Changes in the condition of photosynthetic apparatus of a diatom alga Thallassiosira weisflogii during photoadaptation and photodamage.

Two populations of a diatom alga Thallassiosira weisflogii were grown at photon flux densities (PFD) of 0.8 and 8 µmol/(m2 s). For both diatom populations, the recovery of chlorophyll fluorescence parameters ( F0, Fm, Fv/ Fm, and NPQ) was monitored after nondestructive irradiation by visible light at PFD of 40 µmol/(m2 s) and after high-intensity irradiation by visible light (1000–4000 µmol/(m2 s)). The exposure of diatoms to PFD of 40 µmol/(m2 s)—higher than PFD used for algal growth but still nondamaging to photosynthetic apparatus—induced nonphotochemical quenching (NPQ), which was stronger in algae grown at higher PFD (8 µmol/(m2 s)) than in algae grown at low light. After irradiation with high-intensity light, the recovery of chlorophyll fluorescence parameters was more pronounced in algae grown at elevated PFD level. During short-term irradiation of diatoms with high-intensity visible light (1000 µmol/(m2 s)), a stronger NPQ was observed in the culture adapted to high irradiance. After the treatment of algae with dithiothreitol (an inhibitor of carotenoid deepoxidase in the diadinoxanthin cycle) or NH4Cl (an agent abolishing the proton gradient at thylakoid membranes), a short exposure of algae to PFD of 40 µmol/(m2 s) induced hardly any nonphotochemical quenching. The results indicate the dominant contribution of xanthophyll cycle carotenoids to energy-dependent quenching. [ABSTRACT FROM AUTHOR]