Interspecies electron transfer in methanogenic propionate degrading consortia

Propionate is a key intermediate in the conversion of complex organic matter under methanogenic conditions. Oxidation of this compound requires obligate syntrophic consortia of acetogenic proton- and bicarbonate reducing bacteria and methanogenic archaea. Although H2 acts as an electron-carrier in these consortia, evidence accumulates that formate plays an even more important role. To make energy yield from propionate oxidation energetically feasible for the bacteria and archaea involved, the concentrations of H2 and formate have to be extremely low. On the other hand, the diffusion distance of these carriers has to be small to allow high propionate conversion rates. Accordingly, the high conversion rates observed in methanogenic bioreactors are due to the fact that the propionate-oxidizing bacteria and their methanogenic partners form micro-colonies within the densely packed granules.