Microbial carbon flow in marine methane cold seeps

This thesis investigates microbial carbon flow in methane-rich marine sediments, with a specific focus on a cold seep environment of the Astoria Canyon, USA. Methane is a potent greenhouse gas, but more than 90% of methane generated in marine sediments is consumed through the anaerobic oxidation of methane (AOM) before it can reach the ocean and ultimately the atmosphere. Central to this thesis is the question of how methane-derived carbon is assimilated into microbial biomass by anaerobic methanotrophic archaea (ANME) and their sulfate-reducing partner bacteria (SRB) and how this carbon is subsequently redistributed into microbial metabolites and the dissolved organic carbon (DOC) pool. In my introduction (Chapter 1), I will provide the necessary background information and synthesize recent literature to enable the reader to understand my thesis, with a particular focus on methane and methane-related biogeochemical processes. In the first research project I studied the carbon assimilation by AOM-performing microorganisms (Chapter 2). Therefore, I performed a lipid stable isotope probing (SIP) experiment with 13C-labeled methane and inorganic carbon (IC) in sediments from the Astoria Canyon. The results revealed that both ANME-2 and associated SRB that dominated the cold seep sediments at the Astoria Canyon, primarily assimilate IC rather than directly incorporating methane into their biomass. Our results strengthen earlier studies that also found IC as dominant carbon source. However, a mismatch between SRB and ANME associated lipids hints at another yet undiscovered carbon source for ANME, that is likely supplied by their SRB partners, hence ultimately is also IC derived. Chapter 3 explored the production of disaccharides in methane seep sediments of the Astoria Canyon and their leakage into the porewater. In the sulfate methane transition zone (SMTZ) substantial concentrations and strong 13C depletion of the disaccharides trehalose and sucrose were observed in the porewater and solid phase of the sediment. Further, the 13C-labelling experiment (Chapter 2) demonstrated their synthesis from methane-derived IC by ANME-2/SRB consortia. Their spatial distribution in the sediments aligned with ANME-2 and SRB abundances, providing further evidence for their production during AOM. Given their strong 13C-depletion, elevated concentration and co-occurring 13C depletion of the DOC pool in association with high AOM activity, I propose disaccharides as promising indicators of active AOM. In Chapter 4, these findings were extended to a global survey across six methane-rich environments, including Cascadia Margin seeps, a Hikurangi Margin seep, one Eastern Mediterranean mud volcano, a deep sediment core of the Black Sea, and sediment cores from the Chesapeake Bay estuary. Trehalose and sucrose were consistently detected across all methane-rich and reference sites, with 13C-depleted signatures confirming methane-derived origins in seep sediments, while estuarine and reference sites pointed to additional sources. This establishes disaccharides as widespread products of microbial activity in marine environments and highlights their role as overlooked but significant carriers of carbon into sedimentary carbon pools. Finally, Chapter 5 discusses the findings of preceding chapters. Together, this thesis advances the understanding of carbon flow in methane seep ecosystems. It demonstrates that ANME-2/SRB consortia assimilate primarily IC into biomass, identifies disaccharide production as a direct link between AOM and DOC formation, and documents the global distribution of disaccharides in marine sediments.