Electroactive taxa stabilise a metal overdosed syntrophic propionate oxidising methanogenic consortia

Propionate oxidation is a critical yet rate-limiting step in anaerobic digestion (AD), vulnerable to disruption by metal toxicity. Electroactive bacteria facilitating extracellular electron transfer (EET) may mitigate such toxicities; yet mechanisms remain unclear. This study hypothesized that ethanol feed electroactive taxa mitigate continuous metal overdosing stress by augmenting propionate oxidation and methanogenesis. Duplicate expanded granular sludge bed (EGSB) reactors were operated under two regimes (i) propionate enrichment (PE) or, (ii) with propionate plus ethanol enrichment (EE). A continuous supply of low concentration iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo), tungsten (W), manganese (Mn), and selenium (Se) simulated metal stress. Key indicators - carbon removal, methane production, and microbial community dynamics were systematically monitored. EE reactors exhibited adaptive responses to metal-induced stress, demonstrated by higher propionate removal (58 % vs 27 %) and CO₂-reducing methanogenesis by Day 65. The EE biomass retained significantly more W (290 vs 45.7 mg W kg^-1) (p < 0.01) than PE, indicating a stronger capacity for metal accumulation. These outcomes were associated with (i) enrichment of Uncultured Geobacteraceae, (ii) increased mcrA gene expression (iii) putative cytochrome-based respiration and (iv) potential heme and co-factor biosynthesis modules. Collectively, these findings suggest that ethanol-fed Geobacteraceae plays a role in promoting W retention, stabilizing Co levels and sustaining propionate oxidation/methanogenesis under severe continuous metal exposure. Strengthening such microbial interactions is essential for enhancing AD systems to metal toxicity. These insights help explore the role of electroactive taxa in anaerobic environments and their potential to improve the stability of environmental treatment systems.