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

Nitrogen fixation, the biological conversion of N₂ to NH₃ , is critical to alleviating nitrogen limitation in many marine ecosystems. To date, few measurements exist of N₂ 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 N₂ fixation. We found that N₂ fixation was widespread, yet heterogeneously distributed with sediment depth at all sites. In some locations, rates exceeded previous measurements by > 10×, and provided up to 30% of the community anabolic growth requirement for nitrogen. Diazotrophic activity appeared to be inhibited by pore water ammonium: N₂ fixation was only observed if incubation ammonium concentrations were ≤ 25 μM, and experimental additions of ammonium reduced diazotrophy. In seep sediments, N₂ 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.