Ancient origin and evolution of microbial mercury methylation

The origin and evolution of microbial mercury methylation have long remained a mystery. Here we employed genome-resolved phylogenetic analyses to decipher the evolution of the mercury methylating genes hgcAB, to constrain the ancestral origin of the hgc operon, and to explain its current distribution across the bacterial and archaeal domains. We infer the extent to which vertical descent and horizontal gene transfer have influenced the evolution of this operon and hypothesize that the original function of methylmercury was as an antimicrobial compound on the early Earth. We speculate that subsequent evolution of the detoxifying agent alkylmercury lyase (merB) reduced the selective advantage of the mercury methylation activity, resulting in widespread loss of the hgcAB genes among archaea and bacteria. ### Competing Interest Statement The authors have declared no competing interest.