Influence of Bacterial Strains and Cell Numbers on the Reduction of Fe(III)-Citrate and Ferrihydrite with and without an Electron Shuttle

Fe(III)-reducing bacteria, consisting of different species with varying cell numbers in the environment, are present, together with redox-active electron shuttles and both dissolved and solid Fe(III) species. However, the effect of electron shuttles and Fe(III)-organic-matter(Fe(III)-OM) complexes on microbial Fe(III) reduction is mainly based on a few early isolated model strains. Due to variations in experimental methods among different researchers, it remains unclear whether these two types of compounds influence different Fe(III)-reducing bacteria differently at varying cell numbers. To address this question, we conducted cell suspension experiments with Shewanella oneidensis MR-1, Aeromonas sp. CD and Aeromonas sp. XH, and evaluated reduction rates of Fe(III)-citrate and ferrihydrite with or without AQDS at three different inoculum concentrations. Our results showed that electron shuttles promoted Fe(III) reduction to different extents among these bacteria, along with varying ratios of the rates of Fe(III)-OM reduction and ferrihydrite reduction. Furthermore, cell numbers also influenced the impact of electron shuttle on Fe(III) reduction; ferrihydrite reduction rates were not increasing proportionally with increasing inoculum concentrations when electron shuttles were present. Comparative genomics suggested that differences in the identity of the Fe(III) reductase and the type of Fe(III)-chelators potentially synthesized probably led to these varied promotional effects. These results highlight the significant differences between model and nonmodel Fe(III)-reducing bacteria, suggesting that the presence of Fe(III)-OM complexes and electron shuttles may determine the presence and abundances of certain Fe(III)-reducing bacteria in different environments.