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 r elative importance of water and food as sources of the high Po-210 con centrations usually found in marine organisms. A double-tracer techniq ue was used which enables differentiating the bioaccumulation of Po di ssolved 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 accumu lation in internal tissues through the intake of seawater for osmoregu lation. In contrast, Po-208 in labelled food was readily absorbed and accumulated in the internal tissues of prawns. Similar experiments wit h fish indicated that Po-210 dissolved in seawater did not contribute directly to Po accumulated in internal organs. Instead, nearly all int ernally bound Po resulted from the assimilation of Po-208-labelled foo d. Measured Po absorption efficiencies were approximately 0.35 and 0.0 5 of the ingested dose for prawns and fish, respectively, and roughly corresponded to the assimilation efficiencies of proteins from food. I n both prawns and fish, ingested Po-208 distributed in the tissues in a manner similar to naturally occurring Po-210 in control organisms, w hereas Po-210 dissolved in seawater partitioned differently. Modelling uptake through both pathways demonstrated that Po in the food account ed for >97 % of the Po measured in these 2 organisms. These experiment al results suggested that, in nature, accumulation of Po by marine org anisms in higher trophic levels depends upon the previous Po binding t o organic matter in lower trophic levels; therefore, Po transfer throu gh marine food-chains is primarily a function of the food assimilation efficiency and feeding rate of the organisms. The double-tracer techn ique described here is a useful experimental tool in the study of Po t ransfer pathways in marine organisms, and could also be applied to the study of other aspects of Po biogeochemistry.