A case study of the past CH4 cycle in lakes by the combined use of dual isotopes (carbon and hydrogen) and ancient DNA of methane-oxidizing bacteria: rearing experiment and application to Lake Remoray (eastern France)

This study aims at estimating the potential of the hydrogen stable isotope (delta H-2) analysis of chironomid remains (HC) to reconstruct past changes in the methane (CH4) cycle in lakes. A rearing experiment was first designed to assess the contribution of hydrogen derived from diet to the chironomid biomass and the offset between larvae and their HC. Results from the rearing experiment show that hydrogen stable isotope applied to the chironomid HC seems to be a powerful geochemical tracer because a large proportion (about 85 %) of the total hydrogen in the chironomid biomass is derived from the diet and that the application of a corrected factor to the HC values allows estimating the delta H-2 of the chironomid larvae. We have then tested the interest of the HC delta H-2 analysis in addition to other approaches to reconstruct CH4 production and consumption along a sediment core covering the last 1500 years and collected in the deepest part of Lake Remoray (Jura Mountains, France). delta C-13 and delta H-2 analyses were performed on both HC and sedimentary organic matter and were combined with an analysis of ancient DNA (aDNA) of methane-oxidizing bacteria (MOB). Results show that MOB is an important part of the total bacteria aDNA all along the studied period. Moreover, delta C-13 data confirm that MOB may be an important source of carbon for the chironomid biomass (between 20 and 54 %). Finally, delta H-2 values appear fastidious to obtain in the paleolimnological context; however, this tracer may provide evidence that CO2 reduction is the main dominant process of methanogenesis in Lake Remoray. delta H-2 analysis could be useful to distinguish the long-term evolution and the relative contribution of the two pathways of methanogenesis. However, delta H-2 rearing experiments are needed to better estimate the offset and the contribution values.