Origins and diversification of sulfate-respiring microorganisms.

If the diversification of microbial life can be depicted as a single tree, as inferred by comparative sequencing of ribosomal RNAs, this could provide a framework for defining the order of emergence of new metabolic pathways. However, recent recognition that lateral gene transfer has been a significant force in microbial evolution has created uncertainty about the interpretation of taxonomies based on gene sequences. In this context, the origins and evolution of sulfate respiration will be evaluated considering the evolutionary history of a central enzyme in this process, the dissimilatory sulfite reductase. These studies suggest at least two major lateral transfer events during the early diversification of sulfate respiring microorganisms. The high sequence conservation of this enzyme has also provided a mechanism to directly explore the natural diversity of sulfate-respiring organisms using molecular techniques, avoiding the bias of culture-based identification. These studies suggest that the habitat range and evolutionary diversity of this key functional group of organisms is greater than now appreciated.