Light-dependent kinetic model for microalgae experiencing photoacclimation, photodamage, and photodamage repair

Microalgae naturally are exposed to changing light conditions. While a higher light intensity can promote a faster growth rate, it also can cause photodamage that leads to a temporary or semi-permanent decline in growth rate. We developed a model of photosynthetic growth including photoacclimation, reversible photodamage to photosystem II (PSII), and more severe photodamage to photosystem I (PSI). Phototrophic biomass optimizes its photosynthetic machinery to the light intensity it is experiencing; this is captured in the model by photoacclimation, in which photodamage to PSII caused by absorbed light is balanced by repair. However, repair of PSII photodamage can be overwhelmed by increases of light outside the photoacclimated condition, and this leads to severe PSII photodamage that slows the cells' specific growth rate. Furthermore, very large increases in light intensity can lead to photodamage to PSI, which is semi-permanent in that it can take days to weeks to repair. Our model captures all these phenomena. Example model outputs demonstrate the importance of each phenomenon for increases and decreases in light intensity from the photoacclimated state.