Origin and fate of methane in the Eastern Tropical North Pacific oxygen minimum zone

This work was funded by the Natural Environment Research Council (grant NE/E01559X/1)

Potential carbon fixation via methane oxidation in well-oxygenated river bed gravels

Due to a combination of local methanogenesis and high background concentrations in the groundwater, water in the River Lambourn is 51 times supersaturated with methane (162 nmol CH4 L−1). Pore-water concentrations of methane in the gravels of the riverbed were much lower throughout the year (71 nmol CH4 L−1), suggesting significant methane oxidation. To investigate the potential for methane oxidation as a novel chemosynthetic source of carbon to the food web, we made simultaneous measurements, in laboratory chambers, of primary production, respiration, and methane oxidation associated with the gravels. Biomass-specific net primary production was up to 2.7 µmol O2 mg−1 chlorophyll (Chl) h−1 and was similarly high for respiration (2.7 µmmol O2 mg−1 Chl h−1). We also found active methane (CH4) oxidation with the rate increasing in proportion to concentration. At the maximum rate of 0.18 µmol CH4 mg−1 Chl h−1 and a growth efficiency of 0.8, net carbon fixation via methane oxidation was equivalent to 6% of the carbon fixed via net photosynthetic primary production. However, production via methane oxidation could be proportionately much greater under the shade of the profuse instream or riparian vegetation, deep in the gravels, and especially during winter, when light is limiting (< 25 µmol quanta m−2 s−1)

Диалог. 2012. № 002

Our understanding of the role of freshwaters in the global carbon cycle is being revised, but there is still a lack of data, especially for the cycling of methane, in rivers and streams. Unravelling the role of methanotrophy is key to determining the fate of methane in rivers. Here we focus on the carbon conversion efficiency (CCE) of methanotrophy, that is, how much organic carbon is produced per mole of CH4 oxidised, and how this is influenced by variation in methanotroph communities. First, we show that the CCE of riverbed methanotrophs is consistently high (~50%) across a wide range of methane concentrations (~10–7000 nM) and despite a 10-fold span in the rate of methane oxidation. Then, we show that this high conversion efficiency is largely conserved (50%± confidence interval 44–56%) across pronounced variation in the key functional gene (70 operational taxonomic units (OTUs)), particulate methane monooxygenase (pmoA), and marked shifts in the abundance of Type I and Type II methanotrophs in eight replicate chalk streams. These data may suggest a degree of functional redundancy within the variable methanotroph community inhabiting these streams and that some of the variation in pmoA may reflect a suite of enzymes of different methane affinities which enables such a large range of methane concentrations to be oxidised. The latter, coupled to their high CCE, enables the methanotrophs to sustain net production throughout the year, regardless of the marked temporal and spatial changes that occur in methane