Inhibition of photosynthesis by UV-B exposure and its repair in the bloom-forming cyanobacterium <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis>

The photosynthetic performance of Microcystis aeruginosa FACHB 854 during the process of UV-B exposure and its subsequent recovery under photosynthetic active radiation (PAR) was investigated in the present study. Eight hours UV-B radiation (3.15 W m⁻²) stimulated the increase of photosynthetic pigments content at the early stage of UV-B exposure followed by a significant decline. It suggested that UV-B damage was not an immediate process, and there existed a dynamic balance between damage and adaptation in the exposed cells. Short-term UV-B exposure severely inhibited the photosynthetic capability, but it could restore quickly after being transferred to PAR. Further investigations revealed that the PS II of M. aeruginosa FACHB 854 was more sensitive to UV-B exposure than PS I, and the oxygen-evolving complex of PS II was an important damage target of UV-B. The inhibition of photosynthetic performance caused by UV-B could be recovered to 90.9% of pretreated samples after 20 h exposure at low PAR, but it could not be recovered in the dark as well as under low PAR in the presence of Chloromycetin. It can be concluded that PAR and de novo protein synthesis were essential for the recovery of UV-B-damaged photosynthetic apparatus.