Benthic nitrogen fixation in oxygen minimum zones
The distribution and magnitude of benthic N2 fixation in OMZs, and its relevance for the community inhabiting these sediments was investigated. A possible coupling to sulfate reducers was determined by rate measurements of N2 fixation and sulfate reduction, as well as by molecular analysis in the respective environment. In sediments of the Peruvian OMZ, N2 fixation and sulfate reduction occurred throughout the sediment and depth profiles largely overlapped, suggesting a coupling of both processes. This coupling was further supported by the molecular analysis. In the Mauritanian OMZ N2 fixation activity often overlapped with sulfate reduction, as well as with ferrous iron concentrations. The molecular analysis confirmed the presence of sulfate- and iron-reducing bacteria. Burrowing organisms potentially create a biogeochemical zonation pattern in sediments that enhanced N2 fixation in deeper sediment layers. To examine the marine benthic diazotrophic diversity, samples from OMZs, methane seeps and brackish water sediments were used for high-throughput sequencing. Results showed a small diversity of diazotrophs among sites and a dominance of sulfate reducers. The redundancy analysis showed a positive correlation between diazotrophs and sulfate reduction. To conclude, the detection of benthic N2 fixation in OMZs, as well as the diversity study of benthic diazotrophs from different environments, shows that it is a ubiquitous process in the benthic environment. The global distribution of benthic diazotrophs, as revealed by the diversity study, highlights its previously underestimated role in the benthic N cycle, as well as in the marine N budgets. The controlling factors of diazotrophs in marine sediments are: abundance of sulfate reducers; organic matter content in the sediments; and sulfide concentration in the porewater, as a potential inhibitor.