Nano-selenium ameliorates microplastics-induced injury: Histology, antioxidant capacity, immunity and intestinal microbiota of grass carp (Ctenopharyngodon idella)

Microplastics (MPs) are pollutants widely distributed in the aquatic environments and causing various degrees of aquatic toxicity to aquatic organisms, which has attracted global attention in recent years. Nano-selenium (NSe) has been shown to have the potential to mitigate the harmful impacts of toxic substances. However, there is currently no reported evidence regarding the protective influence of NSe against the adverse effects of MPs. The aim of this study is to determine whether NSe could ameliorate the polystyrene (PS)-MPs-induced injury in grass carp (Ctenopharyngodon idella). The individuals of grass carp were assigned into three groups: (1) the control group fed with basal diet, (2) the PS group fed with basal diet and exposed to PS-MPs, and (3) the NSe group fed with diet supplemented with NSe and exposed to PS-MPs. Our results indicated that NSe administration significantly alleviated the histological damage caused by the PS-MPs in the liver and intestine with lower goblet cell count and larger villus height in the intestine, and significantly lower damage score in the liver. Moreover, NSe mitigated PS-MPs-induced oxidative stress through restoring the activities of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), and malondialdehyde (MDA)) except the intestinal CAT activity. Furthermore, NSe supplementation could help fish maintain lower transcriptional level of the immune-related genes (Toll-like receptor 4 (TLR4) and myeloid differentiation factor 88 (MyD88)), inflammation-related genes (major histocompatibility complex class II (MHC-II) and interleukin 8 (IL-8)) and antioxidant enzyme-related genes (nuclear factor (erythroid-derived 2)-like 2 (Nrf-2) and kelch-like ECH-associated protein 1 (Keap-1)) after PS-MPs exposure. Besides, NSe supplementation dramatically helped maintain the intestinal microbial composition, for example, the proportion of Proteobacteria in the grass carp intestine of the NSe group (41 %) was similar to that of the control group (34 %) while 85 % of the PS group. NSe also played a significant protective role in intestinal microbial diversity, effectively resisting the damage on intestinal microbial diversity due to PS-MPs exposure. PS-MPs reduced the beneficial bacteria and increased the pathogenic microorganism like Aeromonas, which was undeniable signs of intestinal dysbiosis. Functional analysis indicated that PS-MPs affected intestinal microbiota functions like inhibition of metabolism, while NSe could significantly alleviate the damage. Our findings suggested that NSe could ameliorate PS-MPs-induced injury, which could contribute to the better understanding of the ecotoxicological effects of MPs on fish and help develop relevant mitigation strategies.