The microbial and functional reconstruction of instable syntrophic propionate-oxidizing methanogenesis by system recovering and injection modes changing

Abstract

Microbial enrichment is an effective method to obtain highly abundant syntrophic methanogenic propionate-degrading microbial consortia (syntrophic consortia) in complex environments. Nevertheless, there were few researches studying dynamic microbial community change and syntrophic mechanisms over a long-time bioreactor operation. In this study, the highly efficient syntrophic consortia were enriched in the anaerobic bioreactor, and the effect of propionic acid loading (PAL) on microbial community structure and functional pathways was investigated. By single injection mode (30 mL propionic acid was injected into the reactor once per day), acid accumulation occurred severely at the PAL of 2.7 g/(L·d). After the injection mode was changed to continuous injection mode (a syringe pump was used to inject 30 mL propionic acid evenly in 8 h), the reactor performance was more stable and the volumetric biogas production (VBP) reached 0.96 L/(L·d). 16S rDNA sequencing analysis and gene predictions showed that microbial community structures, functional microorganisms and metabolic pathways varied at different PALs and different injection modes. When the propionic acid shock was great, hydrogenic methanogenesis was the major pathway to relieve the acid accumulation. In continuous injection mode, the bacterial structure was more stable. Syntrophobacter became the dominant propionate oxidation bacterium and acetoclastic methanogenesis conducted by Methanosaeta enhanced. This study provides an effective method for the microbial enrichment in the rate-limiting step of anaerobic digestion and lays a foundation for the development of bio-enhancement with high organic loading.