EFFECTS OF UV-A (320 TO 399 NANOMETERS) ON GRAZING PRESSURE OF A MARINE HETEROTROPHIC NANOFLAGELLATE ON STRAINS OF THE UNICELLULAR CYANOBACTERIA SYNECHOCOCCUS SPP

In the open ocean, where turbidity is very low, UV radiation may be an important factor regulating interactions among planktonic microorgani sms, The effect of exposure to UV radiation on grazing by a commonly i solated marine heterotrophic nanoflagellate, Paraphysomonas bandaiensi s, on two strains of the cyanobacteria Synechococcus spp, was investig ated, Laboratory cultures were exposed to a range of irradiances of ar tificially produced UV-B (290 to 319 nm) and UV-A (320 to 399 nm) for up to 10 h, At a UV-B irradiance of 0.19 W m(-2), but not 0.12 W m(-2) , grazing mortality of Synechococcus spp. and nanoflagellate-specific grazing rates were reduced compared to mortality and grazing rates wit h UV-A treatment, Within 6 h of exposure, UV-A alone suppressed grazin g mortality at irradiances as low as 3.02 W m(-2), The extent to which grazing mortality and nanoflagellate-specific grazing rates were supp ressed by UV-A increased with both irradiance and duration of exposure . Over a 6-h exposure period, differences in grazing mortality were la rgely attributable to differential survival of nanoflagellates. Over a longer period of exposure, there was impairment by UV-A alone of nano flagellate-specific grazing rates, Rates of primary productivity of Sy nechococcus spp, were also reduced by UV-A, The extent to which Synech ococcus productivity was reduced, compared to the reduction in Synecho coccus grazing mortality, depended on the duration of UV-A exposure, T hese results support the hypothesis that UV-A alone influences the com position and biomass of marine microbial communities by affecting pred ator-prey interactions and primary production.