Role of ultraviolet radiation on phytoplankton extracellular release and its subsequent utilization by marine bacterioplankton

The influence of ultraviolet (UV) radiation on the production rate and percentage of photosynthetic extracellular release (PER) to total production (PER + particulate) was investigated in the laboratory using the diatom species Chaetoceros muelleri grown under different nutrient and radiation conditions. Only when growing on higher nutrient concentrations (f/2 and f/4 media), were both specific particulate production and PER rates (production per unit chl a) were significantly lower in the presence of photosynthetic active radiation (PhAR) + UV as compared to the treatments receiving only PhAR. When growing on lower nutrient concentrations (f/16 medium), UV radiation had no significant effect on the production rates. The percentage of PER to the total amount of carbon fixed was similar for the PhAR + UV and the PhAR treatments. However, PER produced by C. muelleri during PhAR + UV exposure was taken up by bacterioplankton in the dark at significantly lower rates than PER produced in the absence of UV. This indicates that PER, considered as an important component of the dissolved organic carbon pool, becomes more resistant to bacterial utilization upon UV exposure. Whether this reduced PER-uptake by bacterioplankton is caused by changes in the macromolecular composition of the PER and/or is due to a subsequent photochemically induced alteration of the molecules during UV-exposure remains to be investigated.