Metabolic signatures of adaptation to permanently low
temperatures
Algae exhibit the fastest growth rates under optimal conditions, whereas
stress can threaten their cellular homeostasis and lead to decreased
growth rates and reduced fitness (Borowitzka 2018). The Antarctic alga,
UWO241, experiences low but very stable temperatures of 4-6°C
year-round, but we found that its growth rate is fastest at 10°C and
15°C (Figure 1). This indicates that the biochemical and metabolic
processes operating in UWO241 are better adjusted to higher temperatures
than what it experiences in nature (4°C). Nevertheless, we showed that
UWO241 is, indeed, adapted for life at 4°C, which is reflected in the
fact that light- and CO2-saturated rate of
O2 evolution for UWO241 at this temperature are
comparable to the photosynthetic rate of C. reinhardtii grown at
28°C (Pocock, Vetterli & Falk 2011). The primary metabolome of
4°C-grown UWO241 did not differ significantly from that of cultures at
their optimal growth temperatures of 10°C to 15°C. In contrast, C.
reinhardtii showed a strong temperature dependent response at the level
of the primary metabolome (Figure 2, Figure 3a). The 10°C-grown C.
reinhardtii cultures accumulated increased levels of cryoprotectants
and membrane stabilizers than cultures grown at their optimal
temperature (28°C), a common response in photosynthetic organisms during
cold stress (Wanner & Junttila 1999; Gray & Heath 2005; Kaplanet al. 2007; Guy, Kaplan, Kopka, Selbig & Hincha 2007; Janská,
Maršík, Zelenková & Ovesná 2010; Fürtauer, Weiszmann, Weckwerth &
Nägele 2019). We interpret this as evidence that the psychrophile UWO241
does not exhibit typical cold stress responses when cultured at 4°C,
despite its slow growth rates.
Our data reveal a steady-state re-routing of primary metabolism in
UWO241 when compared to the mesophilic model C. reinhardtii . We
detected the constitutive accumulation of several metabolites with known
stress functions in UWO241, regardless of the growth temperature, while
the same metabolites only accumulated at high levels in 10°C-grownC. reinhardtii . First, soluble sugars have known cryoprotectant
roles in cold-adapted plants and algae (Tulha et al. 2010; Leya
2013; Su et al. 2016). Most detected soluble sugars (trehalose,
sucrose, ribose, maltose, fructose) accumulated at high levels in all
UWO241 cultures but only in 10°C-grown C. reinhardtii (Table 1;
Supplemental Dataset S1). This suggests that the psychrophile has a
re-wired central carbon metabolism and accumulates high amounts of
carbohydrates at the expense of other photosynthetic intermediates,
consistent with previous studies (Cook et al. 2019; Kalraet al. 2020).
Second, we observed similar patterns with ascorbic acid (AsA) and its
oxidized form dehydroascorbic acid (DHA) (Table 1; Supplemental Dataset
S1). The ascorbate-glutathione (AsA-GSH) cycle is a fundamental
metabolic pathway involved in maintenance of cellular redox homeostasis
during cold stress in many photosynthetic species (Dreyer & Dietz 2018;
Hasanuzzaman et al. 2019). Thus, UWO241 and other organisms that
live in perpetually cold environments may need a robust and
constitutively active antioxidant system to cope with high ROS levels.
Third, the non-proteinogenic amino acid ornithine also accumulated at
high levels in UWO241 at all growth temperatures, but only at 10°C inC. reinhardtii (Table 1). Ornithine plays a pivotal role in
polyamine, arginine and proline biosynthesis, and its accumulation has
been linked to increased stress tolerance in plants (Kalamaki et
al. 2009a; Kalamaki, Merkouropoulos & Kanellis 2009b; Ghahremaniet al. 2014). We also detected the increased amount of the
polyamine putrescine in UWO241 compared to C. reinhardtii (and no
increases in arginine or proline, Supplemental Dataset S1). High
ornithine and polyamine levels may play a role in DNA and RNA protection
and stabilization, protein synthesis, and cell cycle progression at low
temperatures (Gill & Tuteja 2010; Minocha, Majumdar & Minocha 2014;
Chen, Shao, Yin, Younis & Zheng 2019). Taken together, we suggest that
in UWO241 the accumulation of stress-related compounds, such as soluble
sugars, antioxidants and polyamines, represents a mechanism to ensure
efficient metabolism, redox homeostasis and cell division at low
temperatures. Unlike C. reinhardtii , where accumulation of
stress-metabolites is induced by cold, in UWO241 these protective
compounds constitutively accumulate across a range of permissive growth
temperatures.