Photoinhibition of photosystem II in Phytoplankton: Processes and patterns

All the diverse lineages of phytoplankters use conserved Photosystem II complexes to mediate photochemical extraction of electrons from water, with concomitant release of oxygen. Photosystem II suffers light-dependent photoinactivation through multiple mechanisms, which is in turn countered by protein subunit removal and replacement. This cycle imposes both direct costs and also delayed opportunity costs. The effective quantum yield for photoinactivation of Photosystem II depends upon taxa, cell size, prior acclimation state, the instantaneous spectral regime and light level. Low temperature or constraints upon metabolism can drive up excitation pressure and photoinactivation, while slowing counteracting repair. Environmental shifts in nutrients or CO2 alter resource allocations and electron flows, with sometimes counter-intuitive effects upon susceptibility to photoinactivation across taxa. These interacting factors generate a wide range of susceptibilities to photoinactivation across phytoplankton cells. Some phytoplankters engage in luxury or pre-emptive recycling of Photosystem II proteins before photoinactivation occurs, and some accumulate large pools of Photosystem II proteins outside active complexes, which may relate to the diverse thylakoid architectures across taxa. These interacting factors generate complex cost-benefit tradeoffs to maintain Photosystem II, and thus the range of conditions where oxygenic photosynthesis is feasible for a given taxa.