Diazotroph community structure in the deep oxygen minimum zone of the Costa Rica Dome.

Oxygen minimum zones (OMZs), characterized by depleted dissolved oxygen concentration in the intermediate depth of the water column, are predicted to expand under the influence of global warming. Recent studies in the Eastern Tropical South Pacific Ocean and Arabian Sea have reported that heterotrophic nitrogen fixation is active in the OMZs. In this study, we investigated the community structure of diazotrophs in the OMZ of the Costa Rica Dome (CRD) upwelling region in the Eastern Tropical North Pacific Ocean, using 454-pyrosequencing of nifH gene amplicons. Comparing diazotroph assemblages in different depth strata of the OMZ (200-1000 m in depth), we found a unique diazotroph community in the OMZ core, which was mainly dominated by methanotroph-like diazotrophs, suggesting a potential coupling of nitrogen cycle and methane assimilation. In addition, some OTUs revealed in this study, especially those belonging to the large sub-cluster Vibrio diazotrophicus, were reported to be abundant and expressing the nifH gene in other OMZs. Our results suggest that the unique hydrographic conditions in OMZs may support similar assemblages of diazotrophs, and heterotrophic nitrogen fixation could also be occurring in our studied region. Our study provides the first insight into the composition and distribution of putative diazotrophs in the CRD OMZ.