Experimental evolution of Chlamydomonas reinhardtii in fluctuating versus constant temperature stress

Natural environmental variations are normal occurrences, however human-driven climatic changes continue to exacerbate the magnitude of these variations. This pushes organisms to their physiological limits where they must adapt or face extinction. The potential for organisms to adapt is of concern; however, the presence of fluctuating regimes to incorporate natural environmental variation is occasionally considered. Here, we investigate the standing variation in thermal optima of ten Chlamydomonas reinhardtii strains along a latitudinal gradient and determine whether adaptation to its temperature extremes is possible via long-term evolution. Despite their broad geographic origins, C. reinhardtii strains exhibited a similar thermal optimum (31.2°C - 32.4°C). We then evolved one strain for ∼900 generations under cold, hot and fluctuating growth conditions. Those grown in constant environmental conditions became specialized to their environment of 20°C or 37°C, whereas those evolved in a fluctuating regime performed comparably to the specialists, suggesting no long-term cost of adaptation via a fluctuating regime. We also found no differences in the amount of variation between fluctuating and constant conditions, suggesting similar amounts of divergence between conditions. Overall, despite our predictions that a fluctuating regime would hinder adaptation, we found that these lineages were able to adapt to this variable environment.