Biodegradable polylactic acid microplastics enhance methylmercury production in soils and promote mercury accumulation in rice grains in Northeastern China

Abstract

Microplastics (MPs), as emerging pollutants, have been accumulating in historically Hg-polluted paddy fields. However, the effects of MPs on mercury (Hg) methylation and subsequent accumulation in rice organs have remained poorly understood. We conducted controlled pot experiments to evaluate the impacts of replacing conventional polyvinyl chloride (PVC) with biodegradable polylactic acid (PLA) on Hg methylation processes in soil-rice systems. Results revealed that the addition of PLA-MPs significantly enhanced methylmercury (MeHg) formation in soil, leading to substantially higher Hg accumulation in rice grains. The average grain Hg concentration reached 130.46 μg∙kg^-1 under PLA treatment - approximately 6-folds higher than in the controls. While MPs amendment reduced bacterial α-diversity (Shannon index), it did not substantially alter the overall microbial community composition. Notably, PLA-MPs significantly upregulated Hg methylation gene expression (hgcA and hgcB). Soil pH was significantly and negatively related to hgcA expression. Desulfobacterota was the predominant Hg-methylation taxon across all treatments. Importantly, hgcA and hgcB expression levels showed strong positive correlations with Hg accumulation in rice grains. Our findings emphasize the necessity for comprehensive risk assessment regarding the implementation of biodegradable MPs in Hg-rich agricultural soils, particularly concerning food safety implications stemming from enhanced Hg enrichment in rice grains.