Photoprotection and photoinhibition in the diatom Phaeodactylum tricornutum

Productivity in marine environments is largely based on the photosynthetic activity of diatoms, microalgae that account for ca. 40% of global oceanic carbon fixation. High photosynthetic rates in diatoms are maintained despite the systematic exposure to changing environmental conditions. Of particular importance is the exposure to changing irradiances, including supersaturating light levels. The success of diatoms in coping with high light has been attributed to the efficiency of their photoprotective mechanisms. In this study, we investigated the effect of light stress on the reaction centre protein D1/psbA from photosystem II of Phaeodactylum tricornutum, which has been shown to be the major target of photodamage. Cultures were grown at 40 μmol photons.m-2.s-1 (used as control) and subjected to 1 h high light (HL) stress of 1,250 μmol photons.m-2.s-1. Lincomycin was added to half of the cultures to infer on PSII repair capacities, by determining D1 concentration with immunoblotting. Pulse-amplitude modulated fluorometry was used to measure stress effects on quantum yield and non-photochemical quenching (NPQ). Pigment concentrations, including the xanthophylls diadinoxanthin and diatoxanthin, were quantified by High Performance Liquid Chromatography (HPLC). It was observed a decrease in D1 in both light treatments, but much more pronounced in HL. Lincomycin affected D1 repair, particularly in HL where almost no D1 was detected. Quantum yield of PSII decreases after 1 h of HL, recovering almost 50%, while lincomycin treated cultures only recovered 25%. NPQ was similar in both treatments, reaching a maximum of 5.7, with diatoxanthin increasing under HL. NPQ’s energy-dependent quenching (qE) dissipated after 13-20 min, while photoinhibitory quenching (qI) was still present after 24 h of recovery. Rapid light curves (RLCs) show a decrease in α, a maintained rETRm which decreases only in lincomycin treated cultures and an increased Ek when lincomycin is added, although it is decreased after recovery. D1 degradation has a damaging effect on PSII repair and recovery, supported by the lowered quantum yields and the high NPQ. P. tricornutum therefore seems to have highly efficient photoprotective mechanisms, with photoinhibition occurring only when repair cannot keep up with the damage inflicted, which was only observed in HL.