Microbial methane oxidation at the redoxcline of the Gotland Deep (Central Baltic Sea)

Abstract

Abstract. Methane concentrations in the stratified water column of the Gotland Deep (Central Baltic Sea) show a strong gradient from high values in the saline deep water (max. 504nM) to low concentrations in the less dense, brackish surface water (about 4 nM). The steepest gradient is present within the redoxcline (between 115 and 135 m water depth) that separates the anoxic deep part from the oxygenated surface water, implying a methane consumption rate of 0.28 nM d−1. The process of microbial methane oxidation within the redoxcline is mirrored by a shift of the stable carbon isotope ratio of methane between the bottom water (δ13C CH4 = −82.4‰) and the suboxic depth interval (δ13C CH4 = −38.7‰). A water column sample from 100 m water depth was studied to identify the microorganisms responsible for the methane turnover at the redoxcline. Notably, methane monoxygenase gene expression analyses for the specific water depth demonstrated that accordant methanotrophic activity was due to only one microbial phylotype. An imprint of these organisms on the particular organic matter was revealed by distinctive lipid biomarkers showing bacteriohopanepolyols and lipid fatty acids characteristic for aerobic type I methanotrophic bacteria (e.g. 35-aminobacteriohopane-30,31,32,33,34-pentol). In conjunction with earlier findings, our results support the idea that biogeochemical cycles in Central Baltic Sea redoxclines are mainly driven by only a few microbial key species.