Anaerobic oxidation of methane in marine sediments

Abstract

Anaerobic oxidation of methane (AOM) is a microbial process in marine sediments, by which sulfate is used as electron acceptor. The current hypothesis is that AOM is mediated by a consortium of methanotrophic archaea and sulfate-reducing bacteria. Research over the past 20 years has shown that AOM retains most of the methane in the sediment. In this thesis, the distribution and relevance of AOM in marine habitats with different methane fluxes was studied. Selected sediments and microbial mats were investigated to explain some of the factors regulating AOM. 1) At Hydrate Ridge, where fossil gases rise and gas hydrates dissociate in the surface sediment, AOM reveals some of the highest rates ever found in marine sediments and is characterized by high heterogeneity. 2) The sediments of Eckernförde Bay are characterized by high methane concentrations due to in situ methane production in organic-rich sediments. We were able to demonstrate that AOM is subject to seasonal changes. The organisms that mediate AOM in Eckernförde Bay apparently belong to the archaeal ANME 2 cluster and form aggregates without bacterial partner. 3) The upwelling system off the Peruvian/Chilean coast is considered to be the most productive marine region. We demonstrated in investigations at three sites along the Chilean continental margin that methane is completely depleted by AOM in the sediment. At all stations the only mechanism of methane transport was molecular diffusion. 4) In the anoxic zone of the Black Sea, we investigated unique microbial reefs above methane seeps. The inner parts of the up to 4 m high reefs are stabilized by porous precipitated carbonates. The outer sides are covered by methanotrophic microbial mats that attain a thickness of several centimeters. The methanotrophic consortium consists of archaea of the ANME 1 cluster and sulfate-reducing bacteria of the Desulfococcus/Desulfosarcina cluster. Our investigations demonstrated that AOM enables the buildup of large biomass.