UPTAKE OF SELENIUM BY FRESH-WATER PHYTOPLANKTON

The uptake of three forms of Se, selenate, selenite and selenomethionine, was examined in three species of freshwater algae, Anabaena flos-aquae (Cyanophyceae), Chlamydomonas reinhardtii (Chlorophyceae), and Cyclotella meneghiania (Bacillariophyceae) in a defined medium using radiotracers at Se concentrations representative of contaminated systems. Based on the relative accumulation by live vs. heat-killed cells, and linear accumulation through time, selenate accumulation by all three species appears to be a physiological process. However, selenite accumulation at these concentrations appears to be due largely to sorption rather than active uptake, as shown by rapid initial accumulation and the fact that accumulation by heat-killed cells was nearly equal to that of dead cells. Both selenate and selenite uptake rates increased linearly with concentration over the range of 1 to 50-mu-g L-1. Selenomethionine uptake is a biological process with saturable uptake kinetics (K(s) ranging from about 2 to 30-mu-g L-1 Se), with much greater uptake rates than the other two forms, and little inactive sorbtion to heat-killed cells.