The bioaccumulation characteristics and combined toxicity effects of aged microplastics with adsorbed Hg(II) in oysters

The ubiquitous microplastics (MPs) in aquatic environments were easily accessible to the highly toxic Hg(II) to form a complex co-pollutant scenario. The bioaccumulation patterns and toxicological effects of such Hg(II)-adsorbed MPs complexes were not clearly revealed and require in-depth investigation. This study employed oysters (Crassostrea hongkonggensis) as a model organism to investigate the joint effect of aged Hg(II)-adsorbed polyethylene MPs (A-PEs) and tire MPs (A-TMPs). Under medium (200 μg L) and high (2 mg L) levels, significant accumulation of A-MPs and Hg(II) was observed in the gill and digestive gland, reaching up to 45-57.18 μg g. The oysters possessed a self-protection mechanism that relied on follicular exclusion and digestive encapsulation, expelling portions of A-MPs and Hg(II). The accumulation of Hg(II) and A-MPs featured the pattern of a rapid initial increase, a peak during mid-to-late stages, and eventual stabilization or gradual decline in organ burdens. Moreover, A-TMPs exhibited accumulation levels generally 3-6 times higher than those of A-PEs. More importantly, the bioaccumulated Hg(II) was predominantly derived from the food phase via the significant vector of the MPs based on the toxicokinetic model. Furthermore, medium and high concentrations induced oxidative damage, immune dysfunction, and the downregulation of associated gene expression. It was noteworthy that the oysters showed neither significant accumulation of A-MPs and Hg(II) nor notable alterations in the enzyme activities after 25 days of exposure at an environmentally relevant low level (20 μg L). These findings offered critical insights into the vector effects of MPs and their synergistic toxic mechanisms with metals in filter-feeding organisms.