Different Interspecies Electron Transfer Patterns during Mesophilic and Thermophilic Syntrophic Propionate Degradation in Chemostats

Propionate is one of the major intermediates in anaerobic digestion of organic waste to CO 2 and CH 4 . In methanogenic environments, propionate is degraded through a mutualistic interaction between symbiotic propionate oxidizers and methanogens. Although temperature heavily influences the microbial ecology and performance of methanogenic processes, its effect on syntrophic interaction during propionate degradation remains poorly understood. In this study, metagenomics and metatranscriptomics were employed to compare mesophilic and thermophilic propionate degradation communities. Mesophilic propionate degradation involved multiple syntrophic organisms ( Syntrophobacter , Smithella , and Syntrophomonas ), pathways, interactions, and preference toward formate-based electron transfer to methanogenic partners (i.e., Methanoculleus ). In thermophilic propionate degradation, one syntrophic organism predominated ( Pelotomaculum ), interspecies H 2 transfer played a major role, and phylogenetically and metabolically diverse H 2 -oxidizing methanogens were present (i.e., Methanoculleus , Methanothermobacter , and Methanomassiliicoccus ). This study showed that microbial interactions, metabolic pathways, and niche diversity are distinct between mesophilic and thermophilic microbial communities responsible for syntrophic propionate degradation.