Methylmercury (MeHg, CH3Hg+) is a potent neurotoxin and bioaccumulates in food webs. Microbial transformation of inorganic mercury produces most of the MeHg in the marine environment. The gene pair hgcAB encodes for Hg methylation, and this process has mainly been attributed to anaerobic bacteria. However, recent studies show the formation of methylmercury in aerobic water columns, although the mechanisms for non-anaerobic mercury methylation remain poorly understood. Our previous work found the marine microaerophilic bacterium Nitrospina as a potential mercury methylator within sea ice. Here, we found Nitrospina-like hgcAB genes were widely distributed in the global oxygen minimum zones (OMZs). We also identified a facultative anaerobe Marinilabilia salmonicolor carrying a putative hgcAB gene cluster that may enable Hg methylation. Furthermore, we found two aerobic Alphaproteobacteria carrying fused hgcAB genes which have very low identities (<40%) to any known fused hgcAB genes. We performed metagenomic analysis of seawater of Antarctic OMZs, to support culturing-based and culture-independent approaches to looking for new genes and pathways for potential non-anaerobic mercury methylation, and to refine existing models for mercury biogeochemical cycling.