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92 COOPER AND JOLLY microorganisms, a property of substantial com- mercial importance. Minimum effective concen- trations are somewhat hard to come by, since most of the literature deals with the amounts required to kill all bacteria rather than with threshold levels at which effects can first be detected. Many investigators have placed Ag at or near the top of the list among heavy metals in toxicity to fungi, slime molds, and bacteria [Porter, 1946]. Water containing 15 X 10-10 g ml-1 Ag from contact with specially prepared metal has exhibited bacteriocidal activity. Water containing 60 X 10-10 g ml-1 Ag has killed E. coli in 2 to 24 hours, depending on numbers of bacteria [Lawrence and Block, 1968]. Bacter- iocidal activity in this context usually implies death of at least 99.9% of the cells present. It has been suggested that Ag interferes more effectively with metabolism of anaerobic bacteria than with aerobic organisms. Evidence to this effect is not conclusive, but this point could be rather easily established in the laboratory. Selec- tive interference with anaerobic bacteria could have significant ecological consequences where water is deoxygenated by organic pollution, in naturally anaerobic sediments, and in sewage treatment processes. SUBLETHAL INHIBITION BY SILVER Very little information is available about sublethal effects of silver or its compounds. There do not seem to be any reported investi- gations of effects of silver concentrations on growth rates of microorganisms or fishes. Nonlethal inhibition is highly probable, how- ever, 51 X 10-10 g ml-1 Ag immobilized Daphnia magna in Lake Erie water at 250 C [McKee and Wolf, 1963]. It has been suggested that silver adversely affects anaerobic processes in sewage treatment, but no concentrations have been cited at which this effect appears. The Office of Pollution Abatement of the Eastman Kodak Company reports (personal communication) that the levels of silver in their effluent are 'so low as to be inconclusive,' and that no definitive tests to determine threshold .levels of toxicity for microorganisms have been conducted. They noted that concentrations in water from which Ag is recovered sometimes reach 100 or even 1000 ppm with no noticeable effect on bacterial decomposition. They stated that no definite conclusions could be drawn about effects of Ag on sewage treatment because of variations in bacterial species and . in the forms of the elements or compounds being tested for toxicity. ^ J COMPARISONS WITH MERCURY Public attention is being increasingly drawn to the possible deleterious effects of mercury in the environment [Novick, 1969]. Some bi- ologists have expressed concern that dispersal of other heavy metals like silver may have similar consequences. There appear to be significant differences between silver and mercury, how- ever. Mercury readily forms volatile, relatively stable organo-metallic compounds such as dimethylmercury with soluble derivatives. The analogous silver compound, methylsilver, de- composes at temperatures above -400 C. Those organo-silver compounds or complexes that are stable at ordinary temperatures tend to be nonvolatile, insoluble, and almost inert. Al- though no statements to this effect have been found in the literature, it would appear that the lower solubility of most silver compounds makes reversal of the bacteriostatic effect of silver easier than similar reversal of mercury's effects. y ENVIRONMENTAL CONCENTRATION PROCESSES Much of the silver in precipitation falling on land will be adsorbed on clays and organic colloids as the water percolates through the soil. However, rain and melted snow moving over the surface of the ground as overland flow, or passing quickly through subsurface layers, makes little contact with soil colloids, and most of the silver in this direct runoff will be de- livered to streams or lakes. Silver in precipita- tion falling directly into lakes and streams is immediately introduced into aquatic systems. Biological concentration in terrestrial sys- tems. Because of the low solubility of most silver salts, and because of the tendency for adsorption of silver by soil colloids, most silver in terrestrial systems will presumably be im- mobilized. Silver uptake by plants seems to be essentially passive and is determined by silver concentrations in the immediate environment [Cannon et al., 1968]. No plants have been found that regularly concentrate silver, and .~ f- I