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 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 g 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 g g^-1 (dry wt). The estimated K_m and V_max 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%.