Photosystem I energy partitioning and activity
At the level of PSI energy partitioning, both organisms exhibited relatively high ΦPSI under all growth conditions, with the exception of HL which resulted a reduction in ΦPSI of 2.2 and 1.2-fold relative to controls in HL-SAG 49.72 and HL-UWO 241, respectively (Figure 1c,d). Both strains exhibited a concomitant increase in donor side limitation (ΦND) in response to HL; however, ΦND was higher in HL-SAG 49.72 relative to HL-UWO 241. Last, acclimation to all stress conditions resulted in lower acceptor side limitation (ΦNA) relative to control conditions in UWO 241 (Figure 1d).
PSI activity was monitored in mid-log cultures of both strains acclimated to control or long-term stress conditions by far red (FR) light inducible P700 photooxidation (Figure 4). The rise in absorbance at 820 nm (ΔA820) is a relative measure of the fraction of photooxidizable P700 reaction centers, while rates of P700 re-reduction in the dark (t½red ) reflect electron donation from alternative donors and mainly CEF (A. G. Ivanov et al., 1998.). UWO 241 exhibited a significantly lower FR-inducible ΔA820 compared to SAG 49.72. Furthermore, ΔA820 values were below detection limits in HL-UWO 241 and HS-UWO 241 (Figure 4a). Last, UWO 241 cells grown in control and LT conditions exhibited significantly fastert½red compared with SAG 49.72 grown under all conditions, with the exception oft½red from HS-SAG 49.72 (Figure 4b).
Since we were unable to detect an FR-induced A820 signal for either HL-UWO 241 or HS-UWO 241, we performed the same measurement in the presence of red actinic light (AL) after treating the cells with DCMU to block electron transfer from PSII to PSI. Under AL, we were able to measure A820 in all UWO 241 cultures (Figure 5). In agreement with the FR measurements of P700photooxidation, acclimation to all three long-term stress conditions caused a significant reduction in ΔA820 relative to the UWO 241 cells grown in control conditions (Figure 5a). Moreover, following AL excitation, t½red was 1.3 to 1.6-fold faster under all stress conditions relative to that of control conditions (Figure 5b).