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

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.