Interstitial meiofauna, organisms smaller than 500 μm that live between sediment grains, are the most abundant animals on Earth. They play crucial roles in biogeochemical cycles, but their responses to microplastics (MPs) remain understudied. Due to their size, meiofauna may be particularly vulnerable to MPs. We quantified how realistic levels of MP contamination affect bioturbation, oxygen penetration depth (OPD), and diffusive oxygen uptake (DOU) in sediment mesocosms over thirteen days. Bioturbation depth and OPD increased, while DOU decreased across all treatments. However, sediments containing MPs had lower bioturbation depth and slightly higher OPD compared to controls. The reduction in bioturbation was likely due to meiofauna stress, while the highest MP contamination caused increased bioturbation depth, likely due to evasion responses. Increased OPD over time was likely due to reduced labile organic matter. This study highlights how bioturbation, OPD, and DOU shift with MP pollution, confirming MPs' impacts on ecosystem functions.