Impact of zinc on biological Fe(III) and nitrate reduction by Shewanella putrefaciens CN32

The impact of zinc on the reductive dissolution of hematite by the dissimilatory metal-reducing bacterium Shewanella putrefaciens CN32 was studied in batch culture mode. Experiments were conducted with hematite (2.0 g L ^-1) in 10 mM PIPES (pH 6.8) and H₂ as the electron donor under nongrowth conditions (10⁸ cell mL^-1), spiked with zinc (0 to 0.23 mM), and incubated for 5 days. Experiments were also conducted with ferric citrate (2.0 mM) and nitrate (1.43 mM NO₃-N L ^-1) to evaluate the effect of zinc with soluble electron acceptors. A 50% inhibitory concentration (IC₅₀) was defined as the zinc concentration that decreased the extent of electron acceptor consumption by 50% compared to a no-zinc biotic control. The total zinc IC₅₀ values for hematite, ferric citrate, and nitrate reduction were 0.21, 0.28, and 0.049 mM, respectively. The free zinc (Zn²⁺) IC₅₀ values for hematite, ferric citrate, and nitrate reduction were 0.13, 0.00016, and 0.049 mM, respectively. The low Zn²⁺ IC₅₀ for ferric citrate was attributed to citrate complexation of zinc. The free Zn²⁺ IC ₅₀ for aerobic cell growth on lactate in zinc-containing defined growth medium was 0.050 mM. Differences between free Zn²⁺ IC ₅₀ values for soluble and solid phase electron acceptors indicate that zinc adsorption to hematite and cell surfaces may control inhibition during solid-phase iron reduction. Greater zinc tolerance observed during solid phase iron reduction compared to soluble electron acceptors may be attributed to the formation of biofilms on hematite surfaces, resulting in higher zinc tolerance compared to freely dispersed cells.