SRP404929 PRJNA895066 Black Sea water microbiome Methylmercury (MeHg) is a neurotoxin detected at high concentrations in certain marine ecosystems, posing a threat to human health. MeHg production is mainly mediated by hgcAB gene-carrying (hgc+) microorganisms. Oxygen is one of the main factors controlling Hg methylation, however its effect on the diversity and ecology of hgc+ microorganisms is still unclear. Being the largest anoxic basin worldwide, with high MeHg concentrations, the Black Sea is an ideal natural laboratory to provide new insights about the link between dissolved oxygen concentration and microorganisms involved in Hg methylation. We combined geochemical and microbial data to assess the abundance, diversity and metabolic potential of hgc+ microorganisms along the vertical redox gradients of the Black Sea water column.