Photosystem-II repair and chloroplast recovery from irradiance stress: relationship between chronic photoinhibition, light-harvesting chlorophyll antenna size and photosynthetic productivity in Dunaliella salina (green algae)

Abstract

High-light (HL) grown Dunaliella salina cells exhibit lower pigment content, a highly truncated chlorophyll (Chl) antenna size, and accumulation of photodamaged PS II centers in the chloroplast thylakoids (chronic photoinhibition). In HL-grown cells, the rate of photosynthesis saturated at higher irradiances and the quantum yield was lower compared to that of normally-pigmented low-light (LL) grown cells. In spite of these deficiencies, the light-saturated rate of photosynthesis for the HL-cells, when measured on a per chlorophyll basis, was ∼3 times greater than that of the LL-grown cells. To delineate the effect of photoinhibition from the Chl antenna size on quantum yield and rate of photosynthesis, HL-acclimated cells were switched to LL-conditions. Repair of photodamaged PS II, estimated from the recovery of functional PS II centers and from the increase in the quantum yield of photosynthesis, occurred with a half-time of ∼1 h. Chlorophyll accumulation in the cells occurred with a half-time of ∼4 h. The differential kinetics in repair versus Chl accumulation provided a ‘window of opportunity’, within about 2–3 h after the HL→LL shift, when cells exhibited a high quantum yield of photosynthesis, a small Chl antenna size and a light-saturated rate that was ∼6–9 times greater than that of the normally pigmented LL-grown cells. The work provides insight on the temporal sequence of events at the chloroplast and thylakoid membrane levels, leading from a chronic photoinhibition of PS II to repair and recovery. It is suggested that it is possible to maximize photosynthetic productivity and light utilization in mass microalgal cultures by minimizing the light-harvesting Chl antenna size of the photosystems.