| The impacts of solar UV radiation (UVR) on the cyanobacterium Anabaena sp. PCC7120 were investigated by examining the growth, photosynthetic performance and morphological changes under three different solar radiation treatments: exposures to constant low PAR (photosynthetic active radiation, 400-700 nm), to natural levels of PAR or PAR plus UV radiation (PAB, 280-700 nm). The results show that solar UVR severely inhibited growth and maximum quantum yield of PSII (Fv/Fm), reduced trichome length and depressed the differentiation of heterocysts. Little amount of UV-absorbing compounds was found and not induced by the exposures. Carotenoid concentrations were raised during the initial phase of exposure but no prolonged increase was evident. Photo-oxidation of chlorophyll a occurred and a slower rate of recovery was observed. Following an initial inhibition under high PAR, growth and maximum quantum yield were stimulated, and heterocyst formation was enhanced and high PAR interfered with the normal pattern of heterocyst differentiation with contiguous heterocysts formed in filaments. Higher concentrations of carotenoids and a two-fold increase in the carotenoids to chlorophyll a ratio provided adequate protection from the high levels of solar PAR. Under the low doses of full solar radiation encountered over the final two days of the experiment, growth and photosynthesis recovered fully in the presence of solar UVR, however, the longer term impact of heterocyst disruption to the survival of Anabaena sp. PCC 7120 requires further study.
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