Polystyrene Nanoplastics Regulate Silicon Cycling and Biosilica Deposition in Marine Synechococcus

Nanoplastics are emerging pollutants with the potential to disrupt the microbial physiology and biogeochemical cycles in marine ecosystems. However, their influence on silicon cycling in cyanobacteria remains poorly understood. Here, we investigate how amine-modified polystyrene nanoplastics (PS-NH₂) regulate silicon transport and biosilica deposition in Synechococcus sp. CC9311, a key contributor to oceanic primary production. At an environmentally relevant concentration (0.1 μg/mL), PS-NH₂ primarily modified the cell surface microenvironment, which significantly increased the level of extracellular biosilica deposition. In contrast, higher PS-NH₂ concentrations (0.5-2.0 μg/mL) induced oxidative stress and membrane damage, leading to a shift toward enhanced intracellular silicon accumulation. Notably, at 2.0 μg/mL, the oxidative damage was the most severe, resulting in the most significant intracellular silicon accumulation compared to 0.5 and 1.0 μg/mL. Transcriptomic analysis revealed that PS-NH₂ exposure downregulated genes involved in energy metabolism and photosynthesis while upregulating stress response pathways, suggesting that silicon accumulation may serve as a protective mechanism against PS-NH₂-induced cellular stress. These findings provide novel insights into the interplay between nanoplastics and microbial silicon metabolism, highlighting a previously unknown pathway by which plastic pollution could influence silicon biogeochemistry in marine ecosystems.