Syntrophic communities contain two or more species (usually single-celled organisms) that are able to degrade and survive on compounds that no single organism could, and feed off each other's waste products in a relatively efficient cycle. Only a small external energy input is required to maintain the cycle continuously, though cellular reproduction is severely hindered. These communities exist where food and energy sources are extremely limited so some of the species involved have evolved unique biochemical pathways and relationships for their survival. Pelotomaculum thermopropionicum is capable of consuming and deriving energy by digesting propionate and many alchohols that ordinarily require an energy input to break down. The waste products from the break down of these alcohols are acetate and hydrogen gas. If the hydrogen gas waste was allowed to build up in its environment, P. thermopropionicum's metabolism would be disrupted and it would not be able to survive. However, P. thermopropionicum lives in syntrophic harmony with Methanothermobacter thermautotrophicus which rapidly consumes hydrogen to produce methane.
"The microorganisms that form methane are physiologically specialized. They can grow only with a few simple substrates, such as H2 and CO2, formate, methanol and acetate, and they therefore depend on other microorganisms that degrade more complex organic compounds for substrate supply...In these communities, reducing equivalents are transferred between bacteria and archaea by shuttle components in a process known as inter-species electron transfer...Neither the archaeon nor the bacterium alone can degrade specific organic compounds; for the degradation of these compounds and growth both microorganisms are essential." (Stams and Plugge 2009:568)
"[A]naerobic microorganisms can use protons as terminal electron acceptors for the oxidation of organic compounds, forming hydrogen. Hydrogen formation is a simple redox reaction, which nevertheless requires enzymes with complex active centres...Hydrogen consumption by methanogens lowers the hydrogen partial pressure, making the conversion exergonic, thus enabling proton reduction and energy conservation." (Stams and Plugge 2009:569) "At low hydrogen partial pressure, NADH oxidation and reduced ferredoxin oxidation coupled to proton reduction is energetically feasible, resulting in hydrogen as the sole reduced product...methanogens are essential to maintain the low concentrations of hydrogen that make the reaction sufficiently exergonic to support energy conservation, cell maintenance and growth." (Stams and Prugge 2009:570)