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.
Propionate is one of the most important intermediates of anaerobic fermentation. Its oxidation performed by syntrophic propionate-oxidizing bacteria coupled with hydrogenotrophic methanogens is considered to be a rate-limiting step for methane production. However, the current understanding of SPOB is limited due to the difficulty of pure culture isolation. In the present study, two anaerobic chemostats fed with propionate as the sole carbon source were operated at different dilution rates (0.05 d−1 and 0.15 d−1). The propionate- and acetate-oxidizing bacteria in the two methanogenic chemostats were investigated combining DNA-stable isotope probing (DNA-SIP) and 16S rRNA gene high-throughput sequencing. The results of DNA-SIP with 13C-propionate/acetate suggested that, Smithella, Syntrophobacter, Cryptanaerobacter, and unclassified Rhodospirillaceae may be putative propionate-oxidizing bacteria; unclassified Spirochaetaceae, unclassified Synergistaceae, unclassified Elusimicrobia, Mesotoga, and Gracilibacter may contribute to acetate oxidation; unclassified Syntrophaceae and Syntrophomonas may be butyrate oxidizers. By DNA-SIP, unclassified OTUs with 16S rRNA gene abundance higher than 62% of total Bacteria in the PL chemostat and 38% in the PH chemostat were revealed to be related to the degradation of propionate. These results suggest that a variety of uncultured bacteria contribute to propionate degradation during anaerobic digestion. The functions and metabolic characteristics of these bacteria require further investigation.
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