Anaerobic propionic acid production via succinate pathway at extremely low pH

Abstract

Propionic acid, a C-3 platform chemical with applications in a wide variety of industries, is currently produced via petroleum-based pathway. Bio-propionic acid can be a promising alternative to petroleum-based one. However, the bio-production of propionic acid is easily inhibited by low pH (<4.5). Typical vegetable wastes, cabbage and Chinese cabbage were fermented for propionic acid production in leaching bed reactors (LBRs) without pH control. The highest propionic acid production reached up to 12.50 g COD/L in LBR with large-size cabbage, corresponding to cumulative yields of 0.27 g COD/g·VSremoval. Without pH control, pH in this reactor was ∼3.5 during the whole operation period. Extremely low pH regulated the intracellular NADH/NAD+ balance and induced a relatively reductive micro-environment. Adenosine triphosphate (ATP) consumption reactions were triggered to maintain the intracellular proton balance and metabolic activity; thus, the relative abundance of H+-ATPase-related genes was increased to maximize ATP production. The low-pH inducible micro-environment selected Lactobacillus fermentum and Lactobacillus plantarum as dominant species in the microbial community. Succinate pathway was identified as the main fermentative pathway for propionic acid production with both cabbage and Chinese cabbage wastes, which maximized energy generation in propionic acid fermentation. This work unveils a unique propionic acid production pathway from vegetable waste biomass and offers a promising route for waste conversion towards a circular bioeconomy.