TRANSURANIC RADIONUCLIDES IN MARINE ENVIRONMENT 529 



It would thus appear that assimilation of plutonium in invertebrates does not 

 constitute an inefficient process nor does it parallel the low gastrointestinal adsorption of 

 plutonium by terrestrial vertebrates where the plutonium has been administered by 

 gavage (Thompson, 1967). What role digestive physiology or the manner in which the 

 label is administered plays in these strikingly dissimilar results remains to be elucidated. If 

 biochemically -bound plutonium is more readily assimilated across gut walls of experi- 

 mental animals than plutonium in either ionic or chelated forms (citrate or tartrate), then 

 it remains to be demonstrated whether substantial differences exist in either the 

 distribution or the elimination of the plutonium retained by the animal. Should such 

 differences be observed, the implications for revising current radiation protection 

 standards are obvious. 



Americium 



As far as we are able to determine, the only published data dealing with the biokinetic 

 behavior of americium in marine organisms are those of Fowler and Heyraud (1974). 

 Using the brine shrimp (Artemia) and the euphausiid (Meganyctiphanes norvegica), 

 Fowler and Heyraud studied ^'*^Am uptake directly from seawater (brine shrimp and 

 euphausiid) and a combined food— seawater pathway (brine shrimp) in a short-term 

 experiment. Artemia accumulated ^'*^Am efficiently from water; concentration factors 

 of 1700 were attained in as little as 48 hr. By contrast, concentration factors from these 

 animals in a labeled phytoplankton suspension reached 400 in the same period of time. 

 Filtration of the seawater showed that some 81% of the ^"^ ^ Am was associated with the 

 algal cells; 2.0% was retained by 0.45-jum filters, and some 17% remained in the filtrate. 

 Recalculation of the concentration factor based on the activity in water and the 0.45-/im 

 filterable fraction alone gave concentration factors virtually identical to those found 

 when accumulation was from water only. Clearly, little ^'*' Am (if any) was assimilated 

 by the food pathway in this experiment. When placed in fresh seawater, both groups of 

 Artemia retained less than 1% of their accumulated ^"^^ Am after 3 hr, which suggested 

 that very little of the ^^^ Am was incorporated metabolically. 



Euphausiids appeared to accumulate ^'^^ Am less efficiently than the brine shrimp and 

 at a somewhat slower rate. After the euphausiids had been exposed for 64 hr to labeled 

 seawater, concentration factors had reached a value of 125, and there was an indication 

 that equilibrium was being approached. When placed in unlabeled seawater, a single 

 euphausiid lost only 40% of its ^'*^Am burden during the first 8 days. At molting, 

 however, virtually all the ^'^^ Am was lost v^th the cast molt. 



Aside from these observations, we have little data concerning the uptake rate, 

 assimilation, and loss of ^"^^Am in marine organisms. However, Fowler and Guary 

 (1977b) have observed relatively high assimilation efficiencies for starfish fed ^"^^Am- 

 labeled mussel tissue, as observed for ^^"^Pu, and retention times appear to be long. 

 Starfish fed a single ration of ^"^^ Am-labeled mussel tissue retained 85 to 95% of the 

 ingested dose 5 weeks postexposure. At dissection, ^90% of the retained ^'^^Am was 

 found in the pyloric caeca. 



The relationships between the behavior of americium and plutonium in the marine 

 environment have recently been summarized by Bowen (1975) and by Livingston and 

 Bowen (1976a). For marine organisms there appears to be no consistent trend regarding 

 their bioavailability, i.e., that one is preferentially taken up by biota in favor of the other. 

 Virtually all the comparisons to date rest with measurements of these isotopes in biota 



