Metallothioneins and cytosolic metals in <i>Neomysis integer</i> exposed to cadmium at different salinities

In the present study the induction of metallothioneins (MTs) and its relation to cytosolic metal concentrations (Zn, Cu and Cd) in the euryhaline crustacean Neomysis integer exposed to Cd at different salinities was studied. N. integer was exposed to the same free cadmium ion activity of 5.74 x — 10-9 mol l-1 (i.e. 1/5 of the 96 h LC50 value expressed as cadmium activity) in hypo-osmotic (5 psu), isosmotic (16 psu) and hyper-osmotic media (25 psu) for 7 days. In this way, the effect of salinity on cadmium speciation was eliminated and therefore the physiological effect of salinity on Cd accumulation and MT induction could be studied. The accumulation of cytosolic Cd in N. integer changed with salinity from 1.11 ± 0.05 µmol l -1 at 5 psu up to 1.43 ± 0.17 µmol l-1 at 25 psu. This could indicate that the physiological response of euryhaline estuarine invertebrates like N. integer to salinity changes can influence the rate of trace metal uptake from solution. While the salinity changes did not cause significant differences in cytosolic Zn concentrations (mean value of all tested salinities: 34.4 ± 2.8 µmol l-1), an inverse relationship between salinity and cytosolic Cu concentration was observed. The highest concentration of 15.7 ± 2.3 µmol Cu l-1 was determined at 5 psu and the lowest 10.9 ± 1.4 µmol Cu l-1 at 25 psu. This could point to a possible relationship between the copper concentration and the hemocyanin metabolism in N. integer. This is the first time that differential pulse voltammetry method was applied to MT assays with N. integer. Although the exposure to Cd resulted in a higher Cd cytosolic concentration, no subsequent MT increase was detected. The significant positive correlation between MT levels and cytosolic Cu concentrations (Spearman correlation coefficient rs = 0.356, p = 0.009) implies a strong relationship between MT and Cu in N. integer