Toxic effects of long-term polystyrene microplastic exposure on gut microbiota, antioxidant capacity, and digestive enzyme activities in <i>Thamnaconus septentrionalis</i>

The rapid growth of aquaculture has supplied abundant high - quality animal protein, yet microplastics (MPs) - induced food safety issues are gaining concern. Recent evidence highlights MPs’ potential toxicity, underscoring the need to study their environmental and health impacts. This study used 6 - month - old Thamnaconus septentrionalis as test subjects, divided into three groups: CA (control), CB (1μm, 1×10⁷ microspheres·L⁻¹), and CC (5μm, 1×10⁷ microspheres·L⁻¹), where ‘C’ denotes the intestinal tract, and ‘A’, ‘B’, ‘C’ represent treatment groups with different microplastic particle sizes (0, 1, and 5μm, respectively). We set up three repeats per group, exposing 20 individuals of T. septentrionalis to water containing polystyrene microplastics for 30 days for each group. Through 16S rRNA sequencing and enzyme activity assays, we investigated MPs’ effects on T. septentrionalis gut microbiota, antioxidant, and digestive enzymes. Results showed that compared to the control, the 1μm and 5μm groups had slightly higher Shannon indices, but with significant Pseudomonadota enrichment (CB: 47.21%, CC: 61.66%; P &lt; 0.05). Bray-Curtis clustering indicated that microbial composition was significantly related to MPs concentration. Vibrio and Acinetobacter were enriched in exposure groups, while Anaerorhabdus and Lactobacillus decreased. MPs may thus disrupt gut barrier function. Both MPs sizes altered gut microbiota structure, with the 5μm group showing a greater impact. In the 5μm group, superoxide dismutase (SOD) activity increased 4.2 - fold, and a significant elevation in glutathione (GSH) activity (P &lt; 0.05), while catalase (CAT) was significantly inhibited (P&lt;0.05), and amylase (AMS) and lipase (LPS) activities were suppressed. Chronic MP exposure impaired nutrient absorption and antioxidant/digestive functions in T. septentrionalis . These findings elucidate the size-dependent ecotoxicological effects of MPs, providing critical insights for assessing marine MPs pollution risks and informing aquaculture safety protocols.