A Rapid Radiochemical Bacterial Bioassay to Evaluate Copper Toxicity in Freshwaters

摘要

A rapid, highly sensitive bacterial bioassay to determine copper toxicity in freshwaters was developed based on the inhibition of cellular assimilation of radiolabeled glucose. The test used a copper-sensitive bacterium isolated from a freshwater stream. Employing sensitive radiochemical techniques enabled environmentally relevant concentrations of the test bacterium (105 cells mL−1) and a short incubation period (4 hours) to be used, which minimized the potential for changes in copper speciation during the test. The 4-hour median effective concentration (EC50) for inorganic copper at pH 7.5 in synthetic freshwater was 0.6 μg L−1 (95% confidence limits 0.4 to 1.0 μg L−1). This compared well with chronic growth inhibition of this bacterium in minimal medium (48-hour EC50 of 0.9 μg L−1 [95% confidence limits 0.7 to 1.0 μg L−1 ]). MINEQL + software (Environmental Research Software) was used to calculate copper (II) ion concentrations in synthetic freshwater at pH 7.5, giving an EC50 value of pCu2+ 8.8. However, using nitrilotriacetic acid metal-ion buffers (Cu-NTA), 50% inhibition occurred at a pCu2+ of 9.7, suggesting this bacterium was markedly more inhibited by copper in these Cu2+-buffered solutions. This may indicate that the Cu-NTA species was contributing to toxicity. The radiochemical bioassay was evaluated further using freshwater samples from both copper-impacted and pristine environments. Measured EC50 values ranged from 3.4 to 34.0 μg L−1 inorganic copper and were strongly correlated with dissolved organic carbon (DOC) concentrations (r = 0.88, p < 0.05).