct for the most part –there was little overlap among regions, between depths, or between cDNA andDNA. In this study of all three OMZ regions, as well as from the few otherpublished reports from individual OMZ sites, the dominance of a few OTUs wascommonly observed. This pattern suggests the dynamic response of the componentsof the overall diverse assemblage to variable environmental conditions.Community composition in most samples was not clearly explained byenvironmental factors, but the most abundant OTUs were differentiallycorrelated with the obvious variables, temperature, salinity, oxygen, andnitrite concentrations. Only a few cyanobacterial sequences were detected.The prevalence and diversity of microbes that harbor nifH genes in the OMZregions, where low rates of N fixation are reported, remains an enigma.

Diversity and distribution of nitrogen fixation genes in the oxygen minimum zones of the world oceans