Response of cattle manure anaerobic digestion to zinc oxide nanoparticles: Methane production, microbial community, and functions

The increasing use of zinc oxide nanoparticles (ZnO NPs) as feed additives has raised huge environmental concerns in the anaerobic digestion (AD) of livestock wastes. In this study the 30-day effect of ZnO NPs on AD performance of cattle manure was investigated under optimal conditions (temperature-55 degrees C, initial pH-10, total solids contents-10%), which were obtained from response surface methodology. Results showed that ZnO NPs (5-100 mg/g TS) promoted the accumulation of soluble chemical oxygen demand, soluble protein and polysaccharide, but inhibited their following fermentation and methane production processes. This inhibition became stronger as ZnO NPs concentration increased, leading to the methane production declined by 84.55% (5 mg/g TS), 92.39% (30 mg/g TS), and 93.72% (100 mg/g TS), respectively. High-throughput sequencing results demonstrated that the decrease in the abundances of functional bacteria from 72.11% to 11.24% (in families Ruminococcaceae and Lachnospiraceae), and of methanogens from 96.82% to < 1% (in genus Methanothermobacter) led to poor fermentation and methanogenesis, respectively. Predictive functional analysis indicated that ZnO NPs can enhance abundances of AD-friendly functional genes instead of possible genetic expression, resulting in the low methane production. Our findings highlight a deep understanding of the role of ZnO NPs in cattle manure AD and are critical for guiding feed additive application.