Genetic systems for hydrogenotrophic methanogens.

Methanogens are obligate anaerobic Archaea that produce energy from the biosynthesis of methane. These lithotrophic microorganisms are widely distributed in oxygen-free environments and participate actively in the carbon cycle. Indeed, methanogenesis plays a major role in the last step of the anoxic degradation of organic substances, transforming acetate, CO(2), and H(2) to methane. The vast majority of the known methanogens are classified as hydrogenotrophic because they use principally H(2) as the electron donor to drive the reduction of CO(2). Unlike many other cultured Archaea, many methanogens thrive in neutral pH, low salinity, and temperate environments. This has been a great advantage in cultivating these organisms in laboratory conditions and in the development of genetic tools. Moreover, the hydrogenotroph Methanococcus maripaludis is currently a model organism among Archaea, not only for its utility in genetic but also for biochemical and physiological studies. Over time, a broad spectrum of genetic tools and techniques has been developed for methanococci, such as site-directed mutagenesis, selectable markers, transformation methods, and reporter genes. These tools have contributed greatly to the overall understanding of this group of microorganisms and the processes that govern its life style. In this chapter, we describe in detail the available genetic tools for the hydrogenotrophic methanogens.