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gb )ournal of Weather Modification Volume 41 <br />6.2 Bioaccumulation of Silver in Oraanisms <br />Bioaccumulation of substances occurs via body <br />surfaces (often referred to as bioconcentration) <br />and through intake of food often referred to as <br />biomagnification). Bioconcentration factors <br />("BCFs") are given by the ratio of the concentra- <br />tion of the compound in the organism with that of <br />the surrounding medium, usually water (also <br />food). For tenestrial plants, uptake is generally <br />through the roots and leaves. For terrestrial ani- <br />mals uptake occurs via the surface or the gastro- <br />intestinal tract ("GIT"). <br />For aquatic vertebrates. uptake is possible via the <br />body surface or the GIT, or in the case of fish, via <br />the gilis. Uptake from water can be due to pas- <br />sive diffusion, activa transport and adsorption. <br />There is no significant evidence of substantial <br />silver uptake via food for aquatic organisms. <br />It is clear that BCFs appear to be correlated with <br />the solubility of the silver compound. <br />In his review of bioaccumulation and toxicity of <br />silver compounds, Ratte (1999) has tabulated the <br />silver bioconcentration factors, the silver com- <br />pound and the species studied. <br />The US EPA (1980) (cited in CICAD 44) has re- <br />ported BCFs of 210 in diatoms, 240 in brown al- <br />gae, 330 in mussels, 2300 in scallops and 18700 <br />in oysters. In contrast, bluegills showed no sig- <br />nificant accumulation when exposed to 0.5 Ng/L <br />silver (Coleman and Cearley, 1974). <br />Concem over the possible accumulation of silver <br />in both marine and freshwater environments has <br />arisen because of bioaccumulation observed in <br />benthic organisms (Bell and Kramer, 1999). <br />Silver is a soft or "b"-class metai and would be <br />expected to coordinate and compiex strongly with <br />soft bases which, in this context, are organic <br />molecules containing sulfur (S) or nitrogen (N) <br />atoms. Silver exhibits a great affinity for sulfur- <br />containing ligands (organic compounds) (Frausto <br />da Silva and Williams 1991). <br />In biological systems thiolate complexes include <br />mercaptans, glutathione and cysteine and for <br />many of these complexes stability constants are <br />available (Bell and Krarner 1999). Higher stabil- <br />ity constants indicate strong binding between <br />silver and the complexing agent which in turn <br />decreases bioavailability. <br />Fisher and Wang(1998), in their review of trophic <br />transfer of silver, note that trophic transfer has <br />been shown to be insignificant in several aquatic <br />animals, for example particularly oysters and <br />shdmp. Oysters were able to accumulate dis- <br />solved silver but did not acquire silver from vari- <br />ous phytoplankton species. Shrimp could accu- <br />mulate dissolved silver but did not acquire silver <br />from planktonic or detrital food. <br />Uptake from marine sediments was reported by <br />Bryan and Langston (1992), leading to the view <br />that sediments are an important source of silver. <br />However, in a reported laboratory exposure ex- <br />periment over some 20 days, net uptake of silver <br />only occurred when the concentration of silver in <br />the sediment exceeded 1 mg/kg. <br />Connel{ et al. (1991) found that incorporated sil- <br />ver was tightly bound to the cell membrane and <br />was not released by mechanical disruption. They <br />also observed that food-chain biomagnification <br />was unlikely at concentrations normally found in <br />the environment. <br />6.3 Bioaccumulation bv Alaae. Pelaqic and <br />Benthic Food Chains <br />The accumulation of dissolved silver into algae is <br />very high. Uptake into algal cells would be ex- <br />pected to influence biogeochemical cycling if the <br />aigae are subsequently consumed by animals. <br />Algae show BCFs, although differences in experi- <br />mental conditions can significantly influence the <br />value. <br />In the pelagic food chain, typically inciuding pra <br />tozoans, rotifers and small crustaceans, signifi- <br />cant bioaccumulation is not likely. For example, <br />bioconcentration was markedly lower in daphnids <br />than in algae. <br />This is reflected in the higher stability constants <br />(K) between silver ion (Ag') and organosulfur <br />compiexes such as thiols, K- 10t3 compared to <br />those of Ag` carboxylate complexes <br />(monodicarboxylic acids, K- 102 - 104 and poly- <br />carboxylates such as EDTA, K- 10'). <br />The benthic food chain, rypically including snails, <br />some insect larvae, bivalves and worms, feed on <br />algae on the bottom of the water body. For gam- <br />marids such as midge larvae and chironomids, <br />the BCF exceeded the BCF found for daphnids. <br />- Scientific Papers -