Cyclic electron flow and ascorbate pathway play a role in survival of
Chlamydomonas sp. UWO241 to long-term photooxidative stress
Abstract
Under environmental stress plants and algae employ a variety of
strategies to avoid damage to the photosynthetic apparatus and maintain
photosynthetic capacity. To date, most studies on stress acclimation
have focused on model organisms possessing limited tolerance to elevated
stress levels. We compared the long-term acclimatory capacities of a
mesophilic alga (Chlamydomonas raudensis SAG 49.72; SAG 49.72) and an
Antarctic halotolerant psychrophile (Chlamydomonas sp. UWO 241; UWO 241)
by monitoring photobiology, cyclic electron flow (CEF) and ROS defense
in cultures acclimated to long-term low temperature, high salinity or
high light stress. SAG 49.72 responded to long-term stress by increasing
chlorophyll a/b ratio and redistributing absorbed light energy from
photosystem II (PSII) to photosystem I (PSI). In contrast, the
psychrophile exhibited faster half times for P700+ re-reduction under
all treatments, suggesting high CEF rates. High CEF was accompanied by
increased capacity for nonphotochemical quenching. Last, UWO 241
exhibited constitutively high activity of two key ascorbate cycle
enzymes, ascorbate peroxidase and glutathione reductase, as well as a
large ascorbate pool. Our results suggest that UWO 241 relies on high
PSI-mediated CEF and ROS detoxification to protect the photosynthetic
apparatus while minimizing energy expenditure on repairs.