Promoting or Inhibiting: New Insights into the Role of Formate in Syntrophic Propionate Metabolism

Anaerobic digestion is a critical technology for pollution control, resource capacity enhancement, and sludge management, necessitating improvements in its efficiency. Formate serves as an electron carrier in syntrophic oxidation of volatile fatty acids (VFAs) during anaerobic digestion. The accumulation of formate can exert an inhibitory effect on the anaerobic digestion process. However, the stress concentration and the mechanism of the formate are not as simple as theoretical calculations based on thermodynamics. Thus, we investigated the response to different concentrations of formate in the syntrophic oxidation of propionate. The anaerobic sludge system and syntrophic co-culture system were applied. The propionate showed more stable degradation when formate dosage ranged from 5 to 10 mM. However, when the formate dosage reached 50 mM, the concentration of propionate was significantly higher than that of CK group, and the propionate metabolism was significantly inhibited. The reduction in functional flora and homogeneous metabolic pathways were found to be unfavorable for the stable progression of syntrophic propionate metabolism. Thus, the enhancement of homoacetogenesis can be a strategy adopted by the sludge system to alleviate formate stress. The methylmalonyl-CoA (MMC) pathway was inhibited under formate stress; the downregulation of RNA transcription of formate dehydrogenase (FDH) and hydrogenase (Hyd) related to MMC pathway may be the main reason for the inhibition of syntrophic propionate oxidation. The anaerobic sludge experiment and the co-culture experiment elucidated the mechanism of action of formate from both macroscopic rules and microscopic molecular mechanisms, respectively.