An integrated ultrasonic and cellulase pretreatment strategy: simultaneously enhancing medium-chain fatty acid production and reducing antibiotic resistance genes in anaerobic digestion of animal manure.

Rapid expansion of animal husbandry poses significant environmental challenges, including waste management and pollution. While medium-chain fatty acid (MCFA) production from livestock manure via chain elongation (CE) process in anaerobic digestion (AD) offers a promising and economically valuable waste management strategy, the associated risks of antibiotic resistance genes (ARGs) propagation through AD products are often overlooked. Additionally, the impact of substrate pretreatments on MCFA production and ARGs abundances in livestock manure during CE process remains unclear. This study assessed synergistic effects of cellulase and ultrasonic pretreatments on MCFA yield, microbial community, and ARGs during CE process of co-digesting lignocellulosic goat manure and corn straw. The results demonstrated that cellulase pretreatment increased caproate yield by 46.72 % (from 6.93 to 10.17 g of chemical oxygen demand (COD)/L). Combined cellulase and ultrasonic pretreatments decreased ARGs relative abundance by 25.28 %. The underlying mechanism was further explored through metagenome, revealing the roles of microbial community shifts and gene regulation in the beneficial outcomes. Specifically, we found that cellulase pretreatment enriched Clostridia and modulated genes related to reverse β-oxidation and ethanol oxidation, contributing to enhanced MCFA production. The reduction in ARGs abundance, particularly with combined pretreatments, was linked to a decrease in Massilibacterium abundance. Additionally, co-occurrence analysis indicated Actinomycetota, Bacillota, Bacteroidota, and Pseudomonadota as primary ARG hosts. This study demonstrates the viability of lignocellulosic goat manure for MCFA production via CE. Pretreatments effectively boost MCFA yield and mitigate ARGs, providing a basis for comprehensive biorefinery systems integrating resource valorization and ARGs control in lignocellulosic animal waste management.