Anaerobic oxidation of methane vs. organic matter facilitated by sulfate reduction: Insights from lipid biomarkers

It is recently estimated that 5.3 Tmol of sulfate is annually reduced in the global ocean to oxidize methane and/or organic matter. The anaerobic oxidation of methane (AOM), an important component in methane cycling driven by consortia of methanotrophic archaea (ANME groups) and sulfate-reducing bacteria, has been a research focus. In contrast, organic matter oxidization facilitated by sulfate reduction is much less well understood. Here we test the sensitivity of lipid biomarkers or the "molecular fossil" to AOM and anaerobic oxidation of organic matter, to better distinguish these two processes in the present and in the past. We used marine sediment samples from hydrate-impacted environments such as the Cascadia Hydrate Ridge (Ocean Drilling Program Leg 204) and analyzed the lipid biomarkers. Modern pore water profile and microbiological evidence suggests the presence of sulfate reduction, high methane flux yet the absence of methanotrophic archaea, implying that the sulfate reduction is induced by oxidation of organic matter. Interestingly, Methane Index (MI) based on the tetraether archaea lipids called GDGTs (glycerol dialkyl glycerol tetraethers), show low values throughout the measured samples from ODP Sites 1244, 1245, and 1248). However, high MI values were found in a few sparsely occurring authigenic carbonate nodules. Similarly, diether archaeal lipids such as archaeol (Ar) and hydroxyarchaeol (OH-Ar), were also in low concentration. These results confirm that AOM is low or absent in the present and in the past (except for when the nodules were precipitated). These data highlight the importance of the biogeochemical processes of sulfate reduction - organic matter oxidation, which perhaps plays a key role in the global cycling of carbon and sulfur.