Repeated fed-batch operations for microbial detoxification of mercury using wild-type and recombinant mercury-resistant bacteria

A wild-type mercury-resistant strain Pseudomonas aeruginosa PU21 (Rip64), and an Escherichia coli PWS1 strain genetically engineered to harbor mercury resistance were examined for their capacity to detoxify soluble mercuric ions with repeated fed-batch operations. The specific mercury detoxification activity for the two strains at different initial mercury concentrations was determined by resting-cell experiments. The fed-batch operations were conducted with different initial culture volumes (V(o)), inoculum sizes (X(o)), and different mercury feeding rates (F(Hg)) to investigate the effects of those operation parameters on the performance of mercury detoxification. The results showed that the wild-type and the recombinant strains had an optimal specific activity of 5x10^-7 and 8x10^-8 μg cell^-1 h^-1, respectively. In fed-batch operation for P. aeruginosa PU21, under the conditions of V(o)=400 ml and X(o)=4.5-4.8x10⁹ cells ml^-1 the overall mercury detoxification efficiency (η) for F(Hg)=16.9 mg Hg h^-1 was 5.26 mg Hg l^-1 h^-1, nearly 35% higher than that for a lower F(Hg) (11.7 mg Hg h^-1). Among the three initial culture volumes examined in this study, the highest η (5.60 mg Hg l^-1 h^-1) was obtained when V(o)=1200 ml and F(Hg)=16.9 mg Hg h^-1. It was also found that an inoculum size higher than 4.0x10⁹ cells ml^-1 enabled a stable fed-batch operation, while as the inoculum was reduced to around 1.6x10⁹ cells ml^-1, the mercury feeding caused severe cell death, leading to an unsuccessful fed-batch operation. In the fed-batch operation for E. coli PWS1 strain with V(o)=1200 ml and F(Hg)=16.9 mg Hg h^-1, the mercury detoxification efficiency was 3.07 mg Hg l^-1 h^-1, only 54% of that for the wild-type P. aeruginosa PU21 strain under the same operating conditions. It was also noticed that the operation with E. coli PWS1 became less efficient at the second fed-batch cycle due to plasmid instability of the recombinant strain. Copyright (C) 1998 Elsevier Science B.V.