An unexpected role for mixotrophs in the response of peatland carbon cycling to climate warming

Jassey, Vincent E. J. and Signarbieux, Constant and Haettenschwiler, Stephan and Bragazza, Luca and Buttler, Alexandre and Delarue, Frederic and Fournier, Bertrand and Gilbert, Daniel and Laggoun-Defarge, Fatima and Lara, Enrique and Mills, Robert T. E. and Mitchell, Edward A. D. and Payne, Richard J. and Robroek, Bjorn J. M. (2015) An unexpected role for mixotrophs in the response of peatland carbon cycling to climate warming. Scientific Reports, 5: 16931. ISSN 2045-2322

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Abstract

Mixotrophic protists are increasingly recognized for their significant contribution to carbon (C) cycling. As phototrophs they contribute to photosynthetic C fixation, whilst as predators of decomposers, they indirectly influence organic matter decomposition. Despite these direct and indirect effects on the C cycle, little is known about the responses of peatland mixotrophs to climate change and the potential consequences for the peatland C cycle. With a combination of field and microcosm experiments, we show that mixotrophs in the Sphagnum bryosphere play an important role in modulating peatland C cycle responses to experimental warming. We found that five years of consecutive summer warming with peaks of +2 to +8 degrees C led to a 50% reduction in the biomass of the dominant mixotrophs, the mixotrophic testate amoebae (MTA). The biomass of other microbial groups (including decomposers) did not change, suggesting MTA to be particularly sensitive to temperature. In a microcosm experiment under controlled conditions, we then manipulated the abundance of MTA, and showed that the reported 50% reduction of MTA biomass in the field was linked to a significant reduction of net C uptake (-13%) of the entire Sphagnum bryosphere. Our findings suggest that reduced abundance of MTA with climate warming could lead to reduced peatland C fixation.