RET,A ee period of time. The detection efficiency of cellulose nitrate film is not quite as high as that of nuclear emulsions because the etching process reaches only those particles which penetrate the top surface of the filn (away from the bone). Since the films were 3 p thick and the etching process removed 1-2 p», a few tracks were missed, On the other hand, this penetration effect should increase resolution (particularly with thicker films) by etching only those alpha tracks which are more perpendicular to the plane of the film. In summary, the cellulose nitrate method provides an alternative to autoradiography with nuclear emulsions. It does not require darkroom facilities. The alpha tracks are enlarged and more easily detectable. Background fogging is eliminated. At the expense of some decrease in efficiency one should be able to obtain better resolution than with emulsions. REFERENCES ite onary ert Re 1. Fleischer, R. L., Price, P. B. and Walker, R. M. Ann. Rev. Nucl. Sct. 15, 1 (1965). 2. Benton, E. V. U.S. Naval Radiological Defense Laboratory Report USNRDL-TR-14 (1968). 3. Becker, K. Health Phys. 16, 113 (1969). 4. Rosenthal, M. W., Marshal, J. H. and Lindenbaum, A. Diagnosis and Treatment of Deposited Radionuctides, Symp. Richland, Washington, May 15-17, 1967. Excerpta Medica Foundation, Amsterdam, 1968, pp. 73-80. Fic. 2—A serially etched cellulose nitrate autoradiograph of trabecular bone from mouse, injected with O1yCi ionic ™Pu, which shows surface alpha tracks. 100 X. THE CONCENTRATION OF RADIUM, THORIUM, AND URANIUM BY TROPICAL ALGAE wee oe fae ine D.N. Edgington, S. A. Gordon,* MI. M. Thommes, and L. R. Almedovart Samples of twenty species of marine algae collected between 1961 and 1968 in Puerto Rico have been analyzed for total Organic material, protein nitrogen, calcium, radium, thorium, and uranium. The results suggest that the concentration of radium, thorium, or uranium by these organisms may be controlled by two mechanisms: (1) ion-exchange or coprecipitation of the ion with the calcium carbonate matrix, or (2) complex formation with either the protein nitrogen or some other component of the organic fraction. Concentration of radium (and possibly thorium) appears to occur by both mechanisms, the dominant one being dependent upon the division. For the Rhodophyceae and the highly calcified Chlorophyceae it is the former, and for the Phaeophycea the latter. Concentration of uranium occurs by the first mechanism. This difference in behavior is consistent with the chemical forms of the ions of these elements in seawater. * Biological and Medical Resarch Division. ¢ Department of Marine Biology, University of Puerto Rico. INTRODUCTION To predict the consequences of the accidental re- lease of radionuclides from nuclear power plants it is necessary to understand the mechanisms by which stable elements are concentrated and transported within the ecosystem. Nuclear power plants, by necessity, are located close to large bodies of water for cooling purposes. Hence, aquatic organisms are among the first most likely to be affected by radionuclide release. Also, data on their natural levels of ra- dieactivity would be useful as reference indices. While some information is available on the concentration of various elements and radionuclides from fallout in marine organisms,‘1-8) little is known concerning their distribution within species, between species, and as a function of collection site. In 1961, before the nuclear reactor “Bonus” at Punta Jiquero