The process in which organic matter is degraded anaerobically by cooperation of several organisms in which some produce H2 and some use H2.

An interesting phenomenon in the bacterial methanogenesis (the consumption of material and release of methane gas) is the coupling of two bacterial classes. The breakdown of complex biological polymers such as cellulose or protein into methane goes through many stages, each of which has specialized microbes that carry it out. H2 producing fatty acid oxidizing bacteria (also known as syntrophs) take fatty acids or alcohols generated by other microbes and use them as growth substrates. These bacteria release H2 during breakdown of the fatty acid which is then used as an energy source for methanogens or sulfate-reducing bacteria. When syntrophs are oxidizing the fatty acids into carbon dioxide and H2, the production of ATP is coupled to this reaction. Thus, buildup of H2 would make this process less efficient. The methanogens and sulfate-reducing bacteria are therefore helpful to the syntrophs, as they consume the excess hydrogen and convert it to methane. A great example of symbiosis.