Mechanism of acute silver toxicity in the euryhaline copepod Acartia tonsa
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Acute silver effects on whole-body ion regulation and Na(+),K(+)-ATPase activity were evaluated in the euryhaline copepod Acartia tonsa. Experiments were run at 20 degrees C, three different salinities (5, 15 and 30 ppt), in either the absence or the presence of food (diatom Thalassiosira weissflogii; 2 x 10(4)cells/mL). Standard static-renewal procedures were used. Copepods were acutely (48 h) exposed to silver (AgNO(3)) concentrations equivalent to the 48-h EC10 (dissolved Ag=3, 49, and 94 microg/L), 48-h EC30 (dissolved Ag=5, 71, and 125 microg/L) or 48-h EC50 (dissolved Ag=7, 83, and 173 microg/L) values in the absence of food or to the 48-h EC50 (dissolved Ag=35, 90, and 178 microg/L) values in the presence of food. These values were previously determined under the same experimental conditions at salinities 5, 15 and 30 ppt, respectively. Endpoints analyzed were whole-body ion concentrations (Na(+), Cl(-), and Mg(2+)) and Na(+),K(+)-ATPase activity. In starved copepods, lower whole-body Na(+) and Mg(2+) concentrations were observed in salinities 5 and 30 ppt, respectively. Also a higher whole-body Na(+),K(+)-ATPase activity was observed in all salinities tested. Data from fed copepods indicate that all these salinity effects were completely associated with starvation. Silver exposure induced a decrease in the whole-body Mg(2+) concentration in starved copepods in salinities 5 and 30 ppt and a Na(+),K(+)-ATPase inhibition in both starved and fed copepods in all salinities tested. Thus, food addition in the experimental media completely protected against silver effects on Mg(2+) concentration, but not against those on Na(+),K(+)-ATPase activity. In starved copepods, enzyme inhibition was dependent on silver concentration and a relationship between this parameter and mortality was observed in all salinities tested. Therefore, Na(+),K(+)-ATPase molecules seem to be a key site for acute silver toxicity in marine invertebrates, as reported for freshwater fish and crustaceans.
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