Temporal changes in the contribution of methane-oxidizing bacteria to the biomass of chironomid larvae determined using stable carbon isotopes and ancient DNA

Freshwater lakes are important sources of methane (CH4) emissions, by organic matter degrada- tion under anaerobic conditions (methanogenesis). Previous studies suggest that lakes contribute up to 16 % of natural emissions. About 60 % of the CH4 produced is used as an energy source by methane- oxidizing bacteria (MOB—methanotrophs), which could support higher trophic levels, especially Chiro- nomidae (Diptera). Because biogenic methane has a very low stable carbon isotope value, evidence of methane-derived organic-matter assimilation can be tracked by stable carbon isotope analysis in consumers such as chironomids. In some cases, however, chiron- omid d 13 C values are not low enough to unambigu- ously demonstrate methanotroph assimilation and an alternative line of evidence is required. Analysis of ancient DNA (aDNA) from the methanotroph community preserved in lake sediment provides reli- able information about past methane oxidation in freshwater lakes. A combination of these two approaches was used to study a sediment core from the deepest zone of Lake Narlay (Jura, France), which covers the last 1,500 years of sediment accumulation. Results show a significant change ca. AD 1600, with an increase in the proportion of MOB in the total bacteria community, and a decrease in chironomid head- capsule d 13 C. These trends suggest assimilation of MOB by chironomid larvae, and account for up to 36 % of the chironomid biomass. The data also provide information about the feeding behavior of chirono- mids, with evidence for preferential assimilation of methanotroph type I and the NC10 phylum. The combination of aDNA analysis and carbon stable isotopes strengthens the reliability of inferences about carbon sources incorporated into chironomid biomass.