Biokinetics of americium and plutonium in the mussel Mytilus edulis

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Abstract
Biokinetics in the mussel M. edulis. L. for Am and Pu taken up from water and labeled diatoms Thalassiosira pseudonana were studied in a series of laboratory experiments. Mussels concentrated Am and Pu 10 to 50 times from both water and labeled diatoms in 5 d [days]. Mussels retained between 1.5 and 16% of the Am injested with labeled diatoms, the highest retention efficiency being reached for the lowest algal cell density tested. Retention efficiency for Am was about twice that for Pu. Pu kinetics in mussels fed Pu(III-IV) and Pu(V-VI) labeled diatoms were identical. Pu taken up from labeled diatoms was lost from soft parts at a slower rate than Am, resulting in decreasing Am/Pu ratios in the soft parts during the depuration phase. Am taken up from labeled diatoms was principally contained in the visceral mass and excreted in feces. Uptake of Pu and Am over 24 h from water was proportional to the ambient concentration for shell, soft parts and whole animal. After 5 d uptake from water the Am/Pu ratio in soft parts was 1.5 times higher than in the ambient seawater; 20 to 30% of the Pu and 30 to 40% of the Am body burden was associated with the soft parts. Following uptake from water, loss from soft parts and shells proceeded at similar rates. Whole body elimination was faster for both elements after uptake from labeled diatoms than from water, and after simultaneous uptake by both routes, Pu was lost slightly faster than Am. After 75 d of loss the biological half-life in the most slowly exchanging compartment ranged from 73-108 d for both elements. Pu was lost from dead shells faster than Am, resulting in increasing Am/Pu ratios during the loss phase. Dissected shells from live mussels showed no increased Am/Pu ratio during the loss phase. Am and Pu leached from fecal pellets at rates (Tb1/2: 2 to 3 w) which increased with temperature and salinity. The results show that uptake from water as well as from food will contribute to the mussels'' Am and Pu body burden and that both routes of uptake could lead to the elevated Am/Pu ratios observed in soft parts of mussels contaminated in the natural environment.

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