pH Influences the Thermal Performance Curves of Protist Populations

Abstract

Global climate change has led to major direct and indirect impacts on ecosystems worldwide. For example, rising temperatures have been linked to direct changes to the microbial community structure and function of peatlands, which in turn affects C-cycling in these ecosystems. But climate change is also associated with other less studied geochemical shifts, such as the acidification of peatlands and other aquatic/semi-aquatic ecosystems. As temperature and pH are recognized to be significant drivers of microbial community composition and ecosystem function, it is important to understand the joint impacts of these abiotic variables on microbial systems. Here, we provide a window into those possible changes by investigating how pH influences the population growth rates of three different protist species — which serve as significant predators within peatland microbial food webs — at rising temperatures. Specifically, we answer the following questions: 1) how do different pH conditions affect thermal performance? And 2) how do microbes adjust their functional traits to cope with temperature and acidity changes in their environment? To address these questions, we quantified the Thermal Performance Curve of population intrinsic growth rate (r-TPC) of populations acclimated to different pH conditions and incubated at different temperatures. We found that the initial pH conditions significantly impacted the thermal performance of the three species, with the effects of pH varying between the smaller, faster growing species and the larger, slower growing species. Initial pH was also found to significantly affect cell size, shape, and cellular contents — three traits previously shown to likely influence r-TPC shape. Our study highlights the importance of evaluating the joint effects of multiple climate change associated abiotic shifts — not just temperature — on microbial communities and their ecosystems.