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 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...

Biogenic methane in freshwater food webs

“…Kankaala, Eller & Jones (2007a) reported that, in laboratory experiments with water from a small, polyhumic lake, densities of MOB and rates of methane oxidation were reduced in the presence of the large bacterivorous cladoceran, Daphnia longispina, providing indirect evidence that Daphnia do graze on MOB. Jones & Lennon (2009) also showed that rates of methane oxidation in laboratory experiments were 25% lower in the presence of Daphnia than in control treatments from which Daphnia were absent. Kankaala et al (2007a) and Jones & Lennon (2009) speculated that, in some circumstances, grazing on methanotrophic bacteria by bacterivorous zooplankton might significantly suppress methane oxidation in situ, and thus that grazer density could influence CH 4 efflux from lakes to the atmosphere.…”

“…Jones & Lennon (2009) also showed that rates of methane oxidation in laboratory experiments were 25% lower in the presence of Daphnia than in control treatments from which Daphnia were absent. Kankaala et al (2007a) and Jones & Lennon (2009) speculated that, in some circumstances, grazing on methanotrophic bacteria by bacterivorous zooplankton might significantly suppress methane oxidation in situ, and thus that grazer density could influence CH 4 efflux from lakes to the atmosphere. However, this has yet to be demonstrated in practice, and the quantitative importance of zooplankton grazers in modifying microbially-mediated ecosystem functions and biogeochemical cycles, such as methanotrophic activity and CH 4 effluxes, needs further study under field conditions.…”

“…Jones & Lennon (2009) did find MOB (3% of the bacterial community) in the water column of a humic lake in Michigan, but found no evidence that these were grazed by Daphnia. They therefore concluded that the moderate 13 C-depletion of zooplankton in the lake might be explained by CO 2 recycling or by consumption of protists that had fed on MOB.…”

The potential of methanotrophic bacteria to compensate for food quantity or food quality limitations in Daphnia

“…Daphnia) via intermediate consumers (i.e. heterotrophic nanoflagellates and ciliates; Jones & Lennon 2009) which could synthesize sterols and hopanoids that subsequently release zooplankton feeding on these intermediate consumers from sterol limitation (Martin-Creuzburg et al 2005a, 2006.…”

Denitrification by sulfur-oxidizing bacteria in a eutrophic lake

“…The bacterial 16S rRNA qPCR reactions (15 µl) contained master mix and 667 nM each of the 340f/533r primers. The total bacterial assay included an initial denaturing step at 95°C for 15 min, followed by 40 cycles of 94°C for 30 s, 68°C for 40 s, 72°C for 50 s, and data collection at 83.5°C (Jones & Lennon 2009). The qPCR amplification efficiencies were always between 0.9 and 1.1, and there was no evidence for primer dimers based on the melting curves.…”