Interactions of marine plankton with transuranic elements

In a series of laboratory experiments, the biokinetics of ²⁴¹Am, an important transuranium element, was studied in Meganyctiphanes norvegica, a euphausiid common in the northwestern Mediterranean. The euphausiids accumulated Am from water by passive adsorption onto exoskeletons, achieving wet weight concentration factors on the order of 10² after 1 wk exposure; concentration factors varied inversely with the size of the euphausiids and linearly with their surface area:wet weight ratios. Essentially all (96±10%) of the Am taken up from water was associated with the exoskeleton, so that negligible Am was retained by the euphausiids after molting. The retention half-time of Am in molts was 2.9 d. Euphausiids could also concentrate Am from feeding suspensions by ingesting Am-labelled diatom cells, although there was negligible Am assimilation (3±2% after 4 d feeding); after passage through the gut, virtually all (99%) of the ingested Am was defecated within 1 wk. The retention half-time of Am in fecal pellets was 41 and 51 d at 13° and 5°C, respectively. In oceanic waters, where the preponderance of ²⁴¹Am is in the dissolved phase, uptake of Am from water by euphausiids would be the dominant route of bioaccumulation. The results underscore the importance of sinking biogenic debris from zooplankters in mediating the vertical transport of Am in the sea. Given their retention half-times for ²⁴¹Am and their rapid sinking rates, fecal pellets and discarded molts have the potential to deliver most of their Am to the sediments.