Nitrogen fixation in sediments along a depth transect through the Peruvian oxygen 1 minimum zone

12 Benthic nitrogen (N2) fixation and sulfate reduction (SR) were investigated in the Peruvian 13 oxygen minimum zone (OMZ). Sediment samples, were retrieved by a multiple corer at six 14 stations (70 1025 m water depth) along a depth transect at 12°S, covering anoxic and 15 hypoxic bottom water conditions. Benthic N2 fixation was detected at all sites using the 16 acetylene reduction assay, with high rates between 70 m and 253 m and lower rates at 17 greater depth. SR rates decreased with increasing water depth.. Benthic N2 fixation and SR 18 depth profiles in sediments showed similar qualitative trends, suggesting a coupling ofboth 19 processes. Potential N2 fixation by sulfate-reducing bacteria was verified by the molecular 20 analysis of nifH genes. Detected nifH sequences, i.e., the key functional gene for N2 21 fixation, encoding for the nitrogenase enzyme, clustered with sulfate-reducing bacteria 22 that have been demonstrated to fix N2 in other benthic environments. Depth-integrated 23 rates of benthic N2 fixation and SR showed no direct correlation along the transect, 24 suggesting that the benthic diazotrophs in the Peruvian OMZ is controlled by additional 25 environmental factors such as organic matter and free sulfide. It was further found that N2 26 fixation in OMZ sediments was not inhibited by high ammonium concentrations. N2 fixation 27 rates in OMZ sediments were similar to rates measured in other organic-rich sediments. 28 Overall, this study improves our knowledge on fixed N sources and N cycling in oxygen 29 deficient environments. 30