Syntrophic H₂ Producing Bacteria - This group is also called obligate H₂ producing or obligate proton reducing bacteria since they oxidise NADH by reducing H+ to H₂ and thereby produce hydrogen. These bacteria breakdown organic acids having greater than two carbon atoms in their chain to produce acetate, CO₂ and H₂.

However, they are able to grow freely and produce H₂ only under low H₂ partial pressure, which is maintained by methanogens. Sewage sludge digesters have about 4 x 106 cells/ml of this group. Examples of these bacteria are Syntrophomonas wolfei, and S. wolinii.