-?h- 
valence, easy oxidation and reduction, and a marked tendency 
to form complexes. Except for the elements carbon, nitrogen, 
and phosphorus, the transition elements are concentrated to 
the greatest degree by marine plants and animals over the 
levels in the water (23826)8 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 ma- 
terial 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. 
In addition to the surface adsorption of non-fission- 
product radioisotopes onto phytoplankton and zooplankton, the 
orocess of direct uptake and assimilation may well be of im- 
portance 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 
imvortance. 
