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, with the exception of high light treatment in SAG 49.72 which exhibited a 3-fold increase in growth rate (Table 2). Chl a/b ratios were consistently lower in UWO 241 vs. SAG 49.72 regardless of 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 and all stress conditions, with the exception of HL-cultures which exhibited a reduction in qL of 1.2- and 4.7-fold relative to controls of UWO 241 and SAG 49.72, respectively. In addition, UWO 241 exhibited a significantly lower qL and ɸPSII relative to SAG 49.72 under control, low temperature and high salt (Table 2; Figure 1a,b). Both strains exhibited a decline 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 in both strains grown under control versus stress conditions by measuring NPQ levels under a range of measuring irradiance levels. The mesophilic strain maintained low NPQ levels over the range of irradiance levels in cells acclimated to either control or stress conditions (Figure 2). In contrast, control UWO 241 cells exhibited a significantly higher NPQ capacity compared with that of SAG 49.72. Acclimation to all stress conditions further enhanced the NPQ capacity of UWO 241 by 1.4 to 2.8-fold, with HL- and HS-UWO 241 exhibiting the highest NPQ values (Figure 2).