Interactions of marine plankton with transuranic elements. 1. Biokinetics of neptunium, plutonium, americium, and californium in phytoplankton

Abstract

The abilities of seven clones of marine phytoplankton, belonging to six different algal classes, to accumulate transuranic elements were evaluated in laboratory culture experiments. Plutonium, americium, and californium were rapidly concentrated by all species, resulting in volume/volume concentration factors generally >10 5 for all species and all isotopes. Two natural assemblages from the coastal Mediterranean behaved like the algal cultures. Isotopes associated with cells by a passive adsorption to cell surfaces, with equilibrium between cells and water reached in 3–4 days. Uptake of isotope was directly proportional to the number of suspended particles and the isotope concentration in the culture. Equilibrium isotope concentrations differed between species, reflecting different numbers of transuranic binding sites on the cell surfaces. Generally, the green and blue‐green cells had less reactive surfaces than the diatoms. Once accumulated, Am was lost more rapidly by green algae than by diatoms. Elimination proceeded in two phases, a rapid initial loss and then a slower release. The biological half‐life for Am turnover in the slowly exchanging compartment in the diatom was 10–12 days. No substantial differences in uptake were noted between cultures receiving Pu in the III–IV and V–VI oxidation states. In contrast to the other elements, neptunium showed no detectable accumulation by any of the cells. The results suggest that Pu, Cf, and Am would associate with marine particles which could transport them vertically, transfer them into the marine food web, or both, while Np would behave essentially conservatively in seawater.