Effects of Ocean Acidification on Toxicity of Heavy Metals in the Bivalve Mytilus edulis L

Abstract

The anthropogenic release of carbon dioxide into the atmosphere leads to an increase in the CO2 partial pressure in the ocean, which may reach 950 μatm by the end of the 21st century. An experiment was performed to test the effects of increased sea water concentrations of CO2 combined with heavy metals on mortality rates, metallothionein, immune funtion, and uptake kinetics of the bivalve Mytilus edulis L. As a result, the mortality rates of Mytilus edulis L fluctuated with pH variety in all heavy metals (Cd, Pb, Cu) test seawater treatments compared to control group. It was demonstrated that in vivo Cd-, Pb-, or Cu pretreatment under acidifications can alleviate metallothionein by in vitro Cd, Pb, and Cu in soft tissues and suggested that acidification can aggravate heavy metals pollution and toxicity for marine organisms. Moreover, the fraction of eosinophilic cells increased over the 21 days period for Cd exposure, then decreased with 28 and 35 days incubation, the fraction of eosinophilic cells increased similar with Cd exposure and pH 8.2. The fraction of eosinophilic cells significantly increased from 32% to 53% of the hemocyte population over the 35-day period for Pb exposure and pH8.2, the fraction of eosinophilic cells increased from 35% to 58% of the hemocyte population over the 35-day period for Pb exposure and pH 6.2. Phagocytosis levels declined with increasing exposure time for Cd alone and phagocytosis levels declined with decreasing pH (from 8.2 to 6.2) with Cd exposure on 35 days. Mytilus edulis L in Pb with reduced pH (6.2) treatments also decreased their phagocytosis levels on 35 days. Further, phagocytosis decreased bigger than that of Pb with pH 8.2. At steady state equilibrium, CF of 109Cd was significantly higher at pH 6.2 and pH 7.7 than at pH 8.2, the accumulation of 203Pb was significantly higher at the lowest pH 6.2 level than at the two exposure levels (pH 7.7 and pH 8.2). Similarly, the lower the seawater pH 6.2, the more 63Cu was accumulated in the hemolymph of Mytilus edulis L. The results indicate that future reductions in pH caused by increased CO2 concentrations in the sea may have an impact on Mytilus edulis L combined with heavy metals pollution.