Recognize the benefit of continuous anaerobic co-digestion of cow manure and sheep manure from the perspective of metabolic pathways as revealed by metatranscriptomics

Anaerobic digestion (AD) of cow manure (CM) has been extensively implemented for simultaneous waste disposal and biogas generation. Within this criteria, AD co-digestion of CM and sheep manure (SM) was proved a viable approach for improved methane production. However, concrete evidence regarding the relationship between substrate catabolism and microbial metabolism is lacking, hindering the understanding of the merits of co-digestion. This study adopted metatranscriptomics to unveil critical metabolisms in the co-digestion scenario. Co-digestion significantly improved cellulase activity (CM + SM: 1.031; CM alone: 0.485–0.675) in lignocellulose hydrolysis. Concurrently, the introduction of SM promoted variable enzymes in protein metabolism (serine and cysteine) to intermediates (pyruvate and Acetyl-CoA) in acidogenesis and acetogenesis. Consequently, co-digestion rendered the improved Coenzyme-B sulfoethylthiotransferase activity (CM + SM: 3.883; CM alone: 1.702–1.864), thus excellent methanogenesis (20% improvement compared with CM alone). The results emphasized that co-digestion of CM and SM improved lignocellulose and certain protein hydrolysis to intermediates, thus enhancing methanogenesis.