Gut microbiota and mercury biotransformation: New perspectives on fish bioaccumulation in China

China is the largest contributor to global mercury (Hg) emissions and the most predominant producer of fish. However, fish Hg concentrations in China are generally maintained at low levels, with localized "hot spots" in specific regions. The biotransformation of Hg, including methylation and demethylation, has been reported for over 50 years and is recognized as a key driver of Hg bioaccumulation in fish. This review provides an overview of Hg accumulation patterns of fish across diverse aquatic ecosystems in China, with focus on how regional heterogeneity in human activities influence Hg speciation, bioavailability, and trophic structure, ultimately contributing to localized "hot spots". We further critically discuss the emerging role of gut microbiota in modulating Hg speciation and toxicity, and try to evaluate the quantitative significance of microbial biotransformation processes in determining spatial and interspecies differences in fish Hg levels in China. The findings highlight the gut microbiota as a critical mediator of Hg biotransformation, which influence its speciation, bioavailability, and ultimate accumulation in fish. Environmental Hg levels (diets and ambient water) and food composition appear to be critical factors affecting the gut microbial composition and biotransformation efficiency, which consequently determine Hg speciation and burdens. Particularly, geographic and species-specific variations in the abundance and activity of Hg-methylating and demethylating microbes contribute to the observed difference in Hg accumulation by fish. The review advances our understanding of Hg accumulation in China's fish from the insight of biotransformation and highlights microbial pathways as key intervention targets for mitigating fish Hg contamination.