A constitutive stress response is a key psychrophilic adaptation.
This study reveals novel insights into psychrophilic adaptation in the green alga UWO241 and potential roles of molecular chaperones and stress metabolism during cold adaptation and heat stress. In this work we showed that many molecular and metabolic signatures commonly used as stress markers in mesophiles (HSPs, carbohydrates, antioxidants) are constitutively accumulated at high levels in UWO241 grown at 4°C. We propose that this is an adaptive advantage to life at extreme conditions, where a permanent exposure to extreme conditions has geared psychrophilic metabolism towards a permanent “stress state”. What UWO241 appears to be lacking is the ability to fine-tune the regulation of this network, particularly the accumulation of HSPs, upon exposure to subsequent heat stress. For instance, in our work we did not detect up-regulation in HSF1 transcript, possibly due to the transient nature of its accumulation (Schmollinger et al. 2013), low protein levels, or low antibody specificity (data not shown). HSF1 has been shown to regulate HSP levels both during heat stress (Schmollinger et al. 2013) and cold stress (Maikova et al. 2016) in C. reinhardtii , but it is not clear whether a similar mechanism operates in UWO241. A more detailed analysis, including expression and protein accumulation of HSF1 using UWO241-specific antibodies is needed.
Our work adds to a growing body of research on how heat stress affects psychrophilic algae (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). Studying the stress networks in psychrophilic algae will provide much needed insight into how these organisms adapt to their extreme environment and whether they are able to respond to temperature stress similarly to their mesophilic relatives.
Acknowledgments: The authors thank Dr. M. Schroda for the kind gift of HSP60A and HSP90C antibodies. We are grateful to Dr. Marc Possmayer for his assistance in algal culturing and RNA extraction.
Author contributions: MC, DRS and NPAH conceptualized the work and designed the experiments. MC wrote the initial draft, performed all cell death assays, microscopy, metabolomics experiments, UWO241 genomic screening and immunoblotting experiments in cooperation with BSM and NM. XZ performed global transcriptomic analysis. GV and SB performed HSP expression analysis and genomic screening of selected species. All authors contributed towards manuscript preparation and editing.