Anaerobic syntrophic associations of fermentative bacteria and methanogenic archaea operate at the ther-modynamic limits of life. The interspecies transfer of electrons from formate or hydrogen as a substrate for the methanogens is key. Contrary requirements of syntrophs and methanogens for growth-sustaining product and substrate concentrations keep the formate and hydrogen concentrations low and within a narrow range. Since formate is a direct substrate for methanogens, a niche for microorganisms that grow by the conversion of formate to hydrogen plus bicarbonate—or vice versa—may seem unlikely. Here we report experimental evidence for growth on formate by syntrophic communities of (i) Moorella sp. strain AMP in coculture with a thermophilic hydrogen-consuming Methanothermobacter species and of (ii) Desulfovibrio sp. strain G11 in coculture with a mesophilic hydrogen consumer, Methanobrevibacter arboriphilus AZ. In pure culture, neither Moorella sp. strain AMP, nor Desulfovibrio sp. strain G11, nor the methanogens grow on formate alone. These results imply the existence of a previously unrecognized microbial niche in anoxic environments. Much attention is paid to the environmental conditions that limit microbial growth and activity (24, 30, 31), such as high salt concentrations, high pressure, high and low pHs, high and low temperatures, and combinations thereof (3, 11, 23, 30, 32, 42). Less attention has been given to the thermodynamic limits of microbial life, although these are the most fundamental limits for any life form (19). These limits are approached in methan-ogenic environments, where syntrophic associations of anaer-obic bacteria and methanogenic archaea obtain energy for growth from catalyzing pathways that operate close to thermo-dynamic equilibrium (⌬G, ϳ0 kJ/mol) (20, 38). Methanogenic communities are generally schematized as four different func-tional groups (or guilds) of bacteria and archaea. Primary fermenters convert complex material into substrates for a group of secondary fermenters, also known as syntrophs. The syntrophs obligately depend on two groups of methanogens, one that uses hydrogen and formate and another that uses acetate (9, 38). For thermodynamic reasons, growth of the syntrophs is sustainable only through the removal of their waste products by the methanogens. Hydrogen is the main electron carrier in such syntrophic associations, but formate is important too, especially in associations where electron fluxes are high (5, 8, 41). It is assumed that formate and hydrogen are in thermodynamic equilibrium (26, 44) (Table 1), but this is not always the case. For instance, measurements in a shallow methanogenic aquifer in Denmark have indicated a potential energy gain of 5 to 10 kJ/mol electrons for the conversion of formate to H 2 and bicarbonate (14). This implies a previously unrecognized niche for organisms that are able to catalyze this reaction. Hydrolytic cleavage of formate to H 2 and bicarbonate has been described before (2, 7, 12, 29), but it has never been shown before that this can be coupled to growth (Table 1). Formate hydrogen lyase has been proposed to be coupled to energy conservation (15). Guyot and Brauman have reported formate-based coupling between a sulfate reducer and a non-formate-using methanogen, but growth was not demonstrated (12). Here we describe experiments that show that bacteria are able to grow by the conversion of formate to H 2 and bicarbon-ate, provided that hydrogen is consumed by a methanogen.
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