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°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.
Highlights
Mixotrophic protists are increasingly recognized for their significant contribution to carbon (C) cycling
Mixotrophy can be found in vastly different taxa using a wide range of mechanisms[2]
We combine a long-term field warming experiment and a laboratory microcosm experiment to determine the effects of warming on the composition of the microbial food web, in particular, mixotrophic testate amoebae (MTA), and the potential consequences of such changes for CO2 uptake of European peatlands
Summary
Total biomass Microalgae Cyanobacteria Ciliates Heterotrophic testate amoebae Mixotrophic testate amoebae Small-sized metazoans (rotifers and nematodes) Bacteria Fungi. Photosynthetic capacity of the bryosphere (Amax, bryo) was significantly lower by 13% after the dark treatment (mean 2.2 mg C g sph−1 h−1) compared to the light treatment (mean 2.5 mg C g sph−1 h−1, F = 5.52, P = 0 .03; Fig. 4c) These data indicate that the decline in photosynthetic CO2 uptake in response to the dark treatment was driven by the decrease in MTA abundance. There was no significant relation between microcosm specific photosynthetic capacity and the abundance of microalgae (Fig. 5b) From these results we conclude that MTA are likely to make a significant contribution to overall bryosphere C fixation and that a reduction in their biomass may reduce Sphagnum photosynthetic capacity.
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