Ecological implications of changes in cell size and photosynthetic capacity of marine Prymnesiophyceae induced by ultraviolet-B radiation

A natural planktonic assemblage of organisms (<240 mu m) was studied in mes ocosm experiments for 7 d under varying conditions of ultraviolet-B radiati on (WB: 280 to 320 nm): UVB excluded, natural radiation and WE enhanced at 2 different levels. Specifically. the changes in Prymnesiophyceae abundance and Light scatter properties attributed to cell size (CS) were investigate d by flow cytometry during the first 72 h with 4 h temporal resolution and were thereafter examined during the 4 following days with twice daily sampl ing. In addition, the specific rate of photosynthesis (p(cell)) of phytopla nkton <5 mu m, ciliate abundance (predator 15 to 35 mu m) and dynamics of n utrients were monitored. Prymnesiophyceae ranged in size between 2.7 and 4 mu m and dominated the phytoplanktonic community <5 mu m (more than 94%). P rymnesiophyceae exhibited marked diel variability with synchronized cell di vision: CS increased during the day and diminished at nighttime, indicating cell division. Shortterm UVB exposures during the first 3 d of the experim ent did not affect CS, probably due to vertical water mixing in the mesocos ms moderating harmful UVB exposure. In contrast, long-term UVB treatments ( 3 to 7 d) induced progressive CS increases in Prymnesiophyceae as a functio n of increasing UVB doses. The successive inhibition of p(cell) of phytopla nkton <5 mu m was also observed as a function of increasing WE doses. The r esults suggest that enhanced UVB provokes the retardation of cell division and inhibition of p(cell) which causes Prymnesiophyceae CS enlargement. CS enlargement and probably a change in the food quality of Prymnesiophyceae c ould result in food limitation for the ciliate population. although ciliate s seem to be directly affected by WE enhancement. This study highlights the ecological implications of CS changes and photosynthetic capacity of phyto plankton, with respect to predator-prey interactions in response to WE enha ncement.