Abstract

Abstract. 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.

Highlights

  • 6 % of nitrogen (N) in seawater is bioavailable (Gruber, 2008)

  • St. 9 (770 m) was below the oxygen minimum zone (OMZ), and sediments were brown to dark olive green with white particles between 0 and 12 cm, and brown to olive green without white particles below this depth

  • We have shown that N2 fixation occurred throughout the sediment and that activity often overlapped with sulfate reduction (SR)

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Summary

Introduction

6 % of nitrogen (N) in seawater is bioavailable (Gruber, 2008). This bioavailable N is mainly present in the form of nitrate (NO−3 ), whereas the large pool of atmospheric dinitrogen gas (N2) is only available for N2 fixing microorganisms (diazotrophs). The quantitative contribution of diazotrophs in the marine N cycle remains unclear and numerous estimates of global sources and sinks of global N have led to an unbalanced budget with deficits of around 200 Tg N yr−1 (Codispoti, 2007). This suggests that either previous N2 fixation rate determinations have been underestimated (Großkopf et al, 2012) or that N loss processes are overestimated (Codispoti, 2007). These budget discrepancies illustrate that the current knowledge on diazotrophy and the marine N cycle is still limited

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