Testing for thermal acclimation in zoospores of an amphibian pathogen.

Thermal acclimation effects on locomotory performance have been widely documented for macroscopic organisms, but such responses remain largely unexplored in microorganisms. Metabolic theory of ecology (MTE) predicts faster responses in smaller organisms, with potential consequences for host-parasite interactions in variable temperature environments. We investigated thermal acclimation effects on zoospores of the amphibian fungal pathogen Batrachochytrium dendrobatidis (Bd), quantifying (1) thermal performance for maximum zoospore velocity and (2) high temperatures needed to immobilize 50% (CT50max) or 100% (CT100max) of zoospores. We obtained measurements within 18 min following a temperature shift. We found significant curvilinear acclimation effects on maximum zoospore velocity and CT50max, although the latter pattern might have been driven by confoundment with zoospore density. We also observed a significant positive effect of the trial start temperature on CT50max, consistent with a rapid acclimation response to the start temperature on a time scale of ~1-6 min (i.e. too rapid for our experimental acclimation treatments to detect), implying that zoospores either have constitutive heat tolerance (i.e. no acclimation) or fully acclimate CTmax to new temperatures within ~10 min. To explore the plausibility of such a rapid response, we analyzed published CTmax acclimation times for macroscopic eukaryotes, resulting in a predicted interquartile range of 3.11-25.98 min when mass-scaled to the size of a Bd zoospore. Taken together, these results suggest that Bd zoospores do exhibit thermal acclimation response on the rapid time scale predicted by MTE, possibly giving Bd an advantage over slower-acclimating hosts in variable-temperature environments.