Microbial and metabolic variations in pH-dependent chain elongation: Co-utilization of lactate and ethanol vs. lactate-based production
Filip Brodowski, Natalia Gutowska, Anna Duber, Roman Zagrodnik, Mateusz Łężyk, Piotr Oleśkowicz-Popiel
Abstract
There is growing interest in utilizing waste streams in mixed culture fermentation (MCF) for carboxylate chain elongation (CE). Lactate and ethanol are the most well-recognized electron donors (ED) in CE; however, previous attempts to co-utilize them have yielded varying results. The impact of ethanol as an additional ED for lactate-based CE on microbiome structure and metabolite production was investigated in continuously-fed bioreactors across a pH range of 5.5–7.0. Caproiciproducens spp. was recognized as the primary potential caproate producer at pH 5.5–6.5, supported by Clostridium sensu stricto 12 spp., Clostridia UCG-014 spp., and Oscillibacter spp. It was demonstrated that ethanol, as an additional ED at pH 6.0 and 6.5, increased caproate concentration by approximately 35% compared to lactate-based CE, while the combination of pH 5.5 and ethanol addition inhibited lactate-based CE and induced solventogenesis, despite the microbiome being enriched with potential chain elongators. The microbiome structure exhibited the greatest shifts at near-neutral pH affecting caproate production. Lactate-based CE was characterized by competition between chain elongators and unidentified microorganisms from the Oscillospiraceae and Lachnospiraceae families. On the other hand, the co-utilization of ethanol and lactate at neutral pH favoured the growth of Anaerotignum propionicum and Clostridium sensu stricto 7 spp. Although neutral pH favoured the growth of chain elongators' competitors, specific ASVs associated with Caproiciproducens spp. still appeared, leading to an increase in its relative abundance up to 19.8%. This study highlights how ethanol as a co-ED for lactate-based CE influenced metabolite profiles and microbial community structure across different pH ranges.