The Hg(2+) resistance mechanism was studied in four freshwater and four coastal marine bacteria that did not hybridize with a mer operonic probe. Inducible Hg(2+)-volatilization was demonstrated for all eight organisms and NADPH-dependent-mercuric reductase activities were detected in crude cell extracts of six of the strains. Hybridization with a merA probe, the gene encoding the mercuric reductase polypeptide, at a stringency permitting hybrid formation between distant merA genes (as exists between gram positive and negative bacteria) detected merA sequences in the genomes of all tested strains. Because these strains represented random selections of bacteria from three aquatic environments, it is concluded that merA encodes a common molecular mechanism for Hg(2+) resistance and volatilization in aerobic heterotrophic aquatic communities.