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

Abstract

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 Lg-1), implies a methane consumption rate of 0.28 nM dg-1. The process of microbial methane oxidation within this zone was evident by a shift of the stable carbon isotope ratio of methane between the bottom water (delta;13C CH4=-82.4% and the redox-zone (delta;13C CH4=-38.7%. Water column samples between 80 and 119 m were studied to identify the microorganisms 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), corroborating their role in aerobic methane oxidation in the redox-zone of the central Baltic Sea. © Author(s) 2012. CC Attribution 3.0 License.