Effects of biological and physicochemical variables on the valve movement responses of freshwater bivalves to manganese, uranium,cadium and copper

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The valve movement responses (VMR) of freshwater unionid bivalves to increasing concentrations of total Mn, U, Cd or Cu, under conditions of varying pH and/or dissolved organic carbon (model fulvic acid) concentrations, were experimentally measured using a computer-based data acquisition system. Specifically, ‘Velesunio angasi’, from Magela Creek in tropical northern Australia, was exposed to Mn and/or U in synthetic freshwater between pH 5.0 and 6.0, both with and without model FA (3.15 or 7.91 mg L⁻¹). In contrast, ‘Hyridella depressa’, from the Hawkesbury-Nepean River in temperate eastern Australia, was exposed to Cd and/or Cu in synthetic freshwater between pH 6.5 and 7.5, both with and without model FA (4.20 or 11.2 mg L⁻¹). Despite differences in bivalve species, geographical location (climate) and water chemistry, valve movement patterns and concentration-response curves were similar for all metals. The results showed that VMR is a quantifiable, sensitive and rapid, real-time endpoint for assessing the toxic effects of metal exposures. For Mn or Cd, VMR was independent (P > 0.05) of pH and/or model FA concentration. In contrast, VMR to U or Cu was highly dependent (P ≤ 0.05) on pH and/or model FA concentration; individuals were more sensitive to U or Cu at low pH and model FA concentrations. Additionally, results from the concentration-response experiments were used to evaluate the free ion activity model (FIAM). Drug-receptor theory was used to develop an extended form of the FIAM to obtain a more rigorous conceptual model. The developed model explains the conditions under which the FIAM will be effective in explaining biological response (BR), but more importantly, precisely quantifies the interaction of metal species at cell receptor sites. Valve movement responses to Mn or Cd were directly proportional to the activity of the free metal ion (Mn²⁺ or Cd²⁺), which is consistent with both the original and extended FIAM. In contrast, VMR to U or Cu were regarded as an ‘exception’ to the original FIAM, since they were a weighted function of the activities of the free metal ion and the 1:1 metal hydroxide species (UO₂²⁺ + UO₂OH+ or Cu²⁺ + CuOH⁺). However, this result is consistent with the extended FIAM, with BR primarily dependent on the activity of UO₂²⁺ or Cu²⁺, and secondarily dependent on the activity of OH⁻. Based on the extended FIAM, this study proposes, for the first time, a quantitative method of uncoupling the biological effects of a metal hydroxide species from that of amelioration of the free metal ion by H⁺. This is a major outcome, since the activities of metal hydroxide and H⁺ cannot be independently varied. Additionally, concentration-response data obtained from the literature, that are considered to be ‘exceptions’ to the original FIAM, were re-examined and found to be consistent with the extended FIAM. Overall, the extended FIAM provides a potentially more useful tool for evaluating metal-organism interaction than the original FIAM.
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