Combined Effects of Microplastics and Heavy Metals, Pesticides, and Antibiotics in Zebrafish (Danio rerio): A Critical Review.

Microplastics (MPs) can modify the partitioning, uptake, and biological effects of co-occurring contaminants, yet zebrafish studies have reported markedly divergent combined responses. This critical review evaluates co-exposure to MPs and metals, pesticides, or antibiotics using two complementary dimensions: confidence in interaction classification and confidence in mechanistic attribution. Across the 86 original research studies, only 12 (13.9%) met the high-confidence criteria for interaction classification and 13 (15.1%) for mechanistic attribution, whereas 28 (32.6%) and 41 (47.7%), respectively, provided limited evidence. Claims of synergy or antagonism are considered robust only when supported by concentration-response data, an explicit additivity model, and quantitative uncertainty analysis. Mechanistic interpretation distinguishes external contaminant partitioning from internal delivery. Sorption to MPs may lower freely dissolved concentrations and produce sequestration, increase internal exposure when ingestion is followed by biologically relevant release, or coexist with stronger toxicity caused by particle-induced sensitization. Across contaminant classes, the observed outcomes are influenced by chemical speciation and ionization, particle size and aging, matrix chemistry, exposure route, gastrointestinal release, tissue burden, and developmental stage. Concentration addition and independent action remain useful reference models, but applications based only on nominal concentrations are difficult to interpret when MPs alter the biologically effective dose, exposure route, or host susceptibility. Future studies should combine complete factorial and concentration-response designs with measurements of freely dissolved, particle-associated, gastrointestinally bioaccessible, and tissue-associated fractions, together with chronic, route-specific exposure and toxicokinetic analyses.