Enhanced Ethanol Degradation Using Geobacter metallireducens – Shewanella oneidensis Coculture Bioanodes in Single-Chamber Microbial Electrolysis Cells

Abstract Ethanol-rich effluents from biorefineries, distilleries, and fermentation industries are abundant but remain difficult to valorize because alcohol oxidation rarely aligns with the extracellular electron-transfer (EET) capacity of single exoelectrogenic species. A coculture bioanode combines Geobacter metallireducens (GM), which carries high-flux c-type cytochrome EET but modest alcohol dehydrogenase activity, with Shewanella oneidensis MR-1 (SO). The coculture was operated in single-chamber microbial electrolysis cells (MECs) for 82 days. The MEC was enriched first with sodium acetate (SA, 1600 mg L–1) and then with the transitional feed (800 mg L–1 SA + 5 mM ethanol, followed by 5 to 20 mM ethanol). The GM+SO coculture outperformed both monocultures in current, with the hierarchy GM+SO > GM > SO preserved across every substrate phase. LSV current density reached 1.66 ± 0.27 A m–2 in GM+SO under 5 mM ethanol, representing 2.7- and 8-fold enhancements over GM and SO, respectively. GM+SO achieved the fastest degradation rate when ethanol was used as the carbon source, at 4.68 ± 0.55 mM d–1; it retained ethanol-removal capacity at 20 mM when GM collapsed under substrate inhibition. End point 16S rRNA sequencing showed that Geobacter and Shewanella accounted for 45% and 20% of reads in the GM+SO biofilm. PICRUSt2 predicted greater potential for alcohol and aldehyde dehydrogenases and c-type cytochrome functions in the coculture, consistent with a hypothesized pathway in which both partners oxidize ethanol to acetate and Geobacter dominates electron delivery to the anode. Thus, the benefit of coinoculation depends on substrate and loading, and substrate removal alone is a poor guide to partner selection for ethanol-treating bioanodes.