Ecological engineering of r/K-selected microbial strategies for enhancing anaerobic propionate degradation

Propionate accumulation is a major bottleneck in anaerobic digestion, requiring cooperation between syntrophic propionate-oxidizing bacteria (SPOB) and methanogens for its degradation. Here, we applied the r/K selection theory as an ecological engineering strategy to enrich SPOB by manipulating sludge retention times (SRTs) and reactor operational modes. Four anaerobic bioreactors, two continuous flow reactors (CFRs) and two sequencing batch reactors (SBRs), were operated at 15- and 30-day SRTs (CFR_15, CFR_30, SBR_15, and SBR_30). Extended SRTs (30 days) enabled efficient organic carbon removal in both SBR_30 and CFR_30, compared with inefficiencies in SBR_15 (39.1 %) and CFR_15 (21.6 %). Microbial community analysis revealed a shift from the putative r-strategist Desulfobulbus (up to 3.3 %) under short SRTs to the putative K-strategist Syntrophobacter (up to 2.2 %) under extended SRTs. Notably, SBR_30 supported the co-enrichment of Pelotomaculum (5.1 %) and Syntrophobacter (2.2 %), forming a complementary consortium that enhanced propionate degradation. Mechanistically, extended SRTs enhanced intermediate metabolism and increased the methanogenic gene abundance, potentially alleviating thermodynamic limitations. Concurrently, SBR mode increased SPOB activity and the relative abundance of propionate oxidation genes. These findings demonstrate that r/K-driven microbial selection, guided by operational control, can strategically enhance SPOB activity and syntrophic metabolism, offering a scalable route to optimize anaerobic bioreactor performance.