Determination of Acute Zn Toxicity in Pore Water from Soils Previously Treated with Sewage Sludge Using Bioluminescence Assays

The effects of increasing concentrations of Zn and Cu in soil pore water from soils of a long-term sewage sludge field experiment on microbial bioluminescence were investigated. Concentrations of total soluble Zn, free Zn 2+, and soluble Cu increased sharply in soil pore water with increasing total soil metal concentrations above 140 mg of Zn kg - 1 or 100 mg of Cu kg - 1 . Two luminescence bioassays were tested, based on two bacteria ( Escherichia coli and Pseudomonas fluorescens ) with the lux genes encoding bacterial luminescence inserted into them. The bioluminescence response of the two microorganisms declined as total soil Zn, soil pore water soluble Zn, and soil pore water free Zn 2+ concentrations increased. The EC 25 values for E. coli and P. fluorescens were 1.3 ± 0.2 and 4.3 ± 0.5 mg L - 1 on a free Zn 2+ basis, respectively. The EC 50 values were 2.5 ± 0.2 and 9.6 ± 0.9 mg of free Zn 2+ L - 1, respectively. Copper had no significant effect on bioluminescence in the two assays, even at the largest soil pore water concentration of about 620 μg L - 1, corresponding to a total Cu concentration in bulk soil of about 350 mg kg - 1 . Thus, the decline in bioluminescence of the two assays was ascribed to increasing soil pore water free Zn 2+ and not soluble Cu.