Growth physiology and PSII photochemistry
To compare the mechanisms by which the psychrophilic UWO 241 and the mesophilic SAG 49.72 acclimate to long-term environmental stress, the two strains were grown under control growth conditions and then shifted to one of three different stress conditions, representing high light (HL), low temperature (LT) and high salt (HS) (Table 1). First, it was confirmed that both strains could fully acclimate to each stress conditions by exhibiting log-phase growth and high photochemical activity in mid-log phase cultures. UWO 241 and SAG 49.72 acclimated to all long-term stress conditions as reflected in their ability to exhibit exponential growth and achieve high PSII photochemical efficiency (FV/FM) under stress conditions (Figure S1; Table 2). Despite the fact that UWO 241 tolerates significantly lower temperature and high salinity levels compared with SAG 49.72, the two strains generally exhibited comparable growth rates under control vs. stress conditions (Table 2). On the other hand, Chl a/b ratios were significantly lower in UWO 241 vs. SAG 49.72 across all growth conditions (Table 2). Moreover, SAG 49.72 cultures grown under all long-term stress conditions exhibited higher Chl a/b ratios compared with control cultures; although, this difference was only significant between the control and low temperature-grown cultures (Table 2).
Both organisms exhibited qL values > 0.70 under control vs. stress conditions, with the exception of HL-cultures which exhibited lower qL in both species relative to controls (Table 2). Both strains exhibited a decrease in ɸPSII in response to HL which was accompanied by an increase in ɸNO and ɸNPQ in HL-SAG 49.72 and HL-UWO 241, respectively (Figure 1a,b). Last, we measured the capacity for NPQ under a range of measuring irradiance levels. The mesophilic strain maintained low NPQ levels over the range of irradiance levels regardless of whether cells were acclimated to control or long-term stress (Figure 2). In contrast, control UWO 241 cells exhibited significantly higher NPQ compared with that of SAG 49.72. Furthermore, acclimation to all stress conditions further increased UWO 241 NPQ capacity, with HL- and HS-UWO 241 exhibiting the highest NPQ values (Figure 2).