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 13 C 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. CO 2 recycling).KEY WORDS: Ecology · Symbiosis · Grazing · Methanotrophs · Methanogens · Dissolved organic carbon · DOC · Subsidy
Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 58: [45][46][47][48][49][50][51][52][53] 2009 derived carbon is incorporated into aquatic food webs (but see Deines et al. 2007).One way that CH 4 -derived carbon may enter aquatic food webs is through the consumption of CH 4 -oxidizing bacteria (methane-oxidizing bacteria [MOB] or methanotrophs). MOB, most of which belong to the alpha-and gamma-Proteobacteria, use CH 4 as a carbon/energy source and oxygen (O 2 ) as a terminal electron acceptor. MOB are commonly found in aquatic sediments, where they co-occur with 13 C-depleted invertebrates (Eller et al. 2005). Isotope labeling experiments have shown that 13 C depletion of chironomids is most likely due to the consumption of MOB . MOB also inhabit the water column of lakes and are abundant at transitions zones where there are high concentrations of O 2 and CH 4 (Carini et al. 2005). Zooplankton that encounter and feed on MOB in the lake water column should have a 13 C-depleted isotopic signature. Support for this hypothesis comes from laboratory experiments that incubated Daphnia under elevated CH 4 concentrations (Kankaala et al. 2006). Moreover, a recent study found that 13 C-depleted Daphnia tend to have a high proportion of MOB phospholipid fatty acids (Taipale et al. 2009b). Together, these results suggest that the consumption of MOB may sometimes be an important trophic pathway for CH 4 -derived carbon in aquatic food webs.A symbiotic association between aquatic invertebrate consumers and MOB is an alternative explanation for patterns of...