Abstract. Diversity and community composition of nitrogen (N) fixing microbes in the three
main oxygen minimum zones (OMZs) of the world ocean were investigated using
operational 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 most
abundant OTUs were affiliated with Alpha- and Gammaproteobacteria. Cluster II, III, and IV assemblages were most diverse at oxygen-depleted depths, and none
of the sequences were closely related to sequences from cultivated
organisms. The OTUs were biogeographically distinct for the most part –
there was little overlap among regions, between depths, or between cDNA and
DNA. In this study of all three OMZ regions, as well as from the few other
published reports from individual OMZ sites, the dominance of a few OTUs was
commonly observed. This pattern suggests the dynamic response of the components
of the overall diverse assemblage to variable environmental conditions.
Community composition in most samples was not clearly explained by
environmental factors, but the most abundant OTUs were differentially
correlated with the obvious variables, temperature, salinity, oxygen, and
nitrite concentrations. Only a few cyanobacterial sequences were detected.
The prevalence and diversity of microbes that harbor nifH genes in the OMZ
regions, where low rates of N fixation are reported, remains an enigma.
Iron cycling and nutrient-limitation patterns in surface waters of the World Ocean
