The impact of bacterial strain on the products of dissimilatory iron reduction

Three bacterial strains from the genus Shewanella were used to examine the influence of specific bacteria on the products of dissimilatory iron reduction. Strains CN32, MR-4 and W3-18-1 were incubated with HFO (hydrous ferric oxide) as the terminal electron acceptor and lactate as the organic carbon and energy source. Mineral products of iron reduction were analyzed using X-ray powder diffraction, electron microscopy, coulometry and susceptometry. Under identical nutrient loadings, iron reduction rates for strains CN32 and W3-18-1 were similar, and about twice as fast as MR-4. Qualitative and quantitative assessment of mineralized end products (secondary minerals) indicated that different products were formed during experiments with similar reduction rates but different strains (CN32 and W3-18-1), and similar products were formed during experiments with different iron reduction rates and different strains (CN32 and MR-4). The major product of iron reduction by strains CN32 and MR-4 was magnetite, while for W3-18-1 it was a mixture of magnetite and iron carbonate hydroxide hydrate (green rust), a precursor to fougerite. Another notable difference was that strains CN32 and MR-4 converted all of the starting ferric iron material into magnetite, while W3-18-1 did not convert most of the Fe³⁺ into a recognizable crystalline material. Biofilm formation is more robust in W3-18-1 than in the other two strains used in this study. The differences in mineralization may be an indicator that EPS (or another cellular product from W3-18-1) may interfere with the crystallization of magnetite or facilitate formation of green rust. These results suggest that the relative abundance of mineral end products and the relative distribution of these products are strongly dependent on the bacterial species or strain catalyzing iron reduction.