Evolutions of antibiotic resistance genes (ARGs), class 1 integron-integrase (intI1) and potential hosts of ARGs during sludge anaerobic digestion with the iron nanoparticles addition

In this work, we investigated the impact of iron nanoparticle, including magnetite nanoparticles (Fe3O4 NPs) and nanoscale zero-valent iron (nZVI), on the anaerobic digestion (AD) performance. Moreover, the evolutions of antibiotic resistance genes (ARGs), class 1 integrons-integrase (intI1) and potential hosts of ARGs were also investigated. The optimal addition of Fe3O4 NPs and nZVI to promote methane production was 0.5 g/L and 1 g/L, which led to 22.07% and 23.02% increase in methane yield, respectively. The degradation rate of organic matter was also enhanced with the addition of Fe3O4 NPs or nZVI. The results of high-throughput sequencing showed that the reactors with iron NPs exhibited significant differences in microbial community structure, compared to the reactors with the non‑iron NPs. Iron NPs have caused the relative abundance of the dominant bacteria (Proteobacteria, Firmicutes and Actinobacteria) generally decreased, while the dominant archaea (Euryarchaeota) increased in AD sludge. Quantitative PCR results revealed that iron NPs accelerated the reductions in total absolute abundance of ARGs, especially a beta-lactamase resistance encoded gene (blaOXA). Network analysis displayed that the attenuation of ARGs was mainly attributed to the decline of potential hosts (Proteobacteria, Firmicutes and Actinobacteria). Meanwhile, environmental factors (such as pH, soluble chemical oxygen demand and heavy metals) were also strongly correlated with ARGs.