Exposure of natural Antarctic marine microbial assemblages to ambient UV radiation: effects on the marine microbial community

The effect of ambient solar UV radiation on natural protist (phytoplankton and protozoan) assemblages from Antarctic coastal waters was determined. Subsamples of the community were exposed to UV radiation attenuated to equivalent water column depths (ED) of 1.0, 10, 3.0, 3,6 and greater than or equal to 20 m for periods of between 8 h and 1 wk. Total concentrations of phytoplankton in treatments exposed at 3.0 and 3,6 m ED were similar to those in control treatments. However, exposure of phytoplankton at less than or equal to 2.0 m ED for greater than or equal to 1 d reduced overall cell size, concentration and biomass, Following UV exposure, some species of phytoplankton died, some flourished and others were unaffected, In contrast, the total concentrations of protozoans at less than or equal to 2.0 m ED for 1 greater than or equal to d were commonly higher than in controls, and 2- to 3-fold higher than at 3.0 and 3.6 m ED. Significant negative correlation was observed between the total concentrations of phytoplankton and protozoa, showing that UV-induced mortality of phytoplankton resulted, directly or indirectly, in an increase in the concentrations of protozoans. Our results showed that UV radiation can change the biomass and species composition of marine microbial communities, altering size and availability of food for higher trophic levels and changing their trophic structure. Thus, increased UVB as a result of ozone depletion is likely to change food web structure and function and may influence biogeochemical cycles.