Chlamydomonas raudensis  H. Ettl (UWO 241) is a psychrophilic green alga endemic to Lake Bonney, Antarctica. The objective of this study was to investigate the response of UWO 241 to incubation at 24°C, a temperature close to optimum for related mesophilic species. Using chl a fluorescence analysis, shifting cells from a growth temperature of 10°C-24°C resulted in a decline in PSII photochemical efficiency with light energy being directed away from photochemistry and toward dissipative pathways. Using the SYTOX Green assay, it was determined that UWO 241 cells die when incubated at 24°C under growth irradiance with a half-time of 34.9 h. The role of light in cell death was minor as cell death occurred in darkness at 24°C with a half-time of 43.7 h. To examine the plasticity of UWO 241 to temperature stress, 10°C-grown cells were shifted to 24°C for 12 h and then returned to 10°C to recover. The 12 h incubation at 24°C, which resulted in <10% cell death, led to declines in both light-saturated rates of photosynthesis and respiration, PSII photochemistry and energy partitioning, and changes to transcript abundances-those associated with the light-harvesting protein of PSII and ferredoxin declining rapidly, whereas transcripts of specific heat-shock proteins (HSPs) increased. Within 24-48 h of being transferred back to 10°C, all parameters returned to levels occurring in 10°C-grown cells. This research shows, for the first time, that 24°C is a temperature that is lethal to UWO 241, and yet this organism displays considerable physiological and molecular plasticity.