The Antarctic green alga Chlamydomonas sp. UWO241 is an obligate psychrophile that thrives in the cold (4-6°C) but is unable to survive at temperatures ≥18°C. Little is known how exposure to heat affects its physiology or whether it mounts a heat stress response in a manner comparable to mesophiles. Here, we dissect the responses of UWO241 to temperature stress by examining its growth, primary metabolome and transcriptome under steady-state low temperature and heat stress conditions. In comparison with Chlamydomonas reinhardtii, UWO241 constitutively accumulates metabolites and proteins commonly considered as stress markers, including soluble sugars, antioxidants, polyamines, and heat shock proteins to ensure efficient protein folding at low temperatures. We propose that this permanent stress metabolism is an adaptive advantage to life at extreme conditions. A shift from 4°C to a non-permissive temperature of 24°C alters the UWO241 primary metabolome and transcriptome, but growth of UWO241 at higher permissive temperatures (10°C and 15°C) does not provide enhanced heat protection. UWO241 also fails to induce the accumulation of HSPs when exposed to heat, suggesting that it has lost the ability to fine-tune its heat stress response. Our work adds to the growing body of research on temperature stress in psychrophiles, many of which are threatened by climate change.

The Antarctic alga Chlamydomonas sp. UWO241 is an obligate psychrophile that thrives in the cold but is unable to survive at moderate, seemingly innocuous temperatures. We dissect the responses of UWO241 to temperature stress using global metabolomic approaches. UWO241 exhibits slow growth at 4°C, a temperature closest to its natural habitat, and faster growth at higher temperatures of 10-15°C. We demonstrate that the slower growth-rate characteristic of UWO241 at 4⁰C is not necessarily a hallmark of stress. UWO241 constitutively accumulates high levels of protective metabolites including soluble sugars, polyamines and antioxidants at a range of steady-state temperatures. In contrast, the mesophile Chlamydomonas reinhardtii accumulates these metabolites only during cold stress. Despite low growth rates, 4°C-grown UWO241 cultures had a higher capacity to respond to heat stress (24°C) and accumulated increased amounts of antioxidants, lipids and soluble sugars, when compared to cultures grown at 10-15°C. We conclude that the slower growth rate and the unique psychrophilic physiological characteristic of UWO241 grown at 4⁰C result in a permanently re-routed steady-state metabolism, which contributes to its increased resistance to heat stress. Our work adds to the growing body of research on temperature stress in psychrophiles, many of which are threatened by climate change.