A DOUBLE-TRACER TECHNIQUE TO DETERMINE THE RELATIVE IMPORTANCE OF WATER AND FOOD AS SOURCES OF PO-210 TO MARINE PRAWNS AND FISH

Accumulation of polonium in the common prawn Palaemon serratus and the sea perch Serranus scriba was studied experimentally to clarify the relative importance of water and food as sources of the high Po-210 concentrations usually found in marine organisms. A double-tracer technique was used which enables differentiating the bioaccumulation of Po dissolved in seawater (as Po-210) and the digestive transfer of Po from food (as Po-208). Accumulation of dissolved Po by prawns was found to occur mainly through adsorption on the exoskeleton with a minor accumulation in internal tissues through the intake of seawater for osmoregulation. In contrast, Po-208 in labelled food was readily absorbed and accumulated in the internal tissues of prawns. Similar experiments with fish indicated that Po-210 dissolved in seawater did not contribute directly to Po accumulated in internal organs. Instead, nearly all internally bound Po resulted from the assimilation of Po-208-labelled food. Measured Po absorption efficiencies were approximately 0.35 and 0.05 of the ingested dose for prawns and fish, respectively, and roughly corresponded to the assimilation efficiencies of proteins from food. In both prawns and fish, ingested Po-208 distributed in the tissues in a manner similar to naturally occurring Po-210 in control organisms, whereas Po-210 dissolved in seawater partitioned differently. Modelling uptake through both pathways demonstrated that Po in the food accounted for >97 % of the Po measured in these 2 organisms. These experimental results suggested that, in nature, accumulation of Po by marine organisms in higher trophic levels depends upon the previous Po binding to organic matter in lower trophic levels; therefore, Po transfer through marine food-chains is primarily a function of the food assimilation efficiency and feeding rate of the organisms. The double-tracer technique described here is a useful experimental tool in the study of Po transfer pathways in marine organisms, and could also be applied to the study of other aspects of Po biogeochemistry.