Novel Syntrophic Isovalerate-Degrading Bacteria and Their Energetic Cooperation with Methanogens in Methanogenic Chemostats

Isovalerate is an important intermediate in anaerobic degradation of proteins/amino acids. Little is known about how this compound is degraded due to challenges in cultivation and characterization of isovalerate-degrading bacteria, which are thought to symbiotically depend on methanogenic archaea. In this study, we successfully enriched novel syntrophic isovalerate degraders (uncultivated Clostridiales and Syntrophaceae members) through operation of mesophilic and thermophilic isovalerate-fed anaerobic reactors. Metagenomics- and metatranscriptomics-based metabolic reconstruction of novel putative syntrophic isovalerate metabolizers uncovered the catabolic pathway and byproducts (i.e., acetate, H₂, and formate) of isovalerate degradation, mechanisms for electron transduction from isovalerate degradation to H₂ and formate generation (via electron transfer flavoprotein; ETF), and biosynthetic metabolism. The identified organisms tended to prefer formate-based interspecies electron transfer with methanogenic partners. The byproduct acetate was further converted to CH₄ and CO₂ by either Methanothrix (mesophilic) and Methanosarcina (thermophilic), which employed different approaches for acetate degradation. This study presents insights into novel mesophilic and thermophilic isovalerate degraders and their interactions with methanogens.