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Nanobubble water (NBW) could improve methane production from anaerobic digestion (AD) of corn straw without secondary contamination. In this study, the effect of carbon dioxide nanobubble water (CO 2 -NBW) volumes (0%, 25%, 50%, 75%, 100%) on methane production from corn straw was investigated. The results showed that addition of CO 2 -NBW could improve methane production and promote substrate degradation in AD process. The highest cumulative methane production of 132.16 mL g -1 VS added was obtained in the 100% CO 2 -NBW added reactor, which was 17% higher than that in the control group. Additionally, the addition of CO 2 -NBW could mitigate the sharp decrease in pH by acting as a buffer. CO 2 -NBW could also enhance microorganism activity throughout the AD process. The electron transport system (ETS) activity was increased by 23%, while the β-glucosidase, dehydrogenase (DHA), and coenzyme F 420 activities were increased by 15%, 23%, and 11%, respectively, at optimum addition of CO 2 -NBW. Meanwhile, addition of CO 2 -NBW accelerated the production and consumption of reducing sugar and volatile fatty acids (VFAs), promoting the reduction rates of TS (Total solid) and VS (Volatile solid)., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)