Metallothionein and cellular energy allocation in the estuarine mysid shrimp <i>Neomysis integer</i> exposed to cadmium at different salinities

In the present study the induction of metallothioneins (MTs) and effects on the cellular energy allocation (CEA) in euryhaline crustacean Neomysis integer exposed to Cd at different salinities were studied. N. integer was exposed to the same sub-lethal concentration of free cadmium ion (1/5 of the cadmium activity of the reported 96 h LC50 value) in hypo-osmotic (7.2 µg Cd/L at 5 psu), isosmotic (23.0 µg Cd/L at 16 psu) and hyper-osmotic media (38.1 µg Cd/L at 25 psu) for 7 days. By using the free Cd concentration as the basis for conducting the exposures, the effect of salinity on cadmium speciation was eliminated and therefore the true effect of salinity as an abiotic factor on the MT induction and CEA could be studied. MT content was quantified by differential pulse voltammetry (DPV); this is the first time that this method is applied to assays with N. integer. No significant differences in MT levels between the control and Cd-exposed groups were observed. The measured MT levels (ranging from 32.3 to 75.7 µg/mg(cytosolicprotein) probably represent the constitutive MT levels responsible for binding essential metals. The differences in MT levels observed at the different salinities indicate a possible relationship between some physiological process (possibly osmoregulation) other than detoxification and MT induction. A decrease in salinity caused an increase in MT level in N. integer, and this was significantly correlated to the CEA values (Spearman correlation coefficient r=-0.56,p= 0.016). Our results indicate that salinity can change the energy status and MT content of N integer. These findings should be taken into account when using these biomarkers - and especially MTs - in field studies and environmental monitoring