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).