Acclimatization of Antarctic natural phytoplankton assemblages when exposed to solar ultraviolet radiation

The effects of solar ultraviolet radiation (WR) on photosynthetic rates of natural assemblages of Antarctic phytoplankton were determined in both 1 day and 2 week incubations during austral spring at Palmer Station, Antarctica. During the first day of the long-term incubations, photosynthetic rates were enhanced by similar to 40% when W-B radiation was excluded from the culture and by an additiona 180% when UV-A radiation was also excluded. In spite of this UVR-induced photoinhibition of photosynthetic rates during the first day of each long-term experiment, cultures with and without exposure to UVR both showed exponential growth after the first few days, so that at the end of the 2 week growth period the chlorophyll-a concentrations in the samples grown with exposure to UVR were quite similar to those in the cultures from which UVR had been excluded. Phytoplankton organic carbon concentrations increased at rates comparable to those of chlorophyll concentrations during the long-term experiments. The effects of UVR were evident, however, in changes in the floristic and chemical composition of the phytoplankton during the long-term incubations. The proportion of the phytoplankton biomass accounted for by diatoms in the cultures exposed to UVR increased, while that of flagellates deceased. Cultures exposed to UVR also showed higher concentrations of UV-absorbing compounds as compared to cultures grown without exposure to UVR. This apparent acclimatization of the cultures to UVR was reflected in decreased sensitivity to UVR when the responses of subsamples were determined in 1 day incubation tests.