Silver in freshwater and seawater fish: toxicity, bioaccumulation, and physiology

Abstract

Freshwater rainbow trout were exposed to 9.2 µg/L total Ag (as AgNO3, a level approximately equal to the 168 h LC50) for 144 h to clarify the toxic mechanism of silver in freshwater teleosts. It was found that silver inhibits active Na+ and Cl- uptake at the gills, resulting in a net loss of both ions from the fish and creating a metabolic acidosis. This leads to a classic stress response (mobilization of cortisol and glucose into the blood plasma), and hyperventilation as a respiratory response to decreased blood pH. Plasma ammonia levels rise without any decrease in ammonia excretion; ammonia excretion later increases. This suggests that the increased plasma levels are due to increased metabolic production. Increased [H+] (decreased pH) results in excess H+ ions in the internal fluids, which are either complexed with ammonia to form NH4+ or are buffered in muscle tissue. The latter results in increased movement of K+ ions into the plasma, which are then excreted at the gills, preventing hyperkalemia. In the end, freshwater teleosts probably die from iono-and osmo-regulatory failure and associated cardiovascular collapse. Seawater teleosts (rainbow trout, tidepool sculpins, English sole, and plainfin midshipmen) and elasmobranchs (Pacific spiny dogfish and long nose skate) were exposed to constant concentrations on total Ag (as AgNO3) ranging from 1.5 to 50.0 µg/L for periods of up to 21 d at salinities of 18 ppt or 30 ppt. These exposure levels are well below those causing acute toxicity in seawater. Silver appears to enter marine teleosts and marine elasmobranchs differently. Seawater teleosts drink the seawater, so the intestines are a major site of silver uptake, along with the gills. Since marine elasmobranchs do not drink, the gills appear to be the sole site of silver uptake from the water. As in freshwater, the liver is the main site for silver accumulation in all marine fish studied. Despite similar terminal liver silver concentrations, marine elasmobranchs have a higher rate of silver accumulation since the livers in elasmobranchs are 10-20 fold larger than in teleosts. Both environmental salinity and exposure concentration play direct roles in determining silver bioaccumulation in marine teleosts. Increasing salinity alters the speciation of silver in the water, which decreases the amount of silver able to enter the fish. Increased silver concentrations mean more silver is available to enter the fish and subsequently cause sublethal toxic effects. Oxygen consumption decreased during the first 7 d of chronic exposure to sublethal silver levels in marine teleosts, with the decrease being more pronounced at higher (still sublethal) silver levels. Ammonia excretion, unaltered during acute exposure (48 h) to high silver levels (250 µg/L), was decreased during the first 7 d of exposure to sublethal silver levels (14.5-50.0 µg/L). This suggests that silver interferes with energy demanding processes such as protein synthesis or iono-regulation. Activity levels of the main enzyme involved in iono-regulation, namely Na+/K+ -ATPase, was affected differently in different fish. In a marine teleost that lives solely in seawater (plainfin midshipmen), silver inhibited the gill ATPase activity after 7 d of exposure, with the inhibition being more effective at higher silver levels. In the tidepool sculpin, a truly euryhaline species, gill ATPase activity increased as the silver levels increased, the latter probably representing a compensatory strategy. Similarly, intestinal ATPase activity was unchanged in the midshipmen, but was increased in the sculpins. Drinking rate in tidepool sculpins, which is involved with both iono-and osmo-regulation, was unaffected by salinity, but was decreased in fish exposed to silver for 8 d. Overall, Ag is far less toxic in seawater than in freshwater, but the mechanisms of toxicity are similar. In both waters, Ag interferes with iono- and osmo-regulation. In seawater, Ag exhibits a significant potential for bioaccumulation and interference with physiological processes during long term low level exposures of marine fish, especially at lower salinity levels.