Conclusion
The extreme and very stable conditions in the perennially ice-covered Antarctic Lake Bonney might appear to be stressful for organismal growth and cellular homeostasis; however, we demonstrate that a slow growth rate, as observed in 4°C-grown UWO241, is a result of a unique psychrophilic cell physiology that constitutively accumulates stress-related compounds as an adaptation to low temperature. We suggest that carbohydrate metabolism, efficient protein synthesis, polyamine accumulation and increased antioxidant capacity are important processes for cold adaptation since these pathways were constitutively operational in UWO241 at all steady-state temperatures (Figure 10). This is in contrast with C. reinhardtii , where these metabolites accumulated only during low temperature stress. In addition, 4°C-grown UWO241 cultures had the highest capacity to respond to heat stress, compared to cultures grown at higher temperatures, and accumulated stress-related metabolites at high levels (Figure 10). Thus, temperatures resulting in a comparatively faster growth rate in this extremophile do not necessarily define the optimal conditions for responding to environmental stress. We suggest that the ability to regulate the partitioning of energy between fast growth rates versus cell maintenance and homeostasis may be a hallmark of photopsychrophily.
Our work adds to a growing body of research on how heat stress affects psychrophilic eukaryotes (Hwang, Jung & Jin 2008; Chong, Chu, Othman & Phang 2011; Possmayer et al. 2011; Boo et al. 2013; Suet al. 2016; Barati, Lim, Gan, Poong & Phang 2018; Poonget al. 2018), a topic that is particularly relevant given recent trends in climate change. The Antarctic continent is one of the most rapidly warming locations on Earth (Chapman & Walsh 2007). Clearly, the responses to stressors in natural environments are much more complex due to the interplay of several conditions, but our study may serve as a baseline for understanding environmental stress responses in Antarctic algae in general.