Horizontal gene transfer in metal and radionuclide contaminated soils.

The horizontal transfer of genes encoded on mobile genetic elements (MGEs) such as plasmids and phage and their associated hitchhiking elements (transposons, integrons, integrative and conjugative elements, and insertion sequences) rapidly accelerate genome diversification of microorganisms, thereby affecting their physiology, metabolism, pathogenicity,and ecological character. The analyses of completed prokaryotic genomes reveal that horizontal gene transfer (HGT) continues to be an important factor contributing to the innovation of microbial genomes. Indeed, microbial genomes are remarkably dynamic and a considerable amount of genetic information is inserted or deleted by HGT mechanisms. Thus, HGT and the vast pool of MGEs provide microbial communities with an unparalleled means by which to respond rapidly to changing environmental conditions and exploit new ecological niches. Metals and radionuclide contamination in soils, the subsurface, and aquifers poses a serious challenge to microbial growth and survival because these contaminants cannot be transformed or biodegraded into non-toxic forms as often occurs with organic xenobiotic contaminants. In this chapter we present cases in which HGT has been demonstrated to contribute to the dissemination of genes that provide adaptation to contaminant stress (i.e., toxic heavy metals and radionuclides). In addition, we present directions for future studies that could provide even greater insights into the contributions of HGT to adaptation for survival in mixed waste sites.