Formate- and Sodium Ion-Dependent Wood-Ljungdahl Pathway Involved in Organo-, Litho-, and Methylotrophic Growth of Tepidanaerobacter acetatoxydans.

Methane production by a biogas digester is a promising alternative to the use of fossil fuels as an energy source. One major substrate for methanogenesis is acetate, which is directly used by acetoclastic methanogens. At high pH and high ammonium concentrations, acetoclastic methanogenesis is inhibited, and acetate is oxidized to H2 + CO2, which are used by hydrogenotrophic methanogens. From such a digester, an ammonium-tolerant syntrophic acetate-oxidizing bacterium, Tepidanaerobacter acetatoxydans, was isolated in 2011 by Westerholm and colleagues, but its physiology and role in a biogas digester are still relatively unknown. We demonstrate that T. acetatoxydans is an acetogenic bacterium using an unusual Wood-Ljungdahl pathway (WLP) lacking a formate dehydrogenase. Therefore, T. acetatoxydans cannot ferment glucose, H2 + CO2, or CO + CO2, but the addition of formate restored acetogenesis. T. acetatoxydans could also grow on methyl groups that were oxidized in the WLP, but due to the absence of a formate dehydrogenase, methyl groups were incompletely oxidized to formate instead of CO2 as an end product. This study gives new insights into the physiology of acetogens that lack formate dehydrogenases and sheds new light on the role of T. acetatoxydans in the food web of a biogas digester.