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.