Photosynthetic adaptation of a bloom-forming cyanobacterium Microcystis aeruginosa (Cyanophyceae) to prolonged UV-B exposure

The bloom-forming cyanobacterium Microcystis aeruginosa Kutz 854 was cultured with 1.05 W.m(-2) UV-B for 3 h every day, and its growth, pigments, and photosynthesis were investigated. The specific growth rates represented by chl a concentration and OD750 were inhibited 8% and 9% by UV-B exposure, respectively. Six days of UV-B treatment significantly reduced cellular contents of phycocyanin and allophycocyanin by 32% and 62%, respectively, and markedly increased the carotenoid content by 27%, but had little effect on the chl a content. The initial values of optimal photosynthetic efficiency for UV-B treated samples were, respectively, 52%, 87%, and 93% of controls on days 4, 7, and 10 of growth. The light-saturated photosynthetic rates at day 6 were significantly lower than controls grown without UV-B. The probability of electron transfer beyond Q(A) decreased during UV-B exposure, and this indicated that the acceptor side of PSII was one of main damage sites. The adaptation of M. aeruginosa 854 to UV-B radiation could be observed from light-saturated photosynthetic rates on day 13 and diurnal changes of chl fluorescence during the late growth phase. When both exposed to higher UV-B, samples cultured under 1.05 W.m(-2) UV-B for 9 days recovered faster than controls. It is suggested that M. aeruginosa 854 had at least three adaptive strategies to cope with the enhanced UV-B: increasing the synthesis of carotenoids to counteract reactive oxidants caused by UV-B exposure, degrading phycocyanin and allophycocyanin to avoid further damage to DNA and reaction centers, and enhancing the repair of UV-B induced damage to the photosynthetic apparatus.