Radiotracer techniques for studying pollutant bioaccumulation in marine organisms

Full text: Nuclear techniques can be used to improve our understanding of the processes involved in the transfer of radionuclides and conventional contaminants through coastal marine environments. In particular, the ability to radio analyse live organisms and the increased sensitivity of radiotracer detection allows reducing considerably biological variation, measuring contaminant biokinetics over the long term in a limited number of individuals, and studying marine organisms and contaminant transfer mechanisms that can not be easily investigated using standard analytical techniques. This overview highlights some of the advantages of radiotracer techniques by illustrating three examples of studies carried out at IAEA-MEL. Jellyfish is a primary example of organisms whose study involves several practical problems due to their gelatinous nature and high water content. Hence, data are extremely sparse on the bioaccumulation of metals and other contaminants in these organisms. The use of radiotracer approaches has allowed us to demonstrate that jellyfish readily take up heavy metals and radionuclides (e.g., Co, Zn, Ag, Cd, 137Cs, 241Am) both from sea water and food (CF up to 4x102 ) and efficiently retain them in their tissues (Tb. up to several weeks). Given the fact that they constitute an important biomass in the oceans and are also efficient metal bio accumulators, our results indicate that jellyfish likely plays an important role in the biological transfer and recycling of heavy metal contaminants in the marine environment. Another example of a major constraint of classical analytical techniques is evaluating contaminant biokinetics over the long term. Using γspectrometric techniques (Ge detector), we were able to follow the heavy metal silver in a small number of crabs (n=20) that were allowed to ingest briefly (single feeding) shrimp previously labeled with 110mAg. Loss kinetics of ingested 110mAg followed in each crab for 4 months indicated that the turnover of silver assimilated by the organisms was extremely slow (Tb1/2 = 7.3 years), underscoring the usefulness of these crabs as an excellent long term recorder of silver contamination. Results also revealed that the assimilation efficiency (AE) of 110mAg was related to the sex of the crab; viz. AE was 71% in male crabs vs. 51% in females. All other biokinetic parameters were not significantly different between males and females. High sensitivity of radio detection may also be useful for investigating bioaccumulation of organic contaminants which occur at very low concentration, such as individual PCB congeners. Uptake of 14C-radiolabelled PCB was investigated in two key echinoderm species, the sea star Asterias rubens and the edible sea urchin Paracentrotus lividus which were exposed to realistic PCB concentrations via sea water, sediments, or food. 14C-PCB biokinetics for both species could be measured in individual organs, most of which are too small to be analysed by standard PCB measurement techniques. The radiotracer results showed that uptake was always the most efficient when organisms were exposed to PCB contaminated sea water. These experiments also allowed determining the main organs which bio accumulate PCB and can be considered for monitoring purposes, viz. body wall and podia of sea stars and body wall and gut of sea urchins. The usefulness of radiotracers to obtain rapid information on bioaccumulation and retention potential of contaminants and its application to monitoring and coastal zone management strategies are discussed. (author)