Nitrogen fixation in sediments along a depth transect through the Eastern Boundary Upwelling Systems off Peru and Mauritania.

Gier, Jessica, Sommer, Stefan, Löscher, Carolin, Dale, Andrew W. , Schmitz, Ruth A. and Treude, Tina (2015) Nitrogen fixation in sediments along a depth transect through the Eastern Boundary Upwelling Systems off Peru and Mauritania. [Talk] In: AGU Fall Meeting 2015. , 14.-18.12.2015, San Francisco, USA .

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Abstract

The distribution of benthic nitrogen (N2) fixation and its relevance for N cycling in the Eastern Boundary Upwelling Systems (EBUS) are still unknown. Recent studies confirm that benthic N2 fixation can be coupled to sulfate reduction (SR) and that several species of sulfate reducing bacteria have the genetic ability to fix N due to the presence of the gene encoding for the nitrogenase enzyme.
We investigated benthic N2 fixation and SR in the Peruvian oxygen minimum zone at 12°S and in the Mauritanian upwelling system at 18°N along a depth transect. Sediments were retrieved by a multicorer and a benthic lander at six stations in both regions.
Benthic N2 fixation occurred throughout the sediment in both EBUS. Off Peru the highest integrated (0-20 cm) N2 fixation rate of 0.4 mmol N/m2/d was measured inside the core of the OMZ at 253 m water depth. Off Mauritania the highest integrated (0-20 cm) N2 fixation rate of 0.15 mmol N/m2/d was measured at 90 m, coinciding with a low bottom water oxygen concentration (30 µM). N2 fixation depth profiles often overlapped with SR activity. Moreover, sequencing data yielded insights into the composition and diversity of the nifH gene pool in EBUS sediments. Interestingly, detected sequences in both EBUS clustered with SR bacteria, such as Desulfovibrio vulgaris and several of the novel detected clades belonged to uncultured diazotrophs.
Our results suggest that N2 fixation and SR were coupled to a large extent in both regions. However, potential environmental factors controlling benthic diazotrophs in the EBUS appear to be the availability of sulfide and organic matter. Additionally, no inhibition of N2 fixation at high ammonium concentrations was found, which highlights gaps in our knowledge regards the interaction between ammonium availability and diazotrophy. Our results contribute to a better understanding of N cycling in EBUS sediments and sources of fixed N.

Document Type: Conference or Workshop Item (Talk)
Keywords: Nitrogen fixation; Upwelling Systems; Peru; Mauritania
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Kiel University
Date Deposited: 04 Jan 2016 08:52
Last Modified: 23 Sep 2019 18:13
URI: http://oceanrep.geomar.de/id/eprint/30737

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