Bioaccumulation and distribution of silver in four marine teleosts and two marine elasmobranchs: influence of exposure duration, concentration, and salinity

Abstract

The bioaccumulation of waterborne silver (added as AgNO 3) was compared amongst drinking (teleosts: rainbow trout, tidepool sculpin, plainfin midshipmen, and English sole) and non-drinking marine fish (elasmobranchs: Pacific spiny dogfish and long nose skate) exposed to 14.5 μg/l Ag for 21 days in 30-ppt seawater. In addition, 21-day exposures were performed on trout, midshipmen, and sculpin at 0 (control), 1.5, 14.5, and 50 μg/l Ag to evaluate the effect of silver concentration, and on sculpins acclimated to 18 and 30 ppt salinity and sampled periodically up to 21 days to evaluate the effects of salinity and exposure duration. A 48-h acute exposure (250 μg/l Ag) was also carried out on sculpins at 10, 18, 24, and 30 ppt. The 1.5-and 14.5-μg/l Ag levels are of regulatory importance, but are several orders of magnitude higher than normal environmental levels. Silver uptake occurred in all exposures, but internal accumulations were less than proportional to exposure concentration (1.5–50.0 μg/l Ag), and tended to saturate over time, suggesting that physiological regulation occurred. Control (non-exposed) fish exhibited measurable levels of silver in all tissues (10–200 μg Ag/kg wet weight), suggesting that they accumulate silver from the natural environment throughout their lifetimes. After 21-day exposure to 14.5 μg/l Ag, silver levels increased 2–20-fold in most tissues of all species, with the greatest concentrations occurring in the livers of teleosts (order: liver>gills≥intestines>white muscle) and the gills of elasmobranchs (order: gills>liver>white muscle>intestines). Rainbow trout accumulated more silver than the other teleosts, and were the only species to suffer significant mortality, effects likely associated with added salinity stress. Accumulations were fairly uniform amongst the other teleosts. Similar concentrations in gills and intestines suggested that both branchial and intestinal uptake occurred, with the latter potentially dominant; indeed sole exhibited no silver build-up in the gills. The two elasmobranchs exhibited no silver build-up in intestines but much higher levels in gills, indicating that in the absence of drinking, only branchial uptake occurs. Nevertheless, based on whole liver content, the elasmobranchs accumulated silver 5–15-fold faster than the three teleosts. Over 21-day exposures (1.5–50.0 μg/l Ag) in sculpin, salinity markedly affected silver accumulation, with tissue-specific levels approximately 6-fold higher at 18, than at 30 ppt. However, there was negligible effect of salinity on silver accumulation during 48 h at 250 μg/l Ag. Silver bioaccumulation appears to be markedly affected by speciation. At lower salinities, or higher [Ag], a neutral charged AgCl aq complex exists in the water, allowing for increased bioaccumulation to occur. At higher salinity, only less bioavailable, negatively-charged AgCl n 1− n complexes are present (AgCl 2 −, AgCl 3 2−, AgCl 4 3−).