ACCUMULATION OF SELENIUM IN A MODEL FRESH-WATER MICROBIAL FOOD-WEB

The transfer of selenium between bacteria and the ciliated protozoan, Paramecium putrinum, was examined in laboratory cultures. The populati on growth of the ciliate was not inhibited in the presence of the high est concentrations of dissolved selenite or selenate tested (10(3) mu g liter-l). Experiments with radioactive (75)selenite or (75)selenate indicated that accumulation of selenium by ciliates through time was l ow when feeding and metabolism were reduced by incubating at 0 degrees C. However, selenium accumulated in ciliate biomass during incubation with dissolved Se-75 and bacteria at 24 degrees C and also when bacte ria prelabeled with Se-75 were offered as food in the absence of disso lved selenium. When Se-75-labeled bacterial food was diluted by the ad dition of nonradioactive bacteria, the amount of selenite and selenate in ciliates decreased over time, indicating depuration by the ciliate s. In longer-term (>5-day) fed-batch incubations with (75)selenite-lab eled bacteria, the selenium concentration in ciliates equilibrated at approximately 1.4 mu g of Se g (dry weight)(-1). The selenium content of ciliates was similar to that of their bacterial food on a dry-weigh t basis. These data indicate that selenium uptake by this ciliate occu rred primarily during feeding and that biomagnification of selenium di d not occur in this simple food chain.