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

Abstract

Reports suggest that ferric iron and electron shuttling molecules will select for Fe -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 Fe respiring populations. Data were similar irrespective of the inoculum source, suggesting that Fe 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.

Ferrihydrite Reduction Exclusively Stimulated Hydrogen Production by Clostridium with Community Metabolic Pathway Bifurcation

“…Ferrihydrite enhanced hydrogen production by elevating the hydrogenase activity and transcription of C. pasteurianum , a typical hydrogen-producing strain . In addition, many iron (oxyhydr)­oxide minerals have been used to stimulate dark fermentation of biohydrogen. − However, Dong et al found that hydrogen generated in the fermentative process was further consumed by hematite reduction, resulting in a decrease in hydrogen . The variations may be attributed to the different inoculums and substrates.…”

“…Clostridium spp. have been identified as fermentative iron reducers with robust dissimilatory iron reduction capacity. − , Popovic et al found that ferrihydrite addition shifted native heterogeneous communities to those predominantly belonging to Clostridiales irrespective of the inoculum source (e.g., woodland marsh sediment, diseased wetwood, or raw septic liquid) . These results indicate that ferrihydrite reduction is capable of targeted enrichment of Clostridium species with fermentation substrate (glucose) supplementation.…”