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

Abstract

Abstract. The potential coupling of nitrogen (N 2 ) 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 N 2 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. N 2 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 N 2 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 N 2 fixation in the Peruvian OMZ is controlled by organic matter (positive) and free sulfide (negative). No correlation was found between N 2 fixation and ammonium concentrations (even at levels > 2022 µM). N 2 fixation rates in the Peruvian OMZ sediments were in the same range as those measured in other organic-rich sediments.

Benthic Dinitrogen Fixation Traversing the Oxygen Minimum Zone Off Mauritania (NW Africa)

“…Overall, the phylogenetic analysis of diazotrophs indicated a potential role of sulfate-reducing bacteria for N 2 fixation in Mauritanian sediments. Sequences clustered with several sulfate reducers related to Desulfovibrio spp., which was found earlier to be involved in benthic N 2 fixation Fulweiler et al, 2013;Gier et al, 2016). Likewise, nifH gene analysis indicated a potential link between N 2 fixation and sulfate reduction in OMZ sediments off Peru .…”

“…While most studies on marine N 2 fixation have focused on pelagic environments (e.g., Zehr and Ward, 2002;Galloway et al, 2004;Riemann et al, 2010;Löscher et al, 2014 and references therein), benthic N 2 fixation gained renewed attention only recently with a few studies demonstrating active N 2 fixation in sediments and identifying diazotrophs by nifH gene analysis (Fulweiler et al, 2007;Bertics et al, , 2013Gier et al, 2016). However, there is uncertainty regarding the environmental factors ultimately controlling benthic N 2 fixation.…”

“…Both microbial processes are often coupled in organic-rich sediments (Bertics and Ziebis, 2010;Bertics et al, 2013;Gier et al, 2016). Additionally, many sulfate reducers carry the nifH gene (Zehr and Turner, 2001;Muyzer and Stams, 2008;Fulweiler et al, 2013;Gier et al, 2016) and actively fix N 2 in culture (Riederer-Henderson and Wilson, 1970), indicating that these bacteria may play a role in supplying bioavailable N to the benthic community Sohm et al, 2011;Fulweiler et al, 2013).…”

“…The sampling procedure ( Table 1) and core slicing details for N 2 fixation have previously been described by Gier et al (2016).…”

Microbial methanogenesis in the sulfate-reducing zone of sediments in the Eckernförde Bay, SW Baltic Sea

“…To determine the possible environmental control parameters of SRZ methanogenesis, a principal component analysis (PCA) was applied according to the approach described in Gier et al (2016). Prior to PCA, the dataset was transformed into ranks to ensure the same data dimensions.…”