Unique geochemical conditions and microbial composition of pockmark sediments in a brackish-water eutrophic continental shelf sea

Abstract

Abstract In marine pockmarks, interactions between outflowing gas, infiltrating freshwater, and increased input of organic matter (OM) affect the supply and utilization of particular electron acceptors required primarily for OM mineralization. Consequently, geochemical processes in pockmark sediments, including the involvement of various groups of microorganisms, may differ substantially from those in surrounding sediments. In the present study, our aim was to investigate the impact of pockmark presence and activity on geochemical processes, microbial community composition, and the functional profile of microbial communities in sediments from three different areas of the SE Baltic Sea. We performed geochemical analyses (methane – CH 4 , porewater sulfate – SO 4 2− , dissolved inorganic carbon – DIC, hydrogen sulfide – H 2 S, dissolved silicate – dSi, ammonium – NH 4 + , iron – Fe, manganese – Mn, sodium – Na + , potassium – K + , magnesium – Mg 2+ , calcium – Ca 2+ , chloride – Cl − , sediment water content – WC, and loss on ignition – LOI) as well as microbiological analyses (16S rRNA) of sediments collected from pockmarks and corresponding reference sites. The obtained data were further analyzed using factor analysis (FA), multidimensional scaling (MDS), and FAPROTAX functional profiling. Pockmarks, especially active ones, were found to contain higher CH 4 concentrations in sediments, increased SO 4 2− consumption, a shallower methane production (MP) zone, and elevated concentrations of OM decomposition products (DIC, dSi and NH 4 + ) in pore waters compared to nearby reference stations. The presence of Fe was detected only in pore waters from deeper sediments within pockmarks, indicating the occurrence of Fe-dependent anaerobic oxidation of methane (Fe-AOM), accompanied by the absence of sulfide. The non-conservative distribution of major cations in pore waters reflected multiple solid-phase and pore-water interactions controlling K + , Ca 2+ and Mg 2+ , with dolomitization potentially acting as a secondary process at one of the investigated pockmarks. Seepage of biogenic gases coupled with freshwater infiltration in deepwater pockmarks resulted in the development of distinct bacterial and archaeal community structures. Specific microbial groups distinguishing pockmarks from the surrounding bottom sediments were identified. Notably, phototrophic microorganisms were also detected despite water depths below the euphotic zone.