Diverse strategies of O(2) usage for preventing photo-oxidative damage under CO(2) limitation during algal photosynthesis

Photosynthesis produces chemical energy from photon energy in the photosynthetic electron transport and assimilates CO(2) using the chemical energy. Thus, CO(2) limitation causes an accumulation of excess energy, resulting in reactive oxygen species (ROS) which can cause oxidative damage to cells. O(2) can be used as an alternative energy sink when oxygenic phototrophs are exposed to high light. Here, we examined the responses to CO(2) limitation and O(2) dependency of two secondary algae, Euglena gracilis and Phaeodactylum tricornutum. In E. gracilis, approximately half of the relative electron transport rate (ETR) of CO(2)-saturated photosynthesis was maintained and was uncoupled from photosynthesis under CO(2) limitation. The ETR showed biphasic dependencies on O(2) at high and low O(2) concentrations. Conversely, in P. tricornutum, most relative ETR decreased in parallel with the photosynthetic O(2) evolution rate in response to CO(2) limitation. Instead, non-photochemical quenching was strongly activated under CO(2) limitation in P. tricornutum. The results indicate that these secondary algae adopt different strategies to acclimatize to CO(2) limitation, and that both strategies differ from those utilized by cyanobacteria and green algae. We summarize the diversity of strategies for prevention of photo-oxidative damage under CO(2) limitation in cyanobacterial and algal photosynthesis.