Widespread nitrogen fixation in sediments from diverse deep-sea sites of elevated carbon loading

Nitrogen fixation, the biological conversion of N-2 to NH3, is critical to alleviating nitrogen limitation in many marine ecosystems. To date, few measurements exist of N-2 fixation in deep-sea sediments. Here, we conducted > 400 bottle incubations with sediments from methane seeps, whale falls and background sites off the western coast of the United States from 600 to 2893 m water depth to investigate the potential rates, spatial distribution and biological mediators of benthic N-2 fixation. We found that N-2 fixation was widespread, yet heterogeneously distributed with sediment depth at all sites. In some locations, rates exceeded previous measurements by > 10x, and provided up to 30% of the community anabolic growth requirement for nitrogen. Diazotrophic activity appeared to be inhibited by pore water ammonium: N-2 fixation was only observed if incubation ammonium concentrations were <= 25 mu M, and experimental additions of ammonium reduced diazotrophy. In seep sediments, N-2 fixation was dependent on CH4 and coincident with sulphate reduction, consistent with previous work showing diazotrophy by microorganisms mediating sulphate-coupled methane oxidation. However, the pattern of diazotrophy was different in whale-fall and associated reference sediments, where it was largely unaffected by CH4, suggesting catabolically different diazotrophs at these sites.