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Abstract

In freshwater food webs, a substantial contribution of methane-derived carbon for the nutrition of larvae of the macroinvertebrate species Chironomus plumosus has been proposed from stable carbon isotope analysis. We investigated if the larvae feed on methanogenic Archaea or methane-oxidising bacteria (MOB), and whether the carbon transfer is based on digestion of the microbial biomass or on a symbiotic relationship. Fluorescence in situ hybridisation (FISH) with larval tissue thin sections showed that the larvae are not inhabited by methanogens or MOB as symbionts or part of their tissue and gut microflora. Furthermore, clone libraries for the bacterial 16S rRNA gene from larval tissue and comparative denaturing gradient gel electrophoresis (DGGE) of sediment and larval DNA samples showed that the eubacterial community present in the larval gut is dominated by anaerobic and facultatively anaerobic bacteria, whereas MOB were only rarely found. Additionally, comparative quantitative PCR assays for Eubacteria and Archaea from sediment samples, larval tissue and gut content revealed a strong enrichment of Eubacteria in the larval gut. Consequently, MOB and methanogens were not detected in amounts high enough to explain the larval δ13C signatures by direct feeding on their biomass. Thus, the microorganisms might be degraded too fast in the larval gut to be detected by the methods used, or the larvae feed indirectly on MOB- or methanogen-derived carbon, e.g. by feeding on meiofauna which itself is grazing on these microorganisms, or by feeding on organisms growing on MOB-derived carbon metabolites like methanol or carbon dioxide.