Can Antarctic algae mount a functional heat shock response?
A defining characteristic of psychrophiles is their inability to grow at
moderate temperatures (Cvetkovska et al. 2017), but the
underlying reasons for this sensitivity are unknown. In this work we
showed that despite living in a permanently cold environment, UWO241 can
mount HSR at the level of the primary metabolome and transcriptome. We
also showed, however, that the response to heat stress in UWO241 is
dependent on the initial culturing temperature. UWO241 grown at 4°C and
then exposed to 24°C resulted in the slowest cell death kinetics (Figure
4a, 4c) and a strong response at the level of its primary metabolome
(Figure 6) and transcriptome
(Figure
7c, 7d). UWO241 cultures grown at 10°C exhibit a strong response when
exposed to heat for 6 hours but only at the level of the transcriptome
(Figure 6, Figure 7). A response at the level of the metabolome is
largely absent, except for increased accumulation of carboxylic acids
and sugar phosphates (Figure 6b). Cultures initially grown at 15°C
exhibited an attenuated response at both the metabolome (Figure 6) and
the transcriptome (Figure 7c, 7d). Both 10°C-grown and 15°C-grown UWO241
cultures were less resistant to 24°C exposure and had faster cell death
kinetics (Figure 4a, 4c). This study demonstrates that UWO241 cultured
at a temperature closest to its natural environment (4°C) has the
highest capacity to respond to heat stress, despite slow growth rates.
The stress metabolism in UWO241 appeared to be routed towards the
accumulation of soluble sugars, antioxidants and cryoprotectants (Figure
6b). Many of these compounds are metabolic markers for cold stress in
mesophiles and are already present in high amounts in non-stressed
UWO241 cells (Figure 3b). The metabolites ergosterol and α-tocopherol
showed the most dramatic increases in accumulation between steady-state
and heat-stressed cultures, with the largest increases seen in the
cultures initially grown at 4°C (Table 2, Supplemental Table S2). The
tocopherol antioxidant system has been studied during high light stress
(Trebst, Depka & Holländer-Czytko 2002; Sirikhachornkit, Shin, Baroli
& Niyogi 2009; Nowicka & Kruk 2012; Szarka, Tomasskovics & Bánhegyi
2012), and to the best of our knowledge this is the first report of its
involvement in heat stress in green algae. In yeast, increased
ergosterol content and re-modelling of the lipid composition of cellular
membranes to has been shown to counteract the deleterious effects of
several stressors, including heat (Swan & Watson 1998; Vanegas,
Contreras, Faller & Longo 2012; Caspeta et al. 2014; Godinhoet al. 2018); however a detailed lipidomic analysis is required
to test whether a similar process is operational in UWO241.
In accordance with the metabolome data, a steady-state culturing
temperature appeared to have only a minor effect on the UWO241
transcriptomic make-up (Figure 7a, 7b), but heat stress induced a
stronger response (Figure 7b, 7c). In UWO241, up to 11.7% of all
nuclear-encoded genes were differentially regulated by heat stress
(10°C-grown cultures, Figure 4a, 4b). Broadly speaking, this response
seems to be of a lesser intensity as compared to that of C.
reinhardtii when exposed to 25 min of heat stress (25°C 42°C), which
results in approximately 19% DEGs (Légeret et al. 2016). When
the green algal halophile Dunaliella bardawil was exposed to heat
stress (25°C 42°C) for 2 hours, it exhibited a similar transcriptomic
response as we observed for UWO241 with 12% of all transcripts
identified as DEGs (Liang, Jiang, Wang & Zhu 2020). Direct comparisons
between studies involving different species, culturing conditions, and
time points are difficult; however, we observed that in UWO241 (Table
3), C. reinhardtii (Légeret et al. 2016) and D.
bardawil (Liang et al. 2020), the few pathways enriched in
up-regulated genes are those involved in protein processing, which
include HSPs. This suggests a conserved role for protein processing
pathways between UWO241 and related mesophilic algae during HSR.