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Abstract

The endpoint of anaerobic degradation of organic compounds in aquatic ecosystems
is methane. This methane-carbon is not necessarily lost for ecosystem processes as it can be utilized
by methane-oxidizing bacteria (MOB), and possibly recycled into benthic and pelagic food
webs. The dominant zooplankton in many lakes are daphnids, which could act as vectors for channeling
methane-carbon from methanotrophic bacteria upwards in the food chain. We demonstrate,
using 13C-enriched diets in laboratory experiments, that methane-carbon can enter the
pelagic food web via filtration of MOB by cladoceran zooplankton. Because carbon use efficiency
in Daphnia appears to be limited by the availability of dietary sterols on prokaryotic diets, we test
the hypothesis that the uptake of MOB, the only prokaryotes possessing sterols and sterol-like
compounds, can lead to a quantitative and qualitative upgrading of phytoplankton diets of Daphnia.
Our results confirm the general superiority of eukaryotic over prokaryotic food sources for
Daphnia growth and reproduction. Although MOB addition compensated for limited food quantity,
we found no evidence for a qualitative upgrading through MOB. Consequently, there was no
direct relationship between the quantity of food available and the fitness (somatic growth) of
Daphnia, but rather a strong food quality effect, independent of MOB addition. Our findings support
the view that methane is an important carbon source to pelagic ecosystems and thus have
strong implications for qualitative and quantitative assessments of carbon recycling pathways in
aquatic ecosystems