Phylum-wide propionate degradation and its potential connection to poly- γ-glutamate biosynthesis in Candidatus Cloacimonadota

The candidate phylum Cloacimonadota is frequently detected in anaerobic environments such as anaerobic digestion (AD) reactors, hydrothermal vents, and deep-sea sediments, yet its metabolism remains poorly understood due to the lack of cultured representatives. Metagenomic evidence suggests capacities for amino acid fermentation, cellulose degradation, and production of carbohydrate-active enzymes, with particular interest in their presumed role in syntrophic propionate oxidation (SPO), a key bottleneck in AD. However, a complete methylmalonyl-CoA (mmc) pathway, central to SPO, has not been previously identified in Cloacimonadota genomes. Here, we report results from a lab-scale anaerobic baffled reactor fed with sugar beet pulp, where a sharp increase in an uncultured Cloacimonadota OTU coincided with recovery of methanogenesis and enhanced methane production. Metagenomic and metatranscriptomic analyses enabled metabolic reconstruction of this OTU, complemented by a curated database of 47 genome-resolved Cloacimonadota species. Comparative genomics revealed conserved protein clusters indicative of an alternative mmc pathway, suggesting that this variant of the SPO pathway is a widespread, phylum-specific trait potentially linked to protein degradation and poly-{gamma}-glutamate biosynthesis. Network analysis identified the methanogenic archaeon Methanothrix as a primary syntrophic partner, an interaction further supported by propionate-fed enrichment cultures showing co-occurrence of Cloacimonadota and Methanothrix species. Our study sheds light on the Cloacimonadota metabolism, advancing our understanding of their ecological roles and potential for biotechnological applications.