Temperature drives microbial community shifts and methanogenic pathways in a carbon fiber fixed-bed reactor for anaerobic digestion

This study investigated the effects of temperature on microbial communities, key enzymes, and energy metabolism in carbon fiber fixed-bed anaerobic reactors under psychrophilic (15 °C), mesophilic (37 °C), and thermophilic (55 °C) conditions. Metagenomic and 16S rRNA gene sequencing revealed that methane content peaked at 80% under psychrophilic conditions, despite lower biogas production (7.5 L/d) compared to mesophilic (25 L/d) and thermophilic (30 L/d) regimes. Bacterial diversity was highest at 15 °C, while archaeal richness peaked at 37 °C. Methanothermobacter and Methanosarcina dominated thermophilic methanogenesis, whereas Methanothrix prevailed at lower temperatures. Genes encoding F420-reducing hydrogenase were most abundant at 55 °C, where biogas production was highest, while CO 2 -reducing methanogenesis genes were well represented at 15 °C, where methane purity reached 80%. ATP synthesis genes shifted from bacteria-associated (37 °C) to archaea-associated (55 °C) dominance. These findings highlight the genetic potential of psychrophilic systems for achieving high methane purity and provide insights for optimizing anaerobic digestion across temperature gradients.