Impact of Exposure of Dairy Cow Feed to Polystyrene Microplastics on 24 h In Vitro Rumen Fermentation Responses, Microbiota Biodegradation Potential and Metabolic Pathways

Microplastic pollution is rising worldwide and may harm both animals and people. Dairy cows rely on a large stomach compartment, called the rumen, to break down feed with the help of many microbes, and this process produces gases and acids that reflect healthy digestion. In this study, we tested whether tiny plastic particles made of polystyrene, in different sizes and amounts, change rumen digestion and whether rumen microbes can break these plastics down. We found that exposure to these particles reduced fermentation activity, shown by lower gas production and lower levels of volatile fatty acids, and the negative effects became stronger as the amount of microplastics increased. At the same time, higher amounts and larger particles increased the activity of some digestive enzymes, suggesting a stress response in the microbial system. Microplastics also lowered the abundance of certain microbes and changed the chemicals produced during digestion. After 24 h, the amount of microplastic particles decreased, indicating that rumen microbes can partially degrade them and generate small chemical products that could further disturb rumen balance. These findings help explain how microplastics may influence ruminant health and suggest a possible biological route to reduce microplastic contamination. Microplastic pollution is increasingly serious worldwide, threatening human and animal health. The cow rumen is a key organ for nutrient digestion and absorption, and its fermentation is closely related to rumen microorganisms. Here, we investigated how polystyrene microplastics (PS-MPs) with varying particle sizes and concentrations affect rumen fermentation and the biodegradability of PS-MPs by rumen fermentation. The results reveal that exposure to PS-MPs lowered gas production and gas concentrations, as well as volatile fatty acid content, and these decreases were positively correlated with PS-MP concentration. However, higher PS-MP concentration and larger particle size increased the activity of carboxymethyl cellulose, β-glucosidase, and xylanase. Furthermore, PS-MP exposure reduced the abundance of certain rumen microorganisms and altered metabolic pathways and metabolites linked to PS-MP biodegradation. It was also found that PS-MP content decreased significantly after 24 h fermentation. Therefore, PS-MPs can inhibit rumen fermentation by affecting the rumen microbiome, and rumen microorganisms and their secreted enzymes can biodegrade PS-MPs to produce styrene and derivatives; such small molecules may further disrupt rumen homeostasis, thereby affecting lactation performance. In addition, rumen microbial degradation of PS-MPs provides a new idea to resolve future microplastic contamination challenges.