| Enhancement of ultraviolet radiation (mainly UV-B) resulted from the depletion of stratospheric ozone has caused considerable attention. Cyanobacteria are regarded as good materials for UV-B researches because of their importance in ecosystem and particularity in evolution process. So far, there are few studies about the effects of UV-B on cyanobacteria in China. In this paper, we have discussed the responses of cyanobacteria to enhanced UV-B, mainly including effects of UV-B on the biomass, photosynthetic apparatus, nitrogen fixation and so on. At the same time, the adaptive mechanisms of cyanobaceteria are emphatically discussed.A number of physiological and biochemical processes of cyanobacteria have been reported to be susceptible to UV-B, although they have evolved efficient mechanisms to cope with the stress of UV exposure. Many investigations have focused the rice-field cyanobacterial species such as Anabaena sp., Nostoc sp., and the model cyanobacteria Synechococcus sp. and Synechocystis sp.. So far, however, there are few studies focusing on the effects of blooming-forming cyanobacteria species. The bloom-forming cyanobacterium Microcystis aeruginosa Ktitz 854 was cultured with 1.05 W-m~-2 UV-B for 3 hours every day, and its growth, pigments and photosynthesis had been investigated. The specific growth rates represented by chl a concentration and OD750 were inhibited 8% and 9% by UV-B exposure, respectively. Six days 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 contents. The initial values of optimal photosynthetic efficiency for UV-B treated samples were respectively 52%, 87% and 93% of controls at the 4th, 7th, and 10th day of growth. The light-saturated photosynthetic rates at 6th day 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 PS II was one of main damage sites. The adaptation of M aeruginosa 854 to UV-B radiation could be observed from light-saturated photosynthetic rates at 13th day and diurnal changes of chlorophyll fluorescence at late growth phase. When both exposed to higher UV-B, samples cultured under1.05 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.In order to investigate how UV-B inhibits M. aeruginosa 854 and how M aeruginosa 854 adapts to UV-B, we carried on the short-term experiments. Our results show that the contents of chlorophyll a, CAR and phycobiliproteins (APC and PC) increased at first, and then dramatically declined during 8 hours UV-B exposure. The photosynthetic capability of M. aeruginosa had been significantly inhibited when the cells being exposured to 80 min UV-B, this could be concluded from both its potential PS II quantum yields (Fv/Fm) and oxygen evolution rate. The decrease of Fv/Fm could be recovered after the exposed cells were transferred to low PAR. However, under the same PAR irradiance but with the addition of CMP, the UV-B induced reduction of Fv/Fm couldn't recover at all, as well as in dark condition. It can be concluded that PAR and protein synthesis are necessary, for the recovery of the damaged photosynthetic apparatus. The electron transport activity of PS II was more sensitive to UV-B than PS I, since PS II activity was inhibited by 63% after 20 min UV-B treatment, while PS I activity inhibited by 30%. The whole chain activity (H2O-MV) was more prone to be inhibited by UV-B exposure than the activity (DPC-MV), so it could be concluded that UV-B might affect the oxygen-evolving c...
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