There is ample evidence in the literature for tolerance to elevated levels of silver by marine invertebrates. Resident clams, Macoma balthica, accumulated only half as much silver as clams transplanted from a pristine area. Transplanted clams retained 90% of the accumulated silver in their tissues while the resident clams lost as much as they gained, and shell closure in the transplanted clams was observed to occur earlier relative to the resident clams (Cain and Luoma 1985).
Mytilus edulis collected from the field and grown in the laboratory in 5, 25 and 50µg/L of silver, grew slower after being exposed for six months, but by 12 months their growth rates equalled that of the controls (Calabrese et al. 1984). A transplanted polychaete, Neanthes virens, from a pristine area accumulated twice as much silver as a population from a contaminated area; transplanted worms also exhibited reduced oxygen consumption, changes in the ionic balance of the coelomic fluid and loss of water proportional to increasing body burdens of silver while the local population did not (Pereira and Kanungo 1981).
In 1992 Bryan and Langston noted that there was no clear evidence that metal body burden is proportional to toxicity of the metal. They cited evidence for tolerance to elevated levels of metals as given by Koechlin and Grasset in 1988 who showed that exposure of the polychaete, Sabella pavonina, to 50 µg/L silver eventually resulted in the deposition of silver into granules where it was associated with sulfhydryls. The granules were then excreted and thus not of toxicological concern.
In 1987 Flemming studied the effects of silver on bacteria growing on ion exchangers. The addition of silver suppressed bacterial growth until a tolerant population developed. Bacteria growing near the area where the silver was applied received sublethal concentrations promoting the development of tolerant populations. Tolerance was shown to fluctuate from 5 to 50 µg/L silver. Many microorganisms can acquire resistance to silver and it appears that the evolution of heavy-metal resistant enzymes is primarily restricted to microorganisms (Klein 1978).
Only two studies were found which indicated that fish could develop a tolerance to silver. In 1974, Coleman and Cearley noted, in a study with largemouth bass, Micropterus salmoides, and bluegill sunfish, Lepomis macrochirus, that silver accumulation was only significant for the first two months of the study, after which a plateau was reached, and no significant mortality was observed except at the highest level of silver exposure in the largemouth bass.
In 1984 Birge et al. studied the effects of time of acclimatization and deacclimatization with the fathead minnow, Pimephales promelas. Acclimatization of the minnow to 1.5 µg/L or 15 µg/L silver for 7 to 14 days significantly increased the LC50 values; however, deacclimatization for >7 days lowered the LC50 value to a level similar to the unacclimatized minnows.
Lab strains of the algaeScenedesmus and Chlorella were inhibited by 100 µg silver/L and 30 µg silver/L, respectively. The Scenedesmus grew at 50 µg silver/L. Strains from lakes in the Sudbury area were much more tolerant. Lake Chlorella grew at 50 µg silver/L and were not completely inhibited until 100 µg silver/L. Lake Scenedesmus was not completely inhibited until 200 µg silver/L. The lab strains exhibited a complete cutoff of growth but growth in lake strains gradually slowed as silver levels rose.
These lake strains were also more tolerant to other metals (Stokes et al. 1973). Chlorella from lakes containing high levels of nickel were nickel tolerant but were also silver tolerant. The lake silver levels were between 1 and 37 µg silver/L (Hutchinson and Stokes 1975).