The mechanism of adaptation to Hg(2+) in four aquatic habitats was studied by correlating microbially mediated Hg(2+) volatilization with the adaptive state of the exposed communities. Structural and functional parameters indicated that adaptation of all four communities was stimulated by exposure to Hg(2+). In saline water communities, adaptation was associated with rapid volatilization after an initial lag period. The mechanism, however, did not promote adaptation in a freshwater environment, where Hg(2+) volatilized slowly, regardless of the adaptive state of the microbial community. Distribution of the mer operon among representative colonies of the communities was not related to adaptation to Hg(2+). Thus, although volatilization is a mechanism which enables some microbial communities to sustain their functions in Hg(2+) stressed environments, it is not coded for by the gene system that mediates the mechanism in pure cultures.