Evidence that UV-B tolerance of the photosynthetic apparatus in microalgaeis related to the D1-turnover mediated repair cycle in vivo

The present study was conceived to elucidate the potential importance of th e D1 turnover-mediated repair mechanism in UV-B tolerance of the photosynth etic apparatus in microalgae. To this end, the lab-identified UV-B sensitiv e and tolerant species of Chlorophyte and Chromophyte algae was used to exa mine photosynthetic response to UV-B exposure in the presence vs. the absen ce of streptomycin, an inhibitor of chloroplast protein synthesis. Measurem ents of photosynthetic O-2 evolution capacity and chlorophyll fluorescence parameters (F-v/F-m, Phi (PSII)) illustrated species-specific UV-B sensitiv ity of the photosynthetic apparatus. Addition of the inhibitor streptomycin caused significant enhancements of UV-B-caused depression of photosynthesi s in UV-B tolerant species, while little effect was observed in the sensiti ve species. In the tolerant species, recovery from UV-B induced 20% decline in F-v/F-m reached completion within 2 hours, much faster than that in the sensitive species. Immunoblotting revealed that exposure to UV-B radiation caused substantial degradation of the D1 protein in the sensitive Heteroco ccus brevicellularis, which was little enhanced by addition of the inhibito r. The same UV-B exposure lead to less D1 degradation in the tolerant Scene desmus sp.. which was significantly enhanced by addition of the inhibitor. This study shows that UV-B tolerance of the photosynthetic apparatus in mic roalgae was associated with a strong capacity for recovery from the UV-B-in duced damage and this capacity related to the DI turnover-mediated repair c ycle, and largely determined UV-B tolerance of the photosynthetic apparatus in these organisms.