Effects of Different Microplastics on Methane Production and Microbial Community Structure in Anaerobic Digestion of Cattle Manure

Microplastics (MPs) are widely distributed in the environment, and they inevitably enter animal bodies during livestock and poultry farming, leading to their presence in livestock and poultry manure. However, there is limited research on the effects of different types of MPs on the anaerobic digestion (AD) performance of livestock and poultry manure. Herein, we investigated the impact of four types of MPs (polyvinyl chloride (PVC), polypropylene (PP), polyethylene (PE), and polyhydroxyalkanoate (PHA)) on AD performance using cattle manure as a substrate. Results demonstrated that the cumulative methane production in the PE group reached 5568.05 mL, exhibiting an 11.97% increase compared to the control group. Conversely, the cumulative methane production was decreased by 5.52%, 9.69%, and 14.48% in the PP, PVC, and PHA groups, respectively. Physicochemical analyses showed that MPs promoted organic matter hydrolysis on day 4 of AD, leading to the accumulation of volatile fatty acids (VFAs) in the initial stage. Specifically, the acetic acid content of PE was 44.48–92.07 mL/L higher than that of the control during the first 8 days. PE MPs also enriched microorganisms associated with methane production. The abundance of Firmicutes was enhanced by 2.89–17.57%, Methanosaeta by 8.42–12.48%, and Methanospirillum by 10.91–16.89% in comparison to the control; whereas PHA MPs decreased the abundance of Methanosaeta by 8.14–31.40%. Moreover, PHA MPs inhibited methane production by suppressing acetate kinase activity while promoting lactate dehydrogenase release from microorganisms involved in the AD process. Based on changes observed in key enzyme functional gene abundances, PHA MPs reduced acetyl-CoA carboxylase functional gene abundance, negatively affecting the acetone cleavage methanogenesis pathway. Meanwhile, PE MPs significantly increased acetate-CoA ligase abundance, thereby promoting the acetic acid methanogenesis pathway. The results provide novel insights into the influence exerted by MPs on AD performance when applied to livestock manure.