Methanogens: from their ecological study to genome analysis
Yoichi Kamagata
National Institute of Bioscience and Human Technology
Agency of Industrial Science and Technology, Tsukuba, Ibaraki 305-8566, Japan
Methane is a utilizable resource if it is properly collected such as in natural gas fields and anaerobic waste (water) treatment processes, but it becomes a greenhouse effect gas once it is emitted into the atmosphere. Most of the naturally occurring methane is derived from methanogenic degradation of organic compounds by methane producing Archaea. Methanogens are characterized by their ability to produce methane and play an important role in global carbon cycle in natural environment. All of the methanogens described so far have been found in a wide variety of anaerobic environments-from animal intestines, wetlands, anaerobic waste (water) treatment plants to extremely high temperature environments such as hot spring and the deep ocean floor. They are strictly anaerobic and phylogenetically placed in the kingdom Euryarchaeota in the domain Archaea. Habitats such as extremophilic environments remind us of the ancient earth environment where plenty of gaseous substrates were available for methanogens. How the origin of life looked like and how the ancient microbes gained energy are still enigmatic, but methanogens are believed to be one of the descendants which had lived in the environments reflecting the early earth.
One of the striking features of methanogens is the fact that most of them are capable of utilizing only very limited number of substrates. Methanobacterium thermoautotrophicum, for instance, is able to use only H2/CO2 and formate as carbon and energy source. Although some methanogens such as Methanosarcina are a little bit more versatile utilizing methyl compounds, acetate, and H2/CO2, most of methanogens have much narrower substrate utilization range.
Biochemistry on methanogenesis was very much enjoying its brilliant era in 1980's through 1990's. Methane formation from H2/CO2 involve a number of complicated steps, and most of the enzymes and cofactors involved are quite unique among Prokaryotes. Recent complete genome sequencing has also convinced us of the uniqueness of this Archaea.
Ecologically, methanogens are responsible for the final step of degradation of organic materials in anaerobic environment. Most of heterotrophic microbes involved in upstream reactions produce hydrogen as one of the endproducts which is subsequently converted to methane by hydrogenotrophic methanogen.
