Aerobic methanotrophy within the pelagic redox-zone of the Gotland Deep (central Baltic Sea)

Water column samples taken in summer 2008 from the stratified Gotland Deep (central Baltic Sea) showed a strong gradient in dissolved methane concentrations from high values in the saline deep water (max. 504 nM) to low concentrations in the less dense, brackish surface water (about 4 nM). The steep methane-gradient (between 115 and 135 m water depth) within the redox-zone, which separates the anoxic deep part from the oxygenated surface water (oxygen concentration 0–0.8 mL L − 1 ), implies a methane consumption rate of 0.28 nM d − 1 . The process of micro- bial methane oxidation within this zone was evident by a shift of the stable carbon isotope ratio of methane between the bottom water ( δ 13 C CH 4 =− 82.4 ‰) and the redox- zone ( δ 13 C CH 4 =− 38.7 ‰). Water column samples be- tween 80 and 119 m were studied to identify the microor- ganisms responsible for the methane turnover in that depth interval. Notably, methane monooxygenase gene expression analyses for water depths covering the whole redox-zone demonstrated that accordant methanotrophic activity was probably due to only one phylotype of the aerobic type I methanotrophic bacteria. 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 methanotrophs (e.g., 35-aminobacteriohopane-30,31,32,33,34-pentol), cor- roborating their role in aerobic methane oxidation in the redox-zone of the central Baltic Sea.