Anaerobic methane oxidation by archaea/sulfate-reducing bacteria aggregates: 2. Isotopic constraints

Recent studies employing novel analytical tools provide detailed, microscopic portraits of archaea/sulfate-reducing bacteria aggregates in sediments from methane seep and vent sites. One of the most striking features of these aggregates is that lipid and cell carbon are highly depleted in ¹³C (δ¹³C < −60‰). Biogenic methane, with δ¹³C values of −50 to −110 permil, is a logical candidate for carbon source of these aggregates. Accordingly, it is widely assumed that the archaea oxidize and assimilate methane, and that methane-derived carbon is transferred to the sulfate-reducing bacteria (SRB) symbionts as CO₂ or as a partially oxidized intermediate. However, methane is not the only possible source of ¹³C-depleted carbon in archaea/SRB aggregates. ΣCO₂ in sediments at seep and vent sites tends to be isotopically “light” due to decomposition of organic matter derived from chemoautotrophic organisms. In addition, CO₂ is depleted in ¹³C by ∼10 permil compared to ΣCO₂ owing to the equilibrium isotope effect. Assimilation of this “light” CO₂ by methanogenic archaea and autotrophic SRB, combined with enzymatic isotope effects, could also yield lipid and biomass that are highly depleted in ¹³C. We derive general equations based on isotope mass-balance and calibrated with laboratory and field data to predict the isotopic composition of archaeal cell carbon and lipids derived from autotrophic methanogenesis and anaerobic methane oxidation. The calculations show that observed δ¹³C values for archaeal biomass and lipids at methane seep and vent sites are readily accounted for by isotope fractionation during methane production from CO₂, and that biomass produced during anaerobic methane oxidation is only slightly depleted in ¹³C relative to methane unless the enzymatic isotope effect associated with the anabolic arm of the assimilation-dissimilation branch point is considerably larger than the isotope effect associated with the catabolic arm. We also apply an isotope diffusion-reaction model to demonstrate that micro-gradients in δ¹³C-CO₂ cannot be maintained within archaea/SRB aggregates. However, ¹³C-depleted carbon in SRB members of the aggregate is readily explained by autotrophic sulfate-reduction with bulk porewater CO₂ as carbon source. These results illustrate that ¹³C-depleted biomass and lipids observed in sediments from methane seep and vent sites may be derived from CO₂-reducing archaea and autotrophic sulfate-reducing bacteria. The inference of anaerobic methanotrophy based on ¹³C depletion in archaeal and sulfate-reducing bacterial cell carbon and/or lipids should be considered tentative unless corroborated by independent, concordant evidence of net methane consumption.