Biochar pre-conditioning reduces nanoplastic toxicity to plant growth-promoting bacteria

Abstract

Nanoplastics are emerging environmental pollutants that threaten soil microbial communities, especially plant growth-promoting bacteria. Here, we investigate whether sugar maple biochar—widely recognized for its soil amendment benefits—can reduce nanoplastic toxicity. Using confocal microscopy, scanning electron microscopy (SEM), and fluorescence spectroscopy, we characterized the interactions between biochar and nanoplastics and observed extensive nanoplastic aggregation on biochar surfaces. Pre-conditioning nanoplastics with biochar (i.e., allowing nanoplastics to interact with biochar before bacterial exposure) lowered their effective concentration in solution and reduced surface coverage on bacterial cells. Growth assays confirmed that biochar pre-conditioning improved both planktonic and biofilm growth of Pseudomonas defensor, a plant growth-promoting bacteria, at nanoplastic concentrations up to 100 μg mL⁻¹. Our results highlight biochar's potential to sequester nanoplastics and mitigate their toxicity, offering a sustainable strategy for protecting microbial communities in plastic-contaminated soils.