Serial passage enrichment of coal-adapted microbial consortia enhances biogenic methane production via structural and functional microbial specialization.

Microbial consortia-driven bioconversion of low-rank coal to methane presents a potential strategy for extracting energy from recalcitrant carbon reservoirs. Current systems achieve a theoretical conversion efficiency of 40-60%; however, hydrolysis poses a significant bottleneck due to its rate limitations. To investigate methane production during serial enrichment, we developed a six-cycle serial-batch enrichment protocol conducted under anaerobic conditions (35 °C, pH 7.2) using lignite-amended activated sludge as microbial seedings. High-throughput 16S rRNA gene sequencing revealed an overall decline in microbial alpha diversity and shifts in community composition across serial passages. The archaeal community was consistently dominated by Methanosarcina, with a mean relative abundance of 81.08% across P1-P6, indicating the establishment of a methanogenic consortium. XPS analysis showed changes in the relative contributions of surface carbon components, whereas SEM observations revealed local differences in coal surface morphology after incubation. Across six serial passages, the activated sludge enrichment consortia retained methane-producing capacity, with faster methane accumulation in later passages but no significant directional increase in day-28 methane yield.