Biological Removal of Inorganic Hg(II) As Gaseous Elemental Hg(0) by Continuous Culture of a Hg-resistant Pseudomonas Putida Strain FB-1

A strain of broad-spectrum, mercury-resistant Pseudomonas putida FB1 was used to remove mercury as the gaseous element (Hg(0)) from a continuous axenic culture, fed with a synthetic medium containing 1 mg Hg l(-1) as HgCl2. Mercury determinations were performed in steady-state cultures using various culture fractions [whole culture, filtered supernatant, bacterial cells (dry wt), recovery trap liquid] in order to determine the removal efficiency at different dilution rates (from 0.1 to 3.0 day(-1)). The removal efficiency ranged from 99.2% to 99.8%, and the residual Hg was maintained below 5 μ l(-1) (the maximum allowable concentration of Hg in liquid wastes according to Italian law) at a dilution rate of 1.0 day(-1), corresponding to a Hg flux of 40 μg l(-1) h(-1). Hg accumulation by cell biomass was negligible for dilution rates under 1.0 day(-1). A progressive accumulation of Hg, both in the liquid phase and in cells, occurred at a higher dilution rate (3.0 day(-1); close to washout), corresponding to a Hg concentration of 25 μg g(-1) (dry wt). The estimated Km and Vmax for Hg reduction were 0.241 mg l(-1) and 9.5 mg g(-1) h(-1), respectively. In batch experiments maximum Hg removal occurred at the optimum growth temperature (28°C) of P. putida. The maximum recovery of Hg in the liquid trap was 78%.