Nitrogen Fixation By Sulfate-Reducing Bacteria in Coastal and Deep-Sea Sediments

Sulfate-reducing bacteria (SRB) can greatly impact benthic nitrogen (N) cycling, by for instance inhibiting coupled denitrification-nitrification through the production of sulfide or by increasing the availability of fixed N in the sediment via dinitrogen (N₂)-fixation. Here, we explored several coastal and deep-sea benthic habitats within the Atlantic Ocean and Baltic Sea, for the occurrence of N₂-fixation mediated by SRB. A combination of different methods including microbial rate measurements of N₂-fixation and sulfate reduction, geochemical analyses (porewater nutrient profiles, mass spectrometry), and molecular analyses (CARD-FISH, HISH-SIMS, "nested" PCR, and QPCR) were applied to quantify and identify the responsible processes and organisms, respectively. Furthermore, we looked deeper into the question of whether the observed nitrogenase activity was associated with the final incorporation of N into microbial biomass or whether the enzyme activity served another purpose. At the AGU Fall Meeting, we will present and compare data from numerous stations with different water depths, temperatures, and latitudes, as well as differences in key geochemical parameters, such as organic carbon content and oxygen availability. Current metabolic and molecular data indicate that N₂-fixation is occurring in many of these benthic environments and that a large part of this activity may linked to SRB.