Copper-induced metabolic variation of oysters overwhelmed by salinity effects

In estuarine environments, Cu (copper) contamination is simultaneously coupled with salinity variation. In this study, ¹H NMR was applied to investigate the metabolic disturbance of estuarine oysters Crassostrea hongkongensis under both Cu and salinity stresses. Oysters were exposed to dissolved Cu (50 μg L^-1) at different salinities (10, 15 and 25 psu) for six weeks, and the Cu accumulation in the oyster tissues was higher at lowered salinity. Based on the NMR-metabolomics results, disturbances induced by Cu and salinity was mainly related to osmotic regulation, energy metabolism and glycerophospholipid metabolism, as indicated by the alteration of important metabolic biomarkers such as alanine, citrate, glucose, glycogen, betaine, taurine, hypotaurine and homarine in the gills. At lower salinity, oysters accumulated higher energy related compounds (e.g., glucose and glycogen) and amino acids (e.g., aspartate, dimethylglycine and lysine), with the enhancement of ATP/ADP production and accumulation of oxidizable amino acids catabolized from protein breakdown. With Cu exposure, the synthesis from glycine to dimethylglycine was observed to cope with severe osmotic stress, together with the elevation of lysine and homarine. The effects induced by Cu were much similar for each salinity treatment, but the combination of Cu and salinity turned out to be consistent with the singular salinity effects. Therefore, salinity played a dominant role in affecting the metabolites of oysters when combined with Cu exposure. This study indicated that salinity should be taken into consideration in order to predict the Cu toxicity in estuarine organisms.