Contribution of methanotrophs to freshwater macroinvertebrates: evidence from stable isotope ratios

Carbon and nitrogen isotopic composition of infaunal invertebrates from pro fundal and littoral zones of Lake Biwa, central Japan, was studied. Samples of chironomid larvae, Stictochironomus sp., showed large variations in bot h delta C-13 values (-64.0 to -23.0 parts per thousand) and delta N-15 valu es (-1.3 to 8.8 parts per thousand) and were significantly depleted in both C-13 and N-15 in comparison to other animals and primary producers from th is lake. Moreover, delta C-13 values lower than -40 parts per thousand ,are unusual for freshwater food webs based on photoautotrophic sources. There was a strong positive correlation between delta C-13 and delta N-15 values of Stictochironomus so., which clearly indicates a mixing of nutrition from 2 isotopically distinct sources. We presume that, other than photoautotrop hic producers, only methanotrophic bacteria, which utilize isotopically lig ht methane, could provide a source that is so depleted in both C-13 and N-1 5. We suspect that the trophic association of some Chironomidae species wit h methanotrophic bacteria may turn out to be: a widespread phenomenon.