Interspecific differences in the bioconcentration of selenite by phytoplankton and their ecological implications

The concentration of Se in algal cells is a primary determinant of Se tissue contents in higher level consumers. Differences in the ability of algal species to concentrate selenite and variability in ambient selenite concentrations may be important determinants of algal Se contents. We compared uptake of 75Se-labeled selenite by 14 algal species at 2 environmentally relevant selenite concentrations. Se content per unit cell volume (Se/Vc) varied by almost 4 orders of magnitude when algae were exposed to 4.5 nM selenite and almost 5 orders of magnitude when exposed to 0.15 nM selenite. Chlorophytes typically exhibited the lowest Se enrichments, while prymnesiophytes, prasinophytes and dinoflagellates exhibited the highest. The Se/Vc of diatoms and cryptophytes varied by >2 orders of magnitude. Even at the lowest selenite concentration, about half the species concentrated Se to an extent that might cause toxicity at higher trophic levels. Within species, the Se cell concentration typically varied by only 2to 3-fold when exposed to selenite concentrations that differed by 30-fold. The Se cell concentration of only 1 species, the diatom Skeletonema costatum, varied in proportion to ambient selenite concentrations. A more detailed study of the dependence between Se cell concentration on ambient selenite concentrations in the diatom Thalassiosira pseudonana revealed an asymptotic approach to a maximum Se cell concentration at high ambient selenite concentrations (>0.1 nM). Given the results for other species exposed to 4.5 and 0.15 nM selenite, such saturation is likely to be a common feature. Our results indicate that the composition of the phytoplankton community could have a pronounced impact on Se concentrations in marine food webs.