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.