Antioxidant responses in Mesopodopsis zeylanica at varying salinity to detect mercury influence in culture ponds

Antioxidant responses in the estuarine mysid, Mesopodopsis zeylanica, were used to assess mercury (Hg) metal contamination in relation to salinity. Natural mysids acclimatized at 5 psu were subjected to laboratory exposures of 5, 15, and 25 psu salinity singly and under the sublethal Hg concentration of 5 µg/L (one-fifth of 24 h LC50 value). Lipid peroxidation in the test species increased with salinity variation from 5 to 15 psu and retained the same up to 25 psu with no change after Hg addition. CAT increased significantly from 5 to 15 psu before Hg influence, and GST increased after addition of the metal. However, a reduction in both these enzymes is evident at higher salinities (25 psu) with or without Hg. The energy cost involved in adjusting ionic concentration with hemolymph during salinity deviation from an isotonic environment could be a cause for the reduction of antioxidants and LPX accumulation at 25 psu. Insignificant changes after Hg addition, however, indicate that Hg2+ free ions did not produce much toxicity to the cellular system. The results suggest that M. zeylanica is comfortable scavenging oxyradicals within the salinity range of 5−15 psu to protect itself from anthropogenic contaminants as evidenced by elevated enzyme activity.