Direction and Progression of Thermal Acclimation Effects on Ciliated Protist Populations Both Depend on Direction of Thermal Change

ABSTRACT Organisms acclimate to environmental temperatures to maintain physiological homeostasis. Acclimation can alter demographic rates, thereby affecting population dynamics. Previous research has demonstrated that acclimation can have positive or negative effects on population growth rates in variable thermal environments, depending on the amount of time acclimation takes. A clear picture of the timescale of acclimation may help identify the consequences of various frequencies of thermal fluctuations for population dynamics. However, the progression of population‐level effects of acclimation over time has not been explored. We used experimental microcosms to test the effects of acclimation on population dynamics of the ciliated protist Colpidium striatum in various thermal regimes. We also observed the progression of these effects over the course of acclimation. Prior acclimation to cooler conditions increased intrinsic growth rates in warm trial conditions relative to the growth rates of warm‐acclimated populations. In contrast, prior acclimation to warm conditions decreased intrinsic growth rates in cool trial conditions relative to the growth rates of cold‐acclimated populations. These results are consistent with an overcompensatory acclimation response and either the “colder is better” or “optimal acclimation temperature” hypothesis, though we cannot distinguish between these two possibilities without an intermediate acclimation temperature treatment. The observed patterns may be due to resource uptake dynamics and/or increased stress at high temperatures with increased exposure duration. The progression of these effects over the course of acclimation also differed in trajectory, and perhaps duration, for populations acclimating to warmer versus cooler conditions. Differences in the effects and progress of thermal acclimation depending on the direction of thermal change suggest that different physiological mechanisms may be driving acclimation to warmer versus cooler conditions.