Since the general recognition of the Archaebacteria, research into the evolution, metabolism, molecular biology and ecological roles of these fastidious anaerobes has proceeded at an ever-increasing pace. All possess a very novel biochemistry and many exploit unique ecological niches. Methanogens, which convert one-and-two carbon compounds into the important atmospheric gas methane, are the largest group among the Archaebacteria. Of all microbial groups, methanogens provide perhaps the best opportunity to study evolution because of their phyologenetic diversity and unique...