Metagenomic insights into the enhanced methane production by hydrochar at varied propionate concentrations

Propionate is a crucial intermediate product in anaerobic digestion (AD), but its degradation is thermodynamically unfavorable. Its accumulation in the AD system can significantly reduce methane production efficiency and may lead to severe acidification. The present study investigated the methane production at various propionate concentrations (15, 30 and 60 mM) with and without hydrochar and also explored the microbial shifts by metagenomic analysis. The findings revealed that higher propionate concentrations reduced methane production and propionate degradation efficiency, leading to acetate accumulation. The addition of hydrochar mitigated the inhibitory effects of increased propionate concentrations, notably reduced the lag phase by 20.13 % and enhanced the methane production rate by 138.43 % under 30 mM propionate. This also resulted in faster degradation of propionate and acetate. Hydrochar impacted the microbial communities’ functional potentials, enhancing methane metabolic potential in all conditions. Genomic-centric analysis revealed that hydrochar selectively enriched syntrophic propionate-oxidizing bacteria (SPOB) candidate MAG79 Syntrophopropionicum sp., which could participate in DIET, across different concentrations of propionate with the most pronounced effect at 30 mM propionate. This was distinct from the dominant species in the control groups, MAG63 Syntrophosphaera sp. and MAG69 Syntrophobacteraceae sp.. Hydrochar further supported the growth of another DIET candidate MAG4 Methanosarcina mazei and increased the abundance of proposed syntrophic acetate-oxidizing bacterium (SAOBs), across all propionate concentrations. These effects collectively led to more efficient propionate degradation and methane production.