text: Syntrophy, in the context of microbial metabolism, refers to the pairing of multiple species to achieve a chemical reaction that, on its own, would be energetically unfavorable.  The best studied example of this process is the oxidation of fermentative end products (such as acetate, ethanol and butyrate) by organisms such as Syntrophomonas.  Alone, the oxidation of butyrate to acetate and hydrogen gas is energetically unfavorable.  However, when a hydrogenotrophic (hydrogen-using) methanogen is present the use of the hydrogen gas will significantly lower the concentration of hydrogen (down to 10 atm) and thereby shift the equilibrium of the butyrate oxidation reaction under standard conditions (ΔGº’) to non-standard conditions (ΔG’). Because the concentration of one product is lowered, the reaction is "pulled" towards the products and shifted towards net energetically favorable conditions (for butyrate oxidation: ΔGº’= +48.2 kJ/mol, but ΔG = -8.9 kJ/mol at 10 atm hydrogen and even lower if also the initially produced acetate is further metabolized by methanogens). Conversely, the available free energy from methanogenesis is lowered from ΔGº’= -131 kJ/mol under standard conditions to ΔG = -17 kJ/mol at 10 atm hydrogen. This is an example of intraspecies hydrogen transfer.  In this way, low energy-yielding carbon sources can be used by a consortium of organisms to achieve further degradation and eventual mineralization of these compounds.  These reactions help prevent the excess sequestration of carbon over geologic time scales, releasing it back to the biosphere in usable forms such as methane and .