Stimulation of methanotrophic growth in cocultures by cobalamin excreted by rhizobia.

Methanotrophs play a key role in the global carbon cycle, in which they affect methane emissions and help to sustain diverse microbial communities through the conversion of methane to organic compounds. To investigate the microbial interactions that cause positive effects on methanotrophs, cocultures were constructed using Methylovulum miyakonense HT12 and each of nine nonmethanotrophic bacteria, which were isolated from a methane-utilizing microbial consortium culture established from forest soil. Three rhizobial strains were found to strongly stimulate the growth and methane oxidation of M. miyakonense HT12 in cocultures. We purified the stimulating factor produced by Rhizobium sp. Rb122 and identified it as cobalamin. Growth stimulation by cobalamin was also observed for three other gammaproteobacterial methanotrophs. These results suggest that microbial interactions through cobalamin play an important role in methane oxidation in various ecosystems.