Additive metal accumulation and possible synergistic pathologies during intermittent exposure to mixtures of mercury plus cadmium in the mussel, Mytilus edulis
摘要
In natural ecosystems, aquatic organisms are intermittently exposed to a complex cocktail of contaminants. Current hazard assessments use only a continuous exposure profile for chemicals, with limited knowledge on the effects of intermittent exposures with mixtures. The study aimed to explore interactive effects of binary mixture of Hg and Cd on metal accumulation in M. edulis compared to each metal alone during intermittent exposure, using biomarkers that characterizes oxidative stress and overall physiological effects. Mytilus edulis were intermittently exposed (2 days exposure, 2 days in clean seawater) to either control (no metals), 50 µg l⁻¹ of mercury (Hg) or cadmium (Cd) alone, or a 50 µg l⁻¹ Hg plus 50 µg l⁻¹ Cd mixture for 14 days. Tissues were collected on days 0, 2, 4, 8, and 14 for metal analysis, and for biochemistry/haematology and histology at the end of the exposure. Significantly higher haemolymph Hg concentrations in Hg alone (~ 280 µg l-1) than in the mixture (~ 110 µg l-1), suggesting an antagonistic effect of Cd on Hg uptake. Gill Cd concentrations were higher in the mixture (~ 110 µg g-1 dw) compared to Cd alone (~ 70 µg g-1 dw) indicating a synergistic effect of Hg on Cd accumulation. However, across all other tissues, the sum of individual metals was not significantly different from the mixture (Student’s t-test, p > 0.05), indicating additive effects. Intermittent exposure did not induce oxidative stress (thiobarbituric acid reactive substance, TBARS, remained ~ 2–3 nmol mg protein-1), although some glutathione depletion occurred in the Hg exposure. Histological observations of gills and digestive gland were similar across treatments, but the pathological effects were generally greater in the digestive gland from the mixture than each metal alone, with some potentially synergistic effects. Haemolymph protein (mean ± SE, mg ml⁻¹) was significantly increased in Hg alone (0.79 ± 0.13) compared to control (0.45 ± 0.13), Cd alone (0.33 ± 0.06), and the mixture (0.38 ± 0.07). Sub-lethal effects included increased haemolymph protein and digestive gland inflammation. In conclusion, the accumulation of Hg and Cd in the tissues were generally additive, although there were some metal and tissue-specific differences in biochemistry, and synergistic pathological effects in the tubules of the digestive gland.