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.