Electron shuttling to ferrihydrite selects for fermentative rather than Fe³⁺—reducing biomass in xylose—fed batch reactors derived from three different inoculum sources

Reports suggest that ferric iron and electron shuttling molecules will select for Fe³⁺—reducer dominated microbial biomass. We investigated the influence of the redox mediators anthraquinone-2,6-disulfonate (AQDS) and riboflavin using xylose as the sole fermentation substrate, with or without ferric iron. Electron shuttling to insoluble ferrihydrite enhanced solventogenesis, acidogenesis, hydrogen production, and xylose consumption, relative to the cells plus xylose controls in fermentations inoculated with woodland marsh sediment, wetwood disease, or raw septic liquid, over multiple transfers in 15-day batch fermentations. 16S rRNA gene based community analyses indicated that ferrihydrite alone, and AQDS/riboflavin plus ferrihydrite, immediately shifted native heterogeneous communities to those predominantly belonging to the Clostdridiales, rather than stimulating Fe³⁺ respiring populations. Data were similar irrespective of the inoculum source, suggesting that Fe³⁺ and/or electron shuttling compounds select for rapid proliferation of fermentative genera when fermentable substrates are present, and increases the extent of xylose consumption and solvent production.