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.