Evidence for limited microbial transfer of methane in a planktonic food web

Abstract

Methane-derived carbon may be an important, but overlooked source of energy fueling food webs in a variety of aquatic ecosystems. Although it is commonly assumed that the flow of methane-derived carbon is regulated by aquatic invertebrate consumption of methane-oxidizing bacteria (MOB), few studies have characterized this trophic interaction. We used stable isotope analysis, bioassay experiments, and PCR-based molecular techniques to investigate the interactions between Daphnia and MOB in the pelagic region of a humic lake located in southwestern Michigan, USA. We observed moderate depletion of 13C in the plankton, but these data alone could not provide evidence for the consumption of MOB by Daphnia. Using quantitative PCR, we determined that MOB attained relatively high densities in the water column (3% of the bacterial community), but we found no evidence that they were grazed upon by Daphnia. Moreover, our results do not support the hypothesis that Daphnia harbored symbiotic MOB. Therefore, the isotopic composition of Daphnia could not be explained by direct trophic interactions with MOB, suggesting the potential importance of indirect trophic interactions (e.g. consumption of MOB-feeding protists) or other processes that alter the isotopic composition of zooplankton resources (e.g. CO2recycling). © Inter-Research 2009.