Effects of UV radiation on grazing by two marine heterotrophic nanoflagellates on autotrophic picoplankton

Phytoplankton <2-3 mu m in diameter, or autotrophic picoplankton, can constitute the majority of the biomass and productivity of photosynthetic organisms in marine and freshwater systems. Indirect evidence has indicated that mortality of autotrophic picoplankton occurs principally at night in the open ocean, but continuously in coastal water. The predominant view of the fate of autotrophic picoplankton production in the ocean is that they are consumed by heterotrophic nanoflagellates. A possible mechanism to explain these observations is that grazing of heterotrophic nanoflagellates on autotrophic picoplankton is inhibited by ultraviolet radiation (UV), at least in clear open-ocean environments. A series of laboratory experiments was conducted to examine the effects of UV radiation on the grazing impact of two heterotrophic nanoflagellates on Synechococcus spp., a commonly occurring genus of autotrophic picoplankton. The two nanoflagellates used were Paraphysomonas bandaiensis and Paraphysomonas imperforata. For both nanoflagellates, there was an inverse relationship between the grazing mortality of Synechococcus and UV irradiance. The grazing mortality of Synechococcus was reduced less with P.imperforata than with P.bandaiensis. In some experiments, the effect of UV on the grazing impact of the nanoflagellate populations was caused in part by UV-related reductions in nanoflagellate survival. However, UV reduced the grazing impact of nanoflagellates primarily by reducing the rates of consumption of Synechococcus by individual nanoflagellates, to a degree directly related to UV irradiance. The results suggest that UV radiation may be an important selection factor in clear open-ocean water, and that in order to predict the effect of increasing UV radiation on marine microbial plankton communities, we must consider interactions between trophic levels as well as effects on single trophic levels.