About 10 to 25 percent of the world's methane emissions come from flooded rice paddies. Methane is a greenhouse gas produced by various groups of microorganisms (methanogenic Archaea). Oxygen is usually highly toxic for these microorganisms. The major producer of methane in the roots of rice plants is what is known as 'Rice Cluster I' (RC-I) Archaea. The mechanisms that give these Archaea a competitive advantage remained unexplained, because it was impossible to get a pure culture of them. Now, scientists from the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany and the Max Planck Institute for Molecular Genetics in Berlin have fully sequenced the genome of an RC-I archaeon from a methane-producing microbial mixed culture. From the genome sequence, the researchers were able to deduce the existence of a number of enzymatic mechanisms, unknown in methanogenic Archaea until now. The mechanisms help the RC-I Archaea to survive when oxygen is present. They allow the RC-I Archaea to adapt specifically to the oxygen-rich area around the roots of the rice plant. The results explain why RC-I Archaea have a selective survival advantage (Science, July 21, 2006).
In the current study, Max Planck researchers from Marburg and Berlin investigated the complete genome sequence of an RC-I archaeon that appears frequently in the mixed culture MRE50. As a rule, the starting point for analysis of a complete microbial genome is a pure culture -- and its corresponding homogeneous component of genetic information. But in the case of RC-I Archaea, no pure culture was available. So all the genetic information of the mixed culture MRE 50 served as the starting point for sequencing the complete RC-I genome. Such heterogeneous genetic information, stemming from various microorganisms in the mixed culture, is called a metagenome.
Contact: Dr. Werner Liesack