Aerobic methane oxidation (AMO) is one of the primary biologic pathways regulating the amount of methane (CH4) released into the environment. AMO acts as a sink of CH4, converting it into carbon dioxide before it reaches the atmosphere. It is of interest for (paleo)climate and carbon cycling studies to identify lipid biomarkers that can be used to trace AMO events, especially at times when the role of methane in the carbon cycle was more pronounced than today. AMO bacteria are known to synthesise bacteriohopanepolyol (BHP) lipids. Preliminary evidence pointed towards 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) being a characteristic biomarker for Type I methanotrophs. Here, the BHP compositions were examined for species of the recently described novel Type I methanotroph bacterial genera Methylomarinum and Methylomarinovum, as well as for a novel species of a Type I Methylomicrobium. Aminopentol was the most abundant BHP only in Methylomarinovum caldicuralii, while Methylomicrobium did not produce aminopentol at all. In addition to the expected regular aminotriol and aminotetrol BHPs, novel structures tentatively identified as methylcarbamate lipids related to C-35 amino-BHPs (MC-BHPs) were found to be synthesised in significant amounts by some AMO cultures. Subsequently, sediments and authigenic carbonates from methane-influenced marine environments were analysed. Most samples also did not contain significant amounts of aminopentol, indicating that aminopentol is not a useful biomarker for marine aerobic methanotophic bacteria. However, the BHP composition of the marine samples do point toward the novel MC-BHPs components being potential new biomarkers for AMO.

Subsurface microbial communities undertake many terminal electron-accepting processes, often simultaneously. Using a tritium-based assay, we measured the potential hydrogen oxidation catalyzed by hydrogenase enzymes in several subsurface sedimentary environments (Lake Van, Barents Sea, Equatorial Pacific, and Gulf of Mexico) with different predominant electron-acceptors. Hydrogenases constitute a diverse family of enzymes expressed by microorganisms that utilize molecular hydrogen as a metabolic substrate, product, or intermediate. The assay reveals the potential for utilizing molecular hydrogen and allows qualitative detection of microbial activity irrespective of the predominant electron-accepting process. Because the method only requires samples frozen immediately after recovery, the assay can be used for identifying microbial activity in subsurface ecosystems without the need to preserve live material. We measured potential hydrogen oxidation rates in all samples from multiple depths at several sites that collectively span a wide range of environmental conditions and biogeochemical zones. Potential activity normalized to total cell abundance ranges over five orders of magnitude and varies, dependent upon the predominant terminal electron acceptor. Lowest per-cell potential rates characterize the zone of nitrate reduction and highest per-cell potential rates occur in the methanogenic zone. Possible reasons for this relationship to predominant electron acceptor include (i) increasing importance of fermentation in successively deeper biogeochemical zones and (ii) adaptation of H2ases to successively higher concentrations of H2 in successively deeper zones.

15 16Multibeam bathymetry revealed the occurrence of numerous craterlike depressions, 17 so-called pockmarks, on the sea floor of the Hammerfest Basin and the Loppa High, 18 south-western Barents Sea. To investigate whether these pockmarks are related to 19 ongoing gas seepage, microbial processes associated with methane metabolism 20 were analyzed using geochemical, biogeochemical and microbiological techniques. 21Gravity cores were collected along transects crossing individual pockmarks, allowing 22 a direct comparison between different locations inside (assumed activity center), on 23 the rim, and outside of a pockmark (reference sites). Concentrations of hydrocarbons 24 in the sediment, particularly methane, were measured as headspace (free) gas, and 25 in the occluded and adsorbed gas fraction. Down to a depth of 2.6 m below sea floor 26 2 (mbsf) sulfate reduction rates were quantified by radiotracer incubations. 27Concentrations of dissolved sulfate in the porewater were determined as well. Neither 28 the sulfate profiles nor the gas measurements show any evidence of microbial activity 29 or active fluid venting. Methane concentrations and sulfate reduction rates were 30 extremely low or even below the detection limit. The results show that the observed 31 sediment structures are most likely paleo-pockmarks, their formation probably 32 occurred during the last deglaciation. 33 34 Introduction 35Pockmarks are craterlike depressions in the seabed that form due to expulsion of 36

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