Metal Toxicokinetics In Two Species Of Mussels Under The Effects Of Tidal Exposure,Body Size,and Exposure Routes

Estuaries are important aquatic ecosystems,and are the main channel of heavy metals and other contaminants entering the sea.Metal contamination in estuary is more serious than in other water bodies.Bivalves are often used as biomonitors to monitor the status of metal contamination in coastal waters as they can accumulate metals to very high concentrations.Thus,measuring metal concentration in aquatic organisms can reflect the level of metal contamination in the environment and evaluate their ecological effects.However,metal bioaccumulation is affected by many factors,and it is necessary to correct the biomonitoring data to accurately evaluate the contamination status and ecological risks.In this study,the black-pigmy mussel Xenostrobus atratus and green mussel Perna viridis were used as model organisms to study the effects of tidal exposure,body size and exposure routes on metal bioaccumulation under the framework of toxicokinetic model.Firstly,we investigated the effects of tidal exposure on cadmium(Cd)bioaccumulation.It is expected that metal bioaccumulation of bivalves living at different tidal heights is different due to the different durations of water immersion.However,there was no significant difference in Cd concentration in tissues of X.atratus collected from different tidal heights,suggesting that tidal exposure had limited effects on Cd bioaccumulation.Studying the effect of tides on the Cd toxicokinetics of X.atratus may explain this phenomenon.Therefore,we quantified Cd toxicokinetics in mussels living at different tidal heights using laboratory simulated tidal conditions,and found that values of uptake rate constant(ku)were very similar among the 6 h,9 h,and 12 h immersion treatments(0.080-0.087 L g-1 d-1),slightly but significantly higher in the continuously immersed treatment(0.109 L g-1 d-1),and intermediate in the 15 h immersion treatment(0.098 L g-1 d-1).Cd bioaccumulation was slightly higher in the continuously immersed mussels than the alternately immersed mussels,but was not proportional to immersion duration,which may be due to the fact that X.atratus can still accumulate Cd when exposed to air.By comparing the Cd bioaccumulation in mussels with or without clean seawater purification after the end of Cd exposure,it was confirmed that the mussels could still absorb Cd dissolved in mantle cavity and Cd adsorbed on tissue surface when exposed to air during low tide.Overall,tidal exposure showed limited effects on Cd bioaccumulation.Secondly,we investigated the effect of body size on Cd bioaccumulation of the two mussels X.atratus and P.viridis.Body size is an important factor affecting bioaccumulation of metals and is the most promising factor to be corrected,but previous studies have found different patterns of size effects on bioaccumulation.For the two mussels selected for this study,sampling results from multiple sites indicated that Cd concentrations are higher in larger X.atratus but in smaller P.viridis.The toxicokinetic model can be used as a framework to study the size effect.Therefore,in order to study inter-individual differences,we developed a double stable isotope method to measure the toxicokinetic parameters of mussel "individuals",and found that the Cd ku was significantly positively correlated with tissue dry weight in X.atratus(0.057-0.297 L g-1 d-1),but was significantly negatively correlated with tissue dry weight in P.viridis(0.155-0.351 L g-1 d-1).In contrast,Cd elimination rate constants(ke)were not influenced by body size in two mussel species,the Cd ke ofX.atratus was 0.004-0.046 d-1,and that of P.viridis was 0.009-0.032 d-1.Overall,the effect of body size on Cd bioaccumulation showed different patterns.The double stable isotope tracer method could be used to obtain metal toxicokinetic parameters for individual organisms,and the results indicated that Cd uptake dominants the size effect and interspecies differences.Finally,we investigated the effect of exposure routes on metal bioaccumulation.The previous chapters focused the uptake of dissolved metals by organisms,whereas both seawater and food particles are important sources of metal bioaccumulation by aquatic organisms in real estuaries.Therefore,in this experiment,mussels were exposed to a mixed solution containing seawater and food particles simulating real estuarine exposure.The toxicokinetics of multiple metals by organisms from both the aqueous and food phase were measured simultaneously using a stable isotope tracer method at environmentally realistic metal concentrations.The results showed that the green mussel had a strong ability to accumulate Pb and Zn and a weak ability to accumulate Ni,and the dissolved metals were the main source of bioaccumulation of Ni,Cu,Zn,Cd,and Pb by the mussel under the experimental conditions of this study.In conclusion,tidal height showed limited effects on Cd bioaccumulation,and the mechanism is that intertidal bivalves can still absorb dissolved metals in the cavity fluid and metals adsorbed on the tissue surface when exposed to air;body size may affect metal bioaccumulation in opposite directions in different mussels,and size effects on tissue Cd concentration are mainly mediated by Cd uptake rather than metal elimination;different exposure routes contributed differently to metal bioaccumulation.In this study,we quantified the effects of tidal height,organism size and exposure routes on metal bioaccumulation using the toxicokinetic model and provided a scientific basis for the assessment of ecological risks of metals in coastal waters.