To assess the short- and long-term impacts of Ultraviolet radiation (UVR, 280–400 nm) on the red tide alga Chaetoceros curvisetus, we exposed cells to three different solar radiation treatments–PAB:280–700 nm, PA:320–700 nm, and P:400–700 nm under 20°C incubated temperature. Short-term exposures were investigated: the photochemical efficiency (ΦPSII) versus irradiance curves under six levels of solar radiation by covering the incubators with a variable number of neutral density screens (the irradiance thus varied from 100 to 3%) lasting 1 h, and longterm exposures were designed to assess how the cells acclimate to solar radiation (the growth, UVabc and ratio of repair to damage rates of D1 protein were detected). A significant decrease in the photochemical efficiency (ΦPSII) at high irradiance (100% of incident solar radiation, 261.6 Wm⁻²) was observed in short-term exposure (1 h). UVR-induced photoinhibition was reduced to 7% in 3% solar radiation (4.08 Wm⁻²), compared with 66% in 100% solar radiation (261.6 Wm⁻²). In long-term experiments (11 days) using batch cultures, cell densities during the first 6 days were relatively constant for treatment P, and decreased slightly under PA and PAB treaments, reflecting a change in the irradiance experienced in the laboratory to that of incident solar irradiance. Thereafter, cell density increased and UV-induced photoinhibition decreased with the following days, indicating acclimation to solar UV. At the end of experiment, cells were found to exhibit both higher ratios of repair to UV-related damage and increased concentrations of UV-absorbing compounds, whose maximum absorption was found to be at 329 nm. Our data indicate that C. curvisetus is sensitive to ultraviolet radiation, but was able to acclimate relatively rapidly (ca. 6 days) by synthesizing UV-absorbing compounds and by increasing the rates of repair processes of D1 protein in PSII.
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