Soil microplastics pollution can reduce viral abundance and have less consistent impacts on bacteria

Viruses play crucial roles for structuring microbial communities and regulating biogeochemical processes. It is yet not clear how viruses interact with their microbial hosts under the stress of emerging contaminants, particularly microplastics (MPs) in soils. This study examined the effects of polyethylene (PE) and polyvinyl chloride (PVC) MPs on soil viruses and bacteria. In a pure culture experiment, soil microcosms in test tubes with only bacterium Pseudomonas fluorescens SBW25, or bacteriophage SBW25Φ2, were exposed PE or PVC pollution. In a microbial community experiment, soils with natural microbes in pots planted with maize were given PE or PVC contamination with a gradient of dosage. In both experiments, PE and PVC significantly reduced viral abundances. However, their effects on bacterial abundances were context-dependent. Bacterial growth in pure culture microcosms was unaffected by MP exposure. In the microbial community experiment, high-dose PE increased bacterial abundance and decreased the virus-to-bacteria ratio (VBR). High-dose PVC markedly reduced bacterial abundance without substantially altering VBR. This study reveals that MP contamination may induce viral inactivation in soil environments, potentially diminishing the top-down control of bacteria by viruses. • Microplastics (PE/PVC) consistently reduce viral abundances in both simplified and complex soil microbial systems. • Bacterial responses to microplastic exposure depend on microplastic type and dosage. • Microplastics may disrupt virus-mediated top-down regulation of soil bacterial populations.