Syngas fermentation to ethanol: CODH/AdhE1 gene expression and microbial community dynamics

Abstract

Biosynthesis of ethanol through syngas fermentation employing enriched microbial consortia offers resilience to operate under non-sterile conditions with energy efficiency. Gas fermentation was performed by varying C1 feedstock with reference to selective enrichment of microbial consortia to maximize ethanol production with simultaneous process parameter optimization. Three different operating conditions (R1-CO₂ + HCO₃⁻, R2-syngas, and R3-syngas + HCO₃⁻) were evaluated using diverse shock-based enrichment strategies (heat-shock, acid-shock and BESA). R2 and R3 systems showed potential for ethanologenic conversion of C1 gases. The highest ethanol (0.38 g L⁻¹) and acetic acid (2.1 g L⁻¹) yields were obtained in the R3 conditions using BESA-treated culture. Upregulation of genes involved in the Wood–Ljungdahl pathway, specifically alcohol dehydrogenase (AdhE1), under the treated conditions correlated with ethanol synthesis. Over-expression of the Ni-protein subunit (CODH) and Fdhl in the treated conditions demonstrated their role in the selective production of ethanol and acetate. Analysis of the microbial community revealed that the R3 conditions showed abundant phyla such as Firmicutes and Actinobacteria, and members of the carboxydotrophs, which possess the coding sequences of the upregulated genes. The treated conditions exhibited a three-fold increase in ethanol biosynthesis with an inorganic carbon fixation rate of 66 mg L⁻¹ h⁻¹, indicating efficient conversion of C1 substrate by the enriched biocatalyst. Higher relative expression and dominance of carboxydotrophs in the microbial diversity depicted the metabolic inclination of enriched biocatalysts towards the conversion of C1 gases.