• Energy Research
• basic medicine close

Deep subseafloor sediments host a microbial biosphere with unknown impact on global biogeochemical cycles. This study tests previous evidence based on microbial intact polar lipids (IPLs) as proxies of live biomass, suggesting that Archaea dominate the marine sedimentary biosphere. We devised a sensitive radiotracer assay to measure the decay rate of ([ 14 C]glucosyl)-diphytanylglyceroldiether (GlcDGD) as an analog of archaeal IPLs in continental margin sediments. The degradation kinetics were incorporated in model simulations that constrained the fossil fraction of subseafloor IPLs and rates of archaeal turnover. Simulating the top 1 km in a generic continental margin sediment column, we estimated degradation rate constants of GlcDGD being one to two orders of magnitude lower than those of bacterial IPLs, with half-lives of GlcDGD increasing with depth to 310 ky. Given estimated microbial community turnover times of 1.6–73 ky in sediments deeper than 1 m, 50–96% of archaeal IPLs represent fossil signals. Consequently, previous lipid-based estimates of global subseafloor biomass probably are too high, and the widely observed dominance of archaeal IPLs does not rule out a deep biosphere dominated by Bacteria. Reverse modeling of existing concentration profiles suggest that archaeal IPL synthesis rates decline from around 1,000 pg⋅mL −1 sediment⋅y −1 at the surface to 0.2 pg⋅mL −1 ⋅y −1 at 1 km depth, equivalent to production of 7 × 10 5 to 140 archaeal cells⋅mL −1 sediment⋅y −1 , respectively. These constraints on microbial growth are an important step toward understanding the relationship between the deep biosphere and the carbon cycle.

SummaryThe anaerobic oxidation of methane (AOM) is a major sink for methane on Earth and is performed by consortia of methanotrophic archaea (ANME) and sulfate‐reducing bacteria (SRB). Here we present a comparative study using in vitro stable isotope probing to examine methane and carbon dioxide assimilation into microbial biomass. Three sediment types comprising different methane‐oxidizing communities (ANME‐1 and ‐2 mixture from the Black Sea, ANME‐2a from Hydrate Ridge and ANME‐2c from the Gullfaks oil field) were incubated in replicate flow‐through systems with methane‐enriched anaerobic seawater medium for 5–6 months amended with either 13CH4 or H13CO3‐. In all three sediment types methane was anaerobically oxidized in a 1:1 stoichiometric ratio compared with sulfate reduction. Similar amounts of 13CH4 or 13CO2 were assimilated into characteristic archaeal lipids, indicating a direct assimilation of both carbon sources into ANME biomass. Specific bacterial fatty acids assigned to the partner SRB were almost exclusively labelled by 13CO2, but only in the presence of methane as energy source and not during control incubations without methane. This indicates an autotrophic growth of the ANME‐associated SRB and supports previous hypotheses of an electron shuttle between the consortium partners. Carbon assimilation efficiencies of the methanotrophic consortia were low, with only 0.25–1.3 mol% of the methane oxidized.