Rates and signatures of methane turnover in sediments of continental margins

Abstract

In this thesis, a variety of different cold seep systems (mud volcanoes and a gas seep) were investigated using a multidisciplinary approach to gain a more systematic understanding of these, methane-driven biogeosystems. The main goals were the detection and quantification of hot spots of methane oxidation as well as an assessment of environmental factors determining the activity and the distribution of methanotrophic communities. Furthermore, key microbial players were identified and the impact of Anaerobic Oxidation Of Methane (AOM) and Aerobic Oxidation Of Methane (MOx) on the surrounding, marine environment was studied. The investigations revealed the following:1. Submarine mud volcanoes are colonized by specialized microbial communities utilizing the fluxes of reduced substrates such as methane and sulphide as energy source. At the actively methane-seeping Haakon Mosby mud volcano (HMMV, Barents Sea), a distinct spatial zonation of several novel clades of free-living and symbiotic aerobic and anaerobic methanotrophs was found. The main selection mechanism determining vertical and horizontal distribution and dominance of the methanotrophic communities were fluid flow rates controlling access to electron acceptors for methane oxidation. 2. The analysis of archaeal and bacterial specific lipid concentrations and their associated delta 13C-values from three seepage areas at HMMV (thermal centre, grey mats and Beggiatoa site) showed a distinct distribution of methanotrophic biomass. At the centre, MOx mediated by a type I methanotroph was the primary biomass-generating process in surface sediments. In patches of reduced sediment, covered by greyish, thiotrophic, microbial mats at the boundary of the centre, a four-fold increase in 13C-depleted lipids specific for anaerobic methanotrophs, gave evidence of active microbial communities, which mediate AOM in the upper 20 cm of sediment. Further away from the centre, in the zone covered by Beggiatoa mats, sharp, vertical gradients of 13C-depleted archaeal and bacterial lipids indicate that AOM communities were restricted to a narrow surface horizon of no more than 4 cm. A combination of molecular techniques (DAPI, FISH, gene libraries) and biomarker fingerprints provided evidence that the AOM community was dominated by a novel strain of archaea (termed ANME-3) and SRB of the Desulfobulbus cluster.3. Biogeochemical investigations at HMMV revealed a high upward flow of sulphate-free subsurface fluids in the centre, strongly limiting the penetration of sulphate and oxygen from seawater. Here, MOx was restricted to the top sediment layer with rates of 0.9 mol m-2 yr-1 and AOM was absent. In the patches of reduced sediments covered with grey mats, a deeper penetration of sulphate was observed, fueling AOM activity down to >12 cm with rates of 12.4 mol m-2 yr-1. Adjacent to the centre at the Beggiatoa site, decreased upward fluid flow allowed for an AOM zone of ca 4 cm at the sediment surface with rates of 4.5 mol m-2 yr-1. At the outer rim of the HMMV, bioventilation of the pogonophoran worms irrigated a much deeper zone with oxygen- and sulphate-rich seawater. Just beneath the roots of the worms, aOM activity was high with 7.1 mol m-2 yr-1. With respect to the area size of the different habitats at HMMV, microbial consumption reduces the methane efflux of HMMV by ca 7* 10-5 Tg yr-1, i.e. 22 to 55 %. 4. The mud volcanoes of the Gulf of Cadiz are currently much less active than the HMMV. Here, thermogenic methane was completely consumed anaerobically in subsurface sediments. AOM and SR rates showed maxima in distinct subsurface sediment horizons between 20 to 200 cm below sea floor. In comparison to other methane dominated environments of the world oceans, AOM activity and diffusive methane fluxes (<0.4 mol m-2 yr-1, respectively) were low to mid range. AOM was generally exceeded by SRR, most likely because methane related, higher hydrocarbons were oxidised anaerobically by SR microbes. Lipid biomarker and 16S rDNA clone library analyses gave evidence that AOM was mediated by a mixed community of ANME-2 and ANME-1 archaea and associated SRB (Seep-SRB1 group). 5. The Tommeliten gas seep is located in the central North Sea. Here, cracks in a buried marl horizon allow methane to migrate into overlying clay-silt sediments. Hydroacoustic sediment echosounding showed several gas flares coinciding with the apex of the marl domes where methane is released into the water column and potentially to the atmosphere. Carbonates in the vicinity of the gas seep contained 13C-depleted, archaeal lipids indicating long-term AOM activity. In the sediment, the zone of active methane consumption was restricted to a distinct horizon of no more than 20 cm. Diagnostic, 13C-depleted archaeal and bacterial lipids as well as 16S rDNA clone libraries provided evidence that AOM was mediated by ANME-1b archaea and SRB most likely belonging to the Seep-SRB1 cluster.