Thirty weeks exposure reveals the damaging effects of microplastics: Histology, immunity and intestinal microbiota of grass carp (Ctenopharyngodon idella).
Microplastic (MP) pollution has become a global aquatic environmental issue, yet its long-term toxic effects on aquatic organisms remain poorly understood. In this study, grass carps (Ctenopharyngodon idella) were subjected to thirty weeks dietary exposure to polystyrene microplastics (PS-MPs) to systematically evaluate the effects on histology, immune signaling, and intestinal microbiota. Histopathological examination revealed that PS-MPs exposure induced focal necrosis, increased melanomacrophage centers, and sinusoidal congestion in the spleen, while the intestine exhibited villus fusion, disordered arrangement, epithelial edema, and lymphocyte infiltration. Transcriptomic analysis further identified 2975 differentially expressed genes (DEGs) in the spleen. Functional enrichment analysis revealed that these DEGs were significantly associated with cytokine-cytokine receptor interaction, Toll-like receptor, RIG-I-like receptor, and NOD-like receptor signaling pathways. Notably, the MAPK-associated gene mapkapk3 was significantly upregulated, whereas the immune-regulatory receptor gene IL20RA was downregulated, suggesting disruption of immune homeostasis and remodeling of innate immune signaling. Gut microbiota analysis revealed marked dysbiosis characterized by increased relative abundances of Fusobacteriota (57%) and Proteobacteria (35%), accompanied by pronounced reductions in Bacteroidota and Firmicutes. All alpha diversity indices (Chao1, Ace, Shannon, Simpson) were significantly decreased, and beta diversity analyses (PCA, PCoA, NMDS) demonstrated clear separation between groups. The opportunistic pathogen Aeromonas veronii was significantly enriched and identified as a key biomarker in the PS-MPs group. Collectively, long-term PS-MPs exposure induced structural tissue damage, altered immune-related transcriptional profiles involving mapkapk3 and IL20RA, and promoted gut microbial dysbiosis with enrichment of opportunistic pathogens, thereby potentially increasing host susceptibility to environmental stressors. These findings provide mechanistic insight into the ecological health risks posed by chronic MP exposure in freshwater fish.