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 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.