Iron oxide nanoparticles (γ-Fe2O3) combined with microcystin-LR induce severe histopathological damage with potential for recovery in the intestinal tissue of Poecilia reticulata.

Iron oxide nanoparticles (γ-Fe₂O₃) (IONPs) have been explored for environmental remediation, particularly in removing cyanotoxins caused by eutrophication in aquatic systems. This study assessed the intestinal toxicity of microcystin-LR (MIC-LR), a hepatotoxic cyanotoxin, and maghemite-type IONPs, both alone and in combination, in adult female Poecilia reticulata. A total of 300 fish were divided into five groups: control, Fe³⁺ ions (IFe, 0.3 mg/L), IONPs (0.3 mg/L), MIC-LR (1 µg/L), and IONPs + MIC-LR (0.3 mg/L and 1 µg/L, respectively). Environmentally relevant concentrations were used, based on regulatory guidelines and recent ecotoxicological literature. Exposure lasted 14 days, followed by a 14-day post-exposure period in reconstituted water. Intestinal samples were collected on days 3, 7, and 14 of both phases for histological analysis, calculating progressive (HIp), regressive (HIr), and total (HIt) histopathological indices. The histopathological indices on all days of exposure and post-exposure showed a p-value < 0.05, indicating the significant difference in the number of pathologies between the groups and the experimental effects. MIC-LR and IFe groups exhibited severe early damage (day 3), while IONPs-treated groups showed significantly milder effects. On day 7, intestinal lesions worsened in the IFe and MIC-LR groups, while IONPs alone or combined with MIC-LR continued to limit damage. Partial recovery was observed during the post-exposure phase. This study addresses a critical knowledge gap, as few investigations have evaluated the intestinal toxicity of MIC-LR or maghemite-type IONPs, particularly in combination, in addition to being a current topic. These current and relevant data provide new insight into the sublethal effects of environmental pollutants, reinforcing the importance of assessing extrahepatic biomarkers. The findings have direct implications for ecotoxicology and public health, given the global increase in nanoparticle use and cyanotoxin contamination.