Polyethylene microplastics affect osmoregulation, overall metabolism and induce anxiety in zebrafish

Polyethylene microplastics (PE-MPs) are widespread aquatic pollutants whose effects on biota vary across species. In fish, MPs can damage gill and digestive structures, with deleterious consequences for health. However, the mechanisms of action and associated physiological responses remain poorly understood. This study examined the effects of an ecologically relevant exposure to 250 μg L^-1 PE-MPs on zebrafish over a period of 15 days. PE-MPs were detected and physically retained on key osmoregulatory tissues (gills and intestine), maintaining stable tissue-associated concentrations throughout the exposure period. After 7 days, disturbances in water-ion balance were evident, including increased muscle water content. These imbalances persisted through day 15, and were accompanied by elevated gill ATPases activity and increased skin permeability. Plasma analysis revealed increased chloride and phosphate levels. As a compensatory response, oxygen consumption and hematocrit increased, indicating heightened metabolic demand. These physiological changes were associated with behavioral alterations indicative of increased anxiety in an open field test. Collectively, these findings demonstrate a cascade of disruptions in ion regulation, metabolism, and behavior caused by PE-MPs exposure, ultimately compromising fish well-being. The methodologies used here may facilitate the assessment of similar contaminants across species and contribute to the standardization of toxicological studies, an essential need given the diversity and methodological variability of current research in this field.