Fermenter-methanogen syntrophy
Fermenter-methanogen syntrophy is a type of H2-coupled syntrophy, in which H2 is produced by fermenters and consumed by methanogens. Fermenters produce H2, when intracellular substrates for fermentation are exhausted; electrons are then transferred from reduced coenzymes to H+ to yield H2. However, this reaction is endergonic and tends to be reversed under natural conditions, leading to the accumulation of reduced coenzymes and resulting growth inhibition. Methanogens benefit from fermenter-produced H2 as an energy source, resulting in the alleviation of the H2-inhibition on the fermenters. Organics are convertedd to methane by the syntrophy.
Syntrophy of Thermosipho globiformans (fermenter ) and Methanocaldococcus jannaschii (methanogen )1
A model 3-member syntrophic system
Thermotogales (bacterial fermenter) have been shown to contain many genes of Thermococclaes (archaeal fermenter)2. We propose a model 3-member syntrophic system to explain this phenomenon. The metabolisms of the two fermenters are not complementary to each other; they produce H2, which inhibits their growth. However, If they build syntrophic relationships with a common methanogen, the H2 is consumed by the methanogen so that they can get close to each other. The methanogen promotes the interaction between them, but may not be essential for their interaction. Conceptually, this relationship may not be restricted to that of Thermococcales and Thermotogales, but may be applicable to all fermentes that are involved in the fermenter-methanogen syntrophy. It could compromise the apparently contradicting two hypotheses for the birth of the first eukaryote cell, the syntrophy hypothesis3 and the eocyte hypothesis4.
References
1) Igarashi, K. and Kuwabara, T. (2013) Submitted.
2) Zhaxybayeva, O. et al. (2009) Proc. Natl. Acad. Sci. USA, 106, 5865-5870.
3) Martin, W. (2005) Curr. Opin. Microbiol., 8, 630-637.
4) Rivera, M.C. and Lake, J.A. (2004) Nature, 431, 152-155.