Long-Term Stability of High-n-Caproate Specificity-Ensuring Anaerobic Membrane Bioreactors: Controlling Microbial Competitions through Feeding Strategies

Abstract

Lactate-based carbon chain-elongating open culture systems have gained increasing research attention as they help to upgrade carbon-rich wastewater to the valuable platform chemical ncaproate. However, microbial competitions occurring in this system have markedly lowered n-caproate specificity. This study investigated possible microbial competitions occurring during long-term operating periods of anaerobic membrane bioreactors (AnMBRs) inoculated with shaped chain-elongating microbiomes to develop L-lactate-to-ncaproate conversion systems with high specificity. We demonstrated both semicontinuously (Sc-AnMBR) and continuously (C-AnMBR) fed AnMBRs and identified three possible competitions between (i) potential chain-elongating bacterial species, (ii) chain-elongating and propionate-fermenting bacterial species, and (iii) chemolithotrophic acetogens and hydrogenotrophic methanogens. In both AnMBRs, Caproiciproducens galactitolivorans achieved an ecological niche among chain-elongating species, occupying more than 79% of the relative abundance. During the Sc-AnMBR operating period, the n-caproate specificity was only 32.4% due to the intervention of the propionate-fermenting species Propionibacterium freudenreichii in L-lactate consumption. In contrast, during the C-AnMBR operating period, C. galactitolivorans prevailed, while P. freudenreichii was thoroughly excluded; thereby, the n-caproate specificity reached 81.3%. Even though propionate fermentation is thermodynamically favored in lactate-fed anaerobic bioreactor systems, this study showed that chain elongation reaction selectivity can be highly improved by controlling the kinetic parameters and suggested a bioreactor operation strategy to ensure high n-caproate specificity.