LB 10-137 - Temperature and carbon dioxide interact to drive divergent shifts in protist community functional composition in SPRUCE peatlands

Background/Question/Methods:

By employing a variety of energy acquisition strategies (e.g. autotrophy, heterotrophy, or mixotrophy), protists play a central role in carbon cycling, especially in peatlands. Understanding how protist communities change with temperature and CO2 concentrations is vital for predicting how ecosystems will respond to climate change. Previous studies have linked protist functional traits to resource acquisition strategies and metabolism. Building upon this work, we examine how protists’ functional traits (e.g. size, shape, cellular contents, resource acquisition) change at the community level with increasing temperatures (+0℃, 2.25℃, +4.5℃, +6.75℃, and +9℃) and atmospheric CO₂ concentrations (+0 PPM, +450 PPM) at the SPRUCE (Spruce and Peatland Responses Under Changing Environments) experiment operated by Oak Ridge National Laboratory. Protist samples were collected from open-chamber mesocosms in this Sphagnum moss-dominated peatlands, cultured in the lab, and measured using fluid imaging to ascertain traits. In accordance with metabolic theory, we hypothesized that increasing temperature will: (1) decrease protist size, (2) increase surface-to-volume ratios through shape changes, and (3) increase cellular contents, all due to higher metabolic demands. Further, we expect that increasing CO₂ concentrations will: (4) increase photosynthesis. Finally, we explore (5) the impact of increased temperature and elevated CO₂ concentrations on protist biomass and abundance.

Results/Conclusions:

We find that protists’ size (volume), surface-to-volume ratio (aspect ratio), resource acquisition strategy (autotrophy vs. heterotropy measured via red/green ratio), and cellular contents/metabolism (sigma intensity) respond as expected across a temperature gradient under ambient CO₂ concentrations; increasing temperature leads to a decrease in volume and surface-to-volume ratio as protist become more ellipsoid, and cellular contents/metabolism increases. Unexpectedly, we find a reversal in all these trends across temperatures and size classes under elevated CO₂, with volume increasing, surface-to-volume ratio decreasing, and cellular contents/metabolism decreasing — all at a greater magnitude than the relative change under ambient conditions. We also found, unexpectedly, that elevated atmospheric CO₂ reduces photosynthesis across temperatures, especially relative to ambient CO₂ concentration treatments. Finally, we find an inverse response in the abundance and biomass of the protist communities. Under ambient CO₂ level treatments, biomass and abundance increase as temperature increases, while under elevated CO₂ levels, the abundance and biomass crash. These results demonstrate that we have yet much to understand about the role not only increasing temperature under climate change has on protist communities, but the interactive and potentially reversing effects that coupled increases in CO₂ concentrations will have on the response of protist-dominated microbiomes under climate change.