Geochemical processes and chemosynthetic primary production in different thiotrophic mats of the Hakon Mosby Mud Volcano (Barents Sea)

We have investigated if in a cold seep methane or sulfide is used for chemosynthetic primary production and if
significant amounts of the sulfide produced by anaerobic oxidation of methane are oxidized geochemically and
hence are not available for chemosynthetic production. Geochemically controlled redox reactions and biological
turnover were compared in different habitats of the Ha°kon Mosby Mud Volcano. The center of the mud volcano
is characterized by the highest fluid flow, and most primary production by the microbial community depends on
oxidation of methane. The small amount of sulfide produced is oxidized geochemically with oxygen or is
precipitated with dissolved iron. In the medium flow peripheral Beggiatoa habitat sulfide is largely oxidized
biologically. The oxygen and nitrate supply is high enough that Beggiatoa can oxidize the sulfide completely, and
chemical sulfide oxidation or precipitation is not important. An internally stored nitrate reservoir with average
concentrations of 110 mmol L21 enables the Beggiatoa to oxidize sulfide anaerobically. The pH profile indicates
sequential sulfide oxidation with elemental sulfur as an intermediate. Gray thiotrophic mats associated with
perturbed sediments showed a high heterogeneity in sulfate turnover and high sulfide fluxes, balanced by the
opposing oxygen and nitrate fluxes so that biological oxidation dominates over geochemical sulfide removal
processes. The three habitats indicate substantial small-scale variability in carbon fixation pathways, either
through direct biological use of methane or through indirect carbon fixation of methane-derived carbon dioxide
by chemolithotrophic sulfide oxidation.