Antibiotic application may raise the potential of methylmercury accumulation in fish.

Mercury (Hg) biotransformation can significantly affect the Hg speciation and bioaccumulation in fish, where gut microbiota play an important role in this process. Antibiotics have been extensively used in aquaculture and can affect gut microbial structure. However, the influence of antibiotics on Hg biotransformation in fish has not been thoroughly understood. The present study investigated the effects of antibiotic (florfenicol) application on gut microbiota and subsequent impacts on Hg biotransformation and bioaccumulation in tilapia (Oreochromis mossambicus). The results showed that the florfenicol treatment did not affect IHg accumulation in the IHg-exposed fish or the MeHg accumulation in the MeHg-exposed fish. However, methylation was significantly weakened (from 0.015% d-1 to 0.005% d-1) and demethylation was completely terminated (from 0.046% d-1 to non-observable level) in the florfenicol-treated fish as compared to the control fish. This can be ascribed to the major shift in the richness of microbial methylators/demethylators in fish gut. Furthermore, florfenicol disturbed the homeostasis of gut microbiome and enhanced the growth of opportunistic pathogens. Our results strongly suggested that antibiotic application significantly altered the gut microbial community, thereby increasing the potential of MeHg accumulation by fish. This study highlights the importance of appropriate use of antibiotics in aquaculture as well as decreasing the environmental risks of Hg contamination in fish.