Benzoate Fermentation by the Anaerobic Bacterium Syntrophus aciditrophicus in the Absence of Hydrogen-Using Microorganisms

The anaerobic bacterium Syntrophus aciditrophicus metabolized benzoate in pure culture in the absence of hydrogen-utilizing partners or terminal electron acceptors. The pure culture of S. aciditrophicus produced approximately 0.5 mol of cyclohexane carboxylate and 1.5 mol of acetate per mol of benzoate, while a coculture of S. aciditrophicus with the hydrogen-using methanogen Methanospirillum hungatei produced 3 mol of acetate and 0.75 mol of methane per mol of benzoate. The growth yield of the S. aciditrophicus pure culture was 6.9 g (dry weight) per mol of benzoate metabolized, whereas the growth yield of the S. aciditrophicus-M. hungatei coculture was 11.8 g (dry weight) per mol of benzoate. Cyclohexane carboxylate was metabolized by S. aciditrophicus only in a coculture with a hydrogen user and was not metabolized by S. aciditrophicus pure cultures. Cyclohex-1-ene carboxylate was incompletely degraded by S. aciditrophicus pure cultures until a free energy change (ΔG′) of −9.2 kJ/mol was reached (−4.7 kJ/mol for the hydrogen-producing reaction). Cyclohex-1-ene carboxylate, pimelate, and glutarate transiently accumulated at micromolar levels during growth of an S. aciditrophicus pure culture with benzoate. High hydrogen (10.1 kPa) and acetate (60 mM) levels inhibited benzoate metabolism by S. aciditrophicus pure cultures. These results suggest that benzoate fermentation by S. aciditrophicus in the absence of hydrogen users proceeds via a dismutation reaction in which the reducing equivalents produced during oxidation of one benzoate molecule to acetate and carbon dioxide are used to reduce another benzoate molecule to cyclohexane carboxylate, which is not metabolized further. Benzoate fermentation to acetate, CO(2), and cyclohexane carboxylate is thermodynamically favorable and can proceed at free energy values more positive than −20 kJ/mol, the postulated minimum free energy value for substrate metabolism

Syntrophism among Prokaryotes

The study of pure cultures in the laboratory has provided an amazingly diverse diorama of metabolic capacities among microorganisms, and has established the basis for our understanding of key transformation processes in nature. Pure culture studies are also prerequisites for research in microbial biochemistry and molecular biology. However, desire to understand how microorganisms act in natural systems requires the realization that microorganisms don t usually occur as pure cultures out there, but that every single cell has to cooperate or compete with other microor macroorganisms. The pure culture is, with some exceptions such as certain microbes in direct cooperation with higher organisms, a laboratory artifact. Information gained from the study of pure cultures can be transferred only with great caution to an understanding of the behavior of microbes in natural communities. Rather, a detailed analysis of the abiotic and biotic life conditions at the microscale is needed for a correct assessment of the metabolic activities and requirements of a microbe in its natural habitat

Methanogens: Syntrophic Metabolism

Syntrophy is a mutualistic interaction in which two metabolically different types of microorganisms are linked by the need to keep metabolites exchanged between the two partners at low concentrations to make the overall metabolism of both organisms feasible. In most cases, the cooperation is based on the transfer of hydrogen, formate, or acetate from fermentative bacteria to methanogens to make the degradation of electron-rich substrates thermodynamically favorable. Syntrophic metabolism proceeds at very low Gibbs’ free energy changes, close to the minimum free energy change needed to conserve energy biologically, which is the energy needed to transport one proton across the cytoplasmic membrane. Pathways for syntrophic degradation of fatty acids predict the net synthesis of about one-third of an ATP per round of catabolism. Syntrophic metabolism entails critical oxidation-reduction reactions in which H2 or formate production would be thermodynamically unfavorable unless energy is invested. Molecular insights into the membrane processes involved in ion translocation and reverse electron transport revealed that syntrophs harbor multiple systems for reverse electron transfer. While much evidence supports the interspecies transfer of H2 and formate, other mechanisms of interspecies electron transfer exist including cysteine cycling and possibly direct interspecies electron transfer as electric current via conductive pili or (semi)conductive minerals