Effects of yeast extract on microbial community structure and methanogenic activity during the syntrophic methanogenesis of propionate

Propionate accumulation represents a critical kinetic bottleneck in anaerobic digestion systems, frequently leading to process instability and reduced carbon conversion efficiency. Although bioaugmentation with functional microbial consortia provides an effective strategy to alleviate propionate inhibition, its engineering application is constrained by the high cost of synthetic culture media and the slow growth of syntrophic microorganisms. This study evaluates low-cost yeast extract as a nutrient substitute to enhance the enrichment efficiency and metabolic activity of propionate-degrading consortia, thereby providing a cost-effective strategy for rapid inoculum production. A propionate-acclimated microbial consortium was obtained from a stable continuous stirred tank reactor. A two-stage enrichment strategy was conducted under mesophilic conditions (37 ℃). In the first stage, semi-continuous reactor experiments were operated at a hydraulic retention time of 15 d and an organic loading rate of 0.5 g/(L·d), with yeast extract concentrations ranging from 0.03 to 2.00 g/L. Based on reactor performance, three representative concentrations (0.13g/L, 0.33g/L and 1.00 g/L) were selected for subsequent validation. In the second stage, batch assays were conducted using three substrates, including sodium propionate, sodium acetate, and a hydrogen/carbon dioxide gas mixture (80/20), to investigate substrate-specific metabolic responses. Control assays containing yeast extract without external carbon substrates were included to quantify background biogas production. System performance was evaluated via gas chromatography with thermal conductivity detection for biogas composition analysis, high-performance liquid chromatography for volatile fatty acid profiling, and soluble chemical oxygen demand measurement. Microbial community dynamics were characterized using 16S rRNA gene high-throughput sequencing, and the absolute abundance of methanogens was quantified by quantitative polymerase chain reaction targeting the mcrA gene. Yeast extract significantly enhanced methanogenic performance, with 1.00 g/L identified as the optimal concentration. At this level, net methane yield reached 380.00 mL/g VS, representing a 4.50-fold increase compared with the control. Quantitative polymerase chain reaction analysis showed that the absolute abundance of methanogens increased by 1.70 times. Microbial community analysis revealed that yeast extract selectively enriched key functional taxa involved in syntrophic metabolism. The relative abundance of the propionate-oxidizing syntroph Syntrophaceticus schinkii increased to 1.70% exclusively in the optimized treatment. Archaeal community structure shifted toward a functionally specialized consortium dominated by Methanosaeta (53.50%) and Methanoculleus (35.30%), indicating enhanced aceticlastic and hydrogenotrophic methanogenesis pathways. Substrate-specific batch assays demonstrated that yeast extract significantly accelerated the degradation kinetics of volatile fatty acids. The acetate degradation rate within the initial 48 h improved by 25.10% to 53.90% compared with the control. In contrast, no significant enhancement was observed in hydrogen/carbon dioxide-fed systems, suggesting that yeast extract primarily stimulates the metabolic activity of volatile fatty acid-oxidizing syntrophs rather than directly enhancing hydrogenotrophic methanogenesis. This study demonstrates a scalable and cost-effective strategy for producing high-activity bioaugmentation inocula, offering a practical approach for mitigating propionate-induced inhibition and improving process stability in industrial anaerobic digestion systems.