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.