Accumulation and cellular distribution of ^'Am, 2'0Po, and "Opb were studied in the marine diatom Thalassiosira pseudonana and the marine green alga Dunaliella tertiolecta, Both species concentrated Am, Po, and Pb from artificially spiked cultures, resulting in wet weight concentration factors of approximately 1 to 4 X lo5 for ^'Am, 3 to 7 X lo4 for 210Po, and 5 to 17 X lo3 for "OPb. These concentration factors are comparable to those based on analyses of natural plankton assemblages. In contrast to 2'0Po, ^Am and "OPb appeared to associate almost exclusively with structural components (cell walls and plasmalemmae) and showed no evidence of protein association. The data, together with field evidence, suggest that ^'Am and "OPb are not bound to cell material easily assimilated by herbivorous zooplankton, while "' Po associates with cellular organic compounds and is assimilated in animals. This may lead to high "OPo turnover rates in surface waters.

Accumulation and toxicity of Cd, Zn, Ag, and Hg were measured in the diatom Thalassiosira pseudonana, the chlorophyte Dunaliella tertiolecta, the coccolithophore Emiliania huxleyi, and the cyanophyte Oscillatoria woronichinii. Bioaccumulation of the metals was measured over a wide (up to lo5) range of metal concentrations, using gamma-emitting radioisotopes of each metal. Metal content of cells was related to total external metal concentration in all cases, in accordance with Freundlich adsorption isotherms. Dead cells accumulated metals comparably to living cells, indicating that the initial association of metal with the cell is governed by adsorption. Volume/volume concentra-

Laboratory experiments employing gamma-emitting radiotracers assessed the retention efficiency of ingested metals in the calanoid copepod Anomalocera patersoni and the retention of excreted metals in fecal pellets. Adult copepods ingested the haptophyte Isochrysis galbana labeled with Io9cd , "Z n, 2 0 3~g , " Â ¥ ' "~ and '-"Pu at a rate of 3.79 mg algal C g-I ammal dry wt h-I. Average retention efficiencies were 30 % for Cd, 48 % for Zn, 21 O/O for Hg, 4.5 % for Am, and 0.8 % for Pu. Algal cells resuspended into unlabeled seawater retained metals to varying extents, with depuration curves conforming to a 2-compartment model; Cd was lost most rapidly. The fecal pellets produced by animals feeding on radioactive I. galbana lost essentially all of their l o g c d , "~n , ~m , and 2 3 7~u to unlabeled seawater within 1 d. ^Hg was lost most slowly, again conforming to a 2-compartment model; its retention half time was about 25 d in the slowly exchanging pool. Bacterial activity did not appear to have an effect on metal retention in the fecal pellets. The retention efficiencies suggest that Cd, Zn, and Hg should be recycled by copepods in surface waters as part of the organic cycle in the sea, while the transuranic elements should be defecated and removed from surface waters by sinking biogemc debris.

In experiments examining the retention of trace elements in decomposing zooplankton debris, the rates of C degradation and metal release from radiolabeled copepod fecal pellets and carcasses were measured for up to 1 mo using radlotracers. Fecal pellets incubated at 18 and 2 "C retained 35 to 40 % and 80 % of their I4C, respectively, after 30 d. Carcasses retained only 13 to 18 % and 28 to 38 % of their 14C after 18 d at 18 and 2 "C, respectively. Leaching of 14C from fecal pellets and carcasses as D0I4C accounted for about half of the 14C loss, even in the absence of microbial activity. Proportionately more of the 14C from carcasses was mlcrobially oxidized to I4CO2 than "C from fecal pellets Release of the particle-reactlve transuranic element 2 4 1~n 1 from fecal pellets was unaffected by microbial activity while release of 75Se and 6 5~n from both fecal pellets and carcasses increased with microbial activity and closely followed I4C loss. Release rates of all elements decreased exponentially over time, wlth the most pronounced decreases occurnng wlthin the first 6 d Retention half-times (t,,,,'~) of 2 4 1~m were 248 d in fecal pellets. By contrast, the t,,12's for "Zn ranged from 2.1 to 13.5 d in fecal pellets and both and had t,,,?'s of only about 1 d in carcasses. The results help explain oceanographic observations that C. Se and Zn are recycled in surface waters while scavenged elements like Am are enriched in fecal pellets and have short residence times in surface waters.

Laboratory radiotracer experiments compared the effects of zooplankton grazing and microbial decomposition on the release of organic C and Ag, Cd, CO, Pb and PO from phytoplankton cells. After 40 h incubation of diatoms with copepods + microorganisms, 70% of the diatom cellular C was remineralized to CO, (40 Yo), assimilated in copepod tissue (20 %), excreted in fecal pellets (5 %) or released as DOC (5 %), microorganisms alone removed half this amount of diatom C. Copepod grazlng enhanced the conversion of Ag from diatom cells to other iorms (particulate and dissolved) by 27 %, PO by 25 %, Pb by 20%, Cd by 13 % and CO by 10% over those cells incubated with only microorganisms; zooplankton grazing increased by 5 to 15 % the release of elements from diatom cells into the dissolved phase. Decomposing copepod fecal pellets, held free-falling on a spinning wheel, lost about 20 % more C, Ag, CO and Pb than did undisturbed fecal pellets, while no appreciable difference was observed for release of metals from copepod carcasses treated similarly. The results suggest that retention of an element contained primarily in the cytoplasm of phytoplankton cells (such as C or Cd) is largely governed by microbial activity and leaching. Microbial activity has a smaller effect on particle-reactive elements (Ag and Pb) bound to structural components of cells; zooplankton are effective in removing these unassimilable elements by grazing and packaging them into rapidly sinking fecal pellets.