Impact of UV-B radiation on microalgae and bacteria:: a mesocosm study with computer modulated UV-B radiation addition

Effects of ambient and enhanced UV-B radiation (UVBR) on marine microbial plankton communities were assessed in a model ecosystem at Kristineberg Marine Research Station (KMRS) on the Swedish west coast. The system consisted of 16 aquaria (401) filled with surface seawater and semicontinuously run by replacing 101 of their contents with filtered seawater twice a day. The aquaria were placed outdoors and the ambient solar radiation was reduced by 70% using neutral screens. Four different levels of UVBR were applied, each in 4 replicates; nothing, ambient, ambient + 10% and ambient + 20%. The enhanced UVBR was supplied by fluorescent tubes whose intensity was modulated by the ambient radiation to give a constant percentage increase, Variables measured were nutrients (N, P, Si), composition of phytoplankton species and pigments, bacterial and primary productivity, and bacterial cell numbers. Statistically significant UVBR effects were found for carbon allocation, size distribution of primary productivity and phytoplankton species composition. It was also found that UVBR exposure during the development of the phytoplankton communities increased their sensitivity to UVBR in short-term carbon dioxide fixation measurements. We propose that this was due to an adaptation of the community to UVBR, including an increased production of components within the photosynthetic apparatus damaged by UVBR. The UVBR had no significant effect on the total biomass of phytoplankton and bacteria.