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

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.

Effects of climate warming on Sphagnum photosynthesis in peatlands depend on peat moisture and species‐specific anatomical traits

“…We did not find any difference in light intensity at the Sphagnum capitula level between control plots (1,509 ± 27 μmol of photons m −2 s −1 ) and the OTCs (1,496 ± 21 μmol of photons m −2 s −1 ). More details on the OTC effect on air and soil temperature can be found in Delarue et al () and Jassey et al (). Furthermore, precipitation at the site was continuously monitored using sensors connected to the data‐logger.…”

High‐throughput environmental sequencing reveals high diversity of litter and moss associated protist communities along a gradient of drainage and tree productivity

“…Among protists, free‐living heterotrophic protists play a central role in the decomposition food web by feeding on bacteria, fungi and other microorganisms. Phototrophic and mixotrophic protists contribute to primary production at the soil surface (Gremmen et al ., ; Jassey et al ., ) and the broad diversity of soil protist parasites could play important roles in soil biotic interactions (Ramirez et al ., ; Dupont et al ., ; Geisen, ; Singer et al ., ; Mahé et al ., ). Protists are not only functionally important, but also are diverse, particularly in moss and litter habitats where morphospecies diversity is often higher than in the underlying soil horizons (Coûteaux, ; Bamforth, ).…”

Testate amoeba records indicate regional 20th‐century lowering of water tables in ombrotrophic peatlands in central‐northern Alberta, Canada

“…Many testate amoebae are predators on bacteria and fungi and accumulate silica in test construction, hence they play a major role in nutrient and silica cycling (Aoki, Hoshino, & Matsubara, ; Mitchell, Charman, & Warner, ). Finally, mixotrophic testate amoebae, which partly rely on photosynthesis, contribute to carbon sequestration and a decrease in abundance of such amoebae in Sphagnum peatlands, for instance due to climate warming, could lead to reduced carbon fixation (Jassey et al., ).…”