Mercury (Hg) is considered by the World Health Organization as one of the top ten chemicals of major public health concern. Its organic form, monomethylmercury (MMHg), poses the highest risk as it can be biomagnified in aquatic systems resulting in elevated exposure among fish and rice consumers. The gut microbiome was identified as a potential factor for the variations among individuals in Hg uptake. However, little is known about the role of gut microbiota in the gastrointestinal tract on Hg transformations and bioavailability. Current literature has shown that the human gut microbiome can demethylate MMHg, however, no known demethylating genes or microbial guilds were identified. Our goal is to determine if the alteration of the gut microbiome by diet amendment would enhance MMHg demethylation. Using a series of batch experiments, we evaluated the effects of changing nutritional component of the diet (by altering relative abundances of carbohydrate or protein) in the gut microbial community structure of two individuals. We tracked Hg methylation and demethylation rates using stable isotope tracers and conducted high throughput 16S rRNA amplicon sequencing to determine the microbial community structure. We observed > 90% decrease in initial MMHg concentration in protein-rich diet mixed with the fecal slurry of individual A; individual B gut microbiota did not exhibit such demethylation trend. No methylation was observed in any treatment. We followed up this experiment by mixing in different ratios of the fecal microbiota of individual A and B. The MMHg demethylation phenotype was conserved, suggesting individual A contains microbiota element necessary for MMHg demethylation that individual B does not possess. Lastly, to identify microbes involved in demethylation, we performed a series of experiments using inhibitors of 2 known demethylation reactions (i.e., sulfate reduction, methanogenesis). None of the inhibitors tested prevented demethylation indicating a new mechanism is involved. Our results indicate that the gut microbiome of individual A contains a microbiota required for demethylation of MeHg that individual B lacks. A comparative metagenomic analysis between the different nutritional amendment and individuals is underway. Our results are important in the understanding of the mechanisms for MMHg demethylation in the human gut and the variations in MMHg absorption and toxicity.