Bioaccumulation dynamics of silver nanoparticles: A comparative study across particle sizes and phytoplankton species

Silver nanoparticles (AgNPs), widely used in consumer products, are inevitably released into aquatic environments. In the water column, AgNPs undergo physicochemical transformations, notably oxidative dissolution, releasing ionic silver (Ag⁺). Both nanoparticulate and ionic Ag species can be toxic to phytoplankton, primarily through bioaccumulation. However, the mechanisms governing Ag bioaccumulation remain poorly understood due to the coexistence of multiple Ag forms and particle sizes, as well as the structural diversity across phytoplankton communities. In this study, we employed a recently proposed methodology to investigate the interactions of Ag⁺ and AgNPs (5 and 20 nm) with two freshwater phytoplankton species exhibiting distinct cellular architectures: the green alga Chlamydomonas reinhardtii and the diatom Cyclotella meneghiniana. Bioaccumulation and adsorption kinetics varied with Ag form, particle size, and cellular architecture. Rapid Ag bioaccumulation was observed during exposure to Ag⁺, whereas accumulation under AgNP exposure was more gradual and sustained. Smaller nanoparticles (5 nm) resulted in higher Ag bioaccumulation in both organisms, with greater dissolution and stronger cell adsorption compared to 20 nm AgNPs. The diatom exhibited higher uptake rates and Ag association than the green alga, consistent with transmission electron microscopy (TEM) observations. AgNPs were embedded in the frustule and detected within cellular compartments of C. meneghiniana, while, in C. reinhardtii, they were mainly observed as aggregates outside the cells, associated with the cell wall and palmelloid envelope. We demonstrate that Ag bioaccumulation is strongly influenced by silver speciation, particle size, and cellular architecture, highlighting the need to integrate these factors in environmental risk assessments.