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

The potential coupling of nitrogen (N2) fixation and sulfate reduction (SR) was explored in sediments of the Peruvian oxygen minimum zone (OMZ). Sediment samples were retrieved by a multiple corer at six stations along a depth transect (70–1025 m water depth) at 12 S, covering anoxic and hypoxic bottom water conditions. Benthic N2 fixation, determined by the acetylene reduction assay, was detected at all sites, with highest rates between 70 and 253 m and lower rates at greater depth. SR rates decreased with increasing water depth. N2 fixation and SR overlapped in sediments, suggesting a potential coupling of both processes. However, a weak positive correlation of their activity distribution was detected by principle component analysis. A potential link between N2 fixation and sulfate-reducing bacteria was indicated by the molecular analysis of nifH genes. Detected nifH sequences clustered with the sulfate-reducing bacteria Desulfonema limicola at the 253 m station. However, nifH sequences of other stations clustered with uncultured organisms, Gammaproteobacteria, and Firmicutes (Clostridia) rather than with known sulfate reducers. The principle component analysis revealed that benthic N2 fixation in the Peruvian OMZ is controlled by organic matter (positive) and free sulfide (negative). No correlation was found between N2 fixation and ammonium concentrations (even at levels > 2022 μM). N2 fixation rates in the Peruvian OMZ sediments were in the same range as those measured in other organic-rich sediments.