Deciphering the role of calcium peroxide on the fate of antibiotic resistance genes and mobile genetic elements during bioelectrochemically-assisted anaerobic composting of excess dewatered sludge

The integration of bioelectrochemical system (BES) into conventional anaerobic composting process can accelerate the degradation of excess dewatered sludge (ES). However, no reports have been made concerning antibiotic resistance genes (ARGs) profile and abundance in BESs fuelled with ES, and the effect of CaO₂ on the fate of ARGs within BESs treating excess sludge has rarely been investigated. The aim of this study was to investigate the effect of CaO₂ and the bioelectrochemical process on the abundances of ARGs and MGEs and to unravel the mechanism of ARGs attenuation. Results showed that CaO₂ addition could enhance the reduction of ARG levels in ES within AnC_BE. The mean ARG abundances in the AnC_BE samples with high CaO₂ doses (0.4 g CaO₂/g VSS and 0.5 g CaO₂/g VSS) were significantly lower than the values in samples with other CaO₂ doses. The abundances of lincosamide nucleotidyltransferase, macB, macrolide transporter ATP-binding proteins, and macrolide-efflux proteins significantly decreased with the increase in the CaO₂ dose. Both CaO₂ addition and bioelectrochemical assistance played important roles in shaping the ARG composition during the AnC_BE process. The variation in the microbial community composition is the most important contributor to the variation in the ARGs composition. The abundances of Actinobacteria and Firmicutes explained 52.8% of the total ARG variance. Of the MGEs, the abundances of plasmids, insertion sequences, and integrons were all reduced in the sludge metagenomes. There was a significantly positive correlation between the abundances of sulI, sulII, tetG, and bla_TEM and those of metal resistance genes (MRGs), which decreased significantly after the AnC_BE process. This study revealed the potential of the combination of CaO₂ and bioelectrogenesis for ARG attenuation.