-2hvalence, easy oxidation and reduction, and & marked tendency to form complexes. and phosphorus, the greatest Except for the elements carbon, nitrogen, the transition elements are concentrated to degree by marine plants and animals over the levels in the water (23,26). These are the elements that form the most stable complexes with organic chelating materials (17,14,27) and it is most likely by this mechanism that the heavy metals zinc, copper, nickel, cobalt, iron, and manga- nese are concentrated in the marine biosphere. If surface binding of these metals to organisms is a major factor for their concentration then & concurrently important factor is that of the extent of the available organic adsorptive surface. Plankton in the sea, especially phytoplankton, provide the greatest surface area with respect to protoplasmic volume and, in addition, probably provide a greater volume of living material than any other of the marine organisms. Thus, the major initial concentration of radioactive zinc, cobalt, iron, and menganese from the environment probably occurs in the phyto- plankton. DOE ARCHIVES In addition to the surface adsorption of non-fissionproduct radioisotopes onto phytoplankton and zooplankton, the nrocess of direct uptake and assimilation may well be of importance in these organisms. The roles of these non-fission elements in the physiology of planktonic organisms are not known but in higher animal forms, at least, they are of major importance. V1