Process stability and microbial adaptation during temperature shift from thermophilic to mesophilic temperature in anaerobic digestion

Thermophilic anaerobic digestion often achieves higher biogas yields than mesophilic conditions, but its high energy demand limits long-term sustainability. As a result, many thermophilic digesters may transition toward mesophilic conditions. This study examined the effects of stepwise temperature shift from thermophilic to mesophilic ranges on biogas production and microbial community dynamics, while identifying factors that support or inhibit methane generation. At 55 ℃ and 50 ℃, methane yields reached 342 and 311 mL CH4/g COD, respectively, with a predominance of hydrogenotrophic Methanobacterium, alongside abundant Coprothermobacter, Defluviitoga, Acetomicrobium, Candidatus Bipolaricaulis, Brachyspira, and Dictyoglomus, suggesting potential syntrophic interactions between these microorganisms. At 45 ℃ and 40 ℃, the methane yield declined to 234 and 219 mL CH4/g COD but recovered to 289 and 251 mL CH4/g COD at 35 ℃ and 30 ℃, respectively. In the mesophilic range, frequent accumulation of acetate and butyrate coincided with a methanogenic shift from hydrogenotrophic Methanobacterium to acetoclastic Methanothrix, indicating a pathway transition. These results reveal that lowering the digestion temperature from thermophilic to mesophilic conditions enables recovery of acetogenic activity and shifts methanogenesis from hydrogenotrophic to acetoclastic pathways, with implications for optimizing biogas production during long-term operation.