Are Sugar Concentrations More Important Than Bacterial Identity in Fe(III) Reduction?

Microbial iron(III) (Fe(III)) reduction plays an important role in the environment and is fundamentally driven by the oxidation of organic matter. However, most studies have primarily focused on how different Fe(III) is reduced by different bacteria, while largely overlooking the oxidation side of the reaction. Sugars as primary organic carbon inputs in soils can be utilized by some Fe(III)-reducing bacteria as electron donors. However, the effect of sugar input on microbial Fe(III) reduction remains poorly understood. In this study, we determined Fe(III) reduction kinetics and the extent of three glucose-metabolizing bacteria (Aeromonas sp. CD, Enterobacter sp. DN, and Bacillus sp. GX) at different glucose concentrations. Our results showed a positive correlation between glucose concentrations and the reduction of Fe(III) minerals (ferrihydrite), with significant increases in both the reduction rate and extent observed at low to moderate glucose levels (5–32 mM). However, compared to ferrihydrite, increasing glucose concentrations had a smaller effect on enhancing the reduction rate and extent of Fe(III)-citrate by the three strains. Glucose concentrations also influenced the promoting effect of an electron shuttle (AQDS), which enhanced ferrihydrite reduction at low glucose concentrations (5 mM) but exhibited weaker or even inhibitory effects at higher glucose concentrations (32–65 mM). Aeromonas sp. CD, with Mtr-based extracellular electron transfer systems (EET), exhibited higher Fe(III)-citrate reduction rates than the other strains, but the difference in ferrihydrite reduction rates was not as pronounced as in reducing Fe(III)-citrate and ferrihydrite with AQDS. Overall, this study highlights the crucial role of sugars and sugar-metabolizing Fe(III)-reducing bacteria in the iron biogeochemical cycle.