Microbiological and biogeochemical processes in a pockmark of the Gdansk depression, Baltic Sea

Comprehensive microbiological and biogeochemical investigation of a pockmark within one of the sites of gas-saturated sediments in the Gdansk depression, Baltic Sea was carried out during the 87th voyage of the Professor Shtokman research vessel. Methane content in the near-bottom water and in the underlying sediments indicates stable methane flow from the sediment into the water. In the 10-m water layer above the pockmark, apart from methane anomalies, elevated numbers of microorganisms and enhanced rates of dark CO2 fixation (up to 1.15 µmol C/(l day)) and methane oxidation (up to 2.14 nmol CH4/(l day)) were revealed. Lightened isotopic composition of suspended organic matter also indicates high activity of the near-bottom microbial community. Compared to the background stations, methane content in pockmark sediments increased sharply from the surface to 40–60 ml/dm3 in the 20–30 cm horizon. High rates of bacterial sulfate reduction (SR) were detected throughout the core (0–40 cm); the maximum of 74 µmol S/(dm3 day) was located in subsurface horizons (15–20 cm). The highest rates of anaerobic methane oxidation (AMO), up to 80 µmol/dm3 day), were detected in the same horizon. Good coincidence of the AMO and SR profiles with stoichiometry close to 1: 1 is evidence in favor of a close relation between these processes performed by a consortium of methanotrophic archaea and sulfate-reducing bacteria. Methane isotopic composition in subsurface sediments of the pockmark (from −53.0 to −56.5‰) does not rule out the presence of methane other than the biogenic methane from the deep horizons of the sedimentary cover.