Dietary HDPE Microplastics Modify Copper Toxicity in Arabian Toad Tadpoles

Microplastics (MPs) and metals can co-occur in amphibian habitats, but their combined effects remain poorly understood. We tested whether dietary high-density polyethylene (HDPE) MPs modified chronic copper (Cu) toxicity in Arabian toad (Sclerophrys arabica) tadpoles. Gosner stage-24 tadpoles were exposed for 28 days to nominal waterborne Cu²⁺ (10 or 100 μg/L), MPs mixed with food, Cu and MP co-exposures, or MPs pre-treated with Cu. Growth (length, mass, and condition) and movement (distance travelled, time motionless, fractal path) were measured after seven, 14, and 28 days, and whole-body Cu burden and metallothionein (MT) were determined at the end of the experiment. Cu-only exposure produced concentration-dependent reductions in growth and altered locomotor activity. MPs alone did not produce sustained effects on growth or behavior. At day 28, composite water samples from the combined Cu-MP treatments contained less Cu than the corresponding Cu-only samples. Whole-body Cu burden was elevated in all high-Cu treatments, whereas MT was highest in the 100 μg/L Cu-only treatment. Tadpoles exposed only to Cu-pre-treated MPs showed higher mass but lower activity than controls at some time points. Under the laboratory conditions tested, dietary HDPE MPs were associated with a reduction of some Cu-associated sublethal responses, suggesting an antagonistic interaction between the MPs and Cu. Still, more work is required to establish this relationship under field conditions.