Influence of complexation with chloride on the responses of a lux-marked bacteria bioassay to cadmium, copper, lead, and mercury

The toxicity of a heavy metal in solution to a microorganism depends not only on its concentration but also on pH and the concentrations of any aqueous complexing ligands in the microorganism's environment. This paper reports on the use of different inorganic resuscitation media and effect of the chloride ion, Cl−, on the bioluminescence response of a bacterial biosensor, Escherichia coli HB101 (pUCD607), to four metals: Cd, Cu, Hg, and Pb. The toxicity tests were conducted at pH 4, using 0.1 M KNO3 as resuscitation medium and adding KCl to investigate effect of Cl− concentration. The species distributions of metals as a function of Cl− concentration were calculated using GeoChem‐PC. Resuscitation in 0.1 M KC1 gave significantly higher light output than that in 0.1 M KNO3, demonstrating that Cl− in the resuscitation medium has a direct effect on the bioluminescence response of the E. coli biosensor. Increasing concentrations of Cl− ions increased the toxicity of Hg, apparently because of the formation of HgCl−3, and increased the toxicity of Pb because of PbCl+ formation. The toxicity of Cu decreased at high Cl− concentrations as free Cu2+ decreased, in accordance with the free ion model. Concentrations of Cl− had no significant effect on the toxicity of Cd. This study clearly demonstrates that the chloro‐complexes of some heavy metals can be toxic and, for Pb and Hg, more toxic than the free ion.