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Microplastics (MPs) are a threat of growing concern for living organisms as they exist in all ecosystems. The bidirectional communication between the gut, its microbiota, and the liver, has been conceptualized as gut-liver axis and may be influenced by environmental factors. MPs can cause intestinal and hepatic injuries, but there is still limited research exploring their impact on gut-liver axis. The aim of this study was to assess the effects of MP ingestion on gut-liver axis balance in gilthead seabream (Sparus aurata) fed with a diet enriched with polystyrene (PS)-MPs (0, 25, or 250 mg/kg b.w./day) for 21 days. PS-MPs affected the composition of gut microbiota, enhancing the evenness of gut microbial species. We also observed the impoverishment of core microbiota, suggesting reduced stability and permanence of microbiota members. Furthermore, PS-MPs reduced predominant bacteria in the gut of gilthead seabreams, increasing low-abundance species, including potential harmful taxa. On the other hand, PS-MPs increased the gene expression of immune and inflammatory mediators (i.e., TLR2, TLR5, and COX-2) in the liver. PS-MP exposure also increased serum triglycerides and bile acids (BAs) without modifying cholesterol. Moreover, the hepatic BA metabolism was impacted by PS-MPs which increased the expression of genes involved in primary BA kinetic (i.e., CYP27A1 and LXRa), which in turn can modulate intestinal microbial community. Indeed, PICRUSt2 mapping of BA-related functions predicted the increase of factors involved in BA metabolism. Specifically, K01442 (choloylglycine hydrolase) and K00076 (7α-hydroxysteroid dehydrogenase) were augmented by PS-MPs, suggesting a possible adaptation or co-evolution of gut microbiota to the modified hepatic BA metabolism. Thus, the obtained results showed that ingested PS-MPs impact the gut microbiota architecture and functions, the hepatic innate immunity, and the BA metabolism, suggesting the involvement of the gut-liver axis in MP-induced toxicity., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)