Phytoplankton on the go! understanding the effects of thermal acclimation across scales on the movement behaviour of C. reinhardtii

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Hannah S. Meier and Anna Ritz, Biology, Reed College, Portland, OR, Tamara Layden and Samuel B. Fey, Department of Biology, Reed College, Portland, OR

Background/Question/Methods
Organisms persist in a mosaic of spatial and temporal environmental conditions that must be successfully navigated at multiple timescales to survive and reproduce. Effective behavioural thermoregulation has been highlighted as a key mechanism by which organisms can persist amidst the backdrop of directional environmental warming, as global environmental temperatures increase across ecosystems; however, how organisms can sense and respond to thermal cues on timescales exceeding their generation time remain underexplored. Here, we investigate the capacity for both short-term reversible and transgenerational behavioural plasticity in response to changes in the thermal environment (i.e. thermal acclimation), using the motile freshwater green algae Chlamydomonas reinhardtii as a model organism. We acclimated populations of C. reinhardtii to 12.5℃, 25℃ and 37.5℃ thermal environments for a period of two weeks. We then evaluated a number of specific movement behaviours, including speed of movement, turning angle, and frequency of movement within a given population over a period of two-weeks following one of three temperature perturbations: 12.5℃, 25℃ and 37.5℃. We evaluated movement behaviour by running video clips of each sample through the TrackMate plugin of FIJI, followed by processing through the moveHMM package in R (version 1.3.1093).
Results/Conclusions
Our results show that thermal acclimation influences the movement behaviour of C. reinhardtii populations in novel environments. Relative to populations acclimated in cold environments, hot-acclimated populations had a higher frequency of moving particles while generally being the slowest and significantly larger in size. Additionally, results from a hidden Markov model revealed that hot acclimated populations spend more time engaged in directional movement and less time engaged in small scale non-linear movement. The effects of thermal history dissipated over the two-week interval, yet the decay of this effect varied across movement responses such that different metrics of movement behaviour reached their acclimated values at different time points. Collectively, these results show that thermal history is an important factor in predicting how populations will respond to new thermal environments and that thermal history can have persistent impacts on species across timescales exceeding their generation time. Thus, understanding the effectiveness of behavioural thermoregulation in novel thermal environments may require integrating an understanding of both past and present experienced thermal environments.