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). In other words,
their physiology is very sensitive to increased temperatures, but the
underlying reasons for this sensitivity are unknown. We showed that the
response to heat stress in UWO241 is dependent on the initial culturing
temperature. UWO241 grown at 4°C (with slowest growth rates), shows a
strong response to exposure to a non-permissive growth temperature of
24°C at the level of its primary metabolome (Figure 7, Table 3 – 273
DAMs), and has the slowest cell death kinetics as compared to those
grown at higher temperatures (Figure 4A, 4B; Figure 5). In UWO241
cultures grown at 10°C, a response at the level of the metabolome is
largely absent, except for increased accumulation of carboxylic acids
and sugar phosphates (Figure 7B, Table 3 – 71 DAMs). In accordance,
10°C-grown UWO241 cultures were less resistant to 24°C exposure and
exhibited rapid cell death kinetics. Cultures initially grown at 15°C
with the fastest growth rates, also had an attenuated metabolic response
(Figure 7; Table 3 – 26 DAMs), and rapid cell death kinetics (Figure
4A, 4B; Figure 5). Exposure to higher (but not lethal) temperatures to
generate a timely heat stress signal has been demonstrated to have a
protective effect in mesophiles (Song et al. 2012; Horváthet al. 2012; Yeh et al. 2012; Kishimoto et al.2019), but this is clearly not the case for the psychrophile UWO241.
This study demonstrates that when UWO241 is cultured at a temperature
closest to its natural environment (4°C), it has a higher capacity to
respond to high temperature stress than when cultured at temperatures
that provide faster growth rate.
We suggest that UWO241 exhibits a constitutive routing of steady-state
metabolism towards soluble sugars, antioxidant and cryoprotectant
accumulation. Many of these metabolites are characteristic of
low-temperature adaptation and are maintained or even increased during
short-term heat stress, including high levels of carbohydrates
(particularly sucrose) during heat stress (Figure 7B, Table 4). The
antioxidants ascorbate and α-tocopherol (Table 4, Supplemental Table S2)
were dramatically increased during exposure to 24°C. Tocopherol and
ascorbate have 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 involvement of the
tocopherol-ascorbate antioxidant system in heat stress in green algae.
We also detected evidence for the re-modeling of lipid composition of
cellular membranes due to heat stress and increased amounts of saturated
palmitic (16:0), stearic (18:0) and monounsaturated oleic (18:1) FAs
(Table 4, Supplemental Dataset S2). The most dramatic increase was for
the lipid ergosterol (Table 4). Increased membrane rigidity and high
ergosterol content has been shown to counteract the deleterious effects
of several stressors, including heat, in yeast (Swan & Watson 1998;
Vanegas, Contreras, Faller & Longo 2012; Caspeta et al. 2014;
Godinho et al. 2018). Efficient photosynthesis and maintenance of
energy metabolism could be the driving forces behind soluble sugar,
fatty acid and antioxidant synthesis during short-term heat stress in
UWO241. The constitutively up-regulated carbon metabolism due to cold
adaptation could provide intermediates and energy for mounting a heat
stress response in 4°C-grown UWO241.