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).