Syntrophy and interspecies hydrogen transfer
Syntrophy is the name given to the association between two species of microorganisms, growing in the same culture environment, where each exhibits growth characteristics that depend on the presence of the other organism. The monograph by Fenchel and Finlay describes a number of examples.  In the reticulo-rumen there are cases where methanogens grow in syntrophic relations with other species of microorganisms. The methanogens depend on the hydrogen and carbon dioxide produced by other species and some of these other species (such as Ruminococcus) grow better in the presence of the methanogens because of the altered patterns of redox balance associated with reduced partial pressure of hydrogen in the growth environment due to interspecies hydrogen transfer.