Energetics of methanogenic benzoate degradation by Syntrophus gentianae in syntrophic coculture

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Abstract
Summary: Growing cocultures of Syntrophus gentianae with Methanospirillum hungatei degraded benzoate to CH4 and acetate. During growth, the change of free energy available for Syntrophus gentianae ranged between -50 and -55 kJ mol−1. At the end-point of benzoate degradation, a residual concentration of benzoate of 0.2 mM was found, correlating with a free energy change of -45 kJ mol−1 available to the fermenting bacterium. Benzoate thresholds were also observed in dense cell suspensions. They corresponded 1 a final energy situation in the range -31.8 to -45.8 kJ mol−1 for the fermentin bacterium. Addition of a H2-oxidizing sulfate reducer to the methanogenic coculture inhibited by bromoethanesulfonate (BES) resulted in benzoate degradation to below the limit of benzoate detection (10 μM). Accumulated acetate proved to be thermodynamically inhibitory; removal of acetate by Methanosaeta concilii in methanogenic or molybdate-inhibited sulfate-reducing cocultures led to degradation of residual benzoate with a final δG’ -45.8 kJ mol−1. In methanogenic cocultures, the residual Gibbs free energy (δG’) available for the fermenting bacterium at the end of benzoate degradation correlated with the concentration of acetate built up during the course of benzoate degradation; higher concentrations led to more positive values for δG’. Addition of different concentrations of propionate resulted in different values for δG when benzoate degradation had ceased; higher concentrations led to more positive values for δG’. Addition of acetate or propionate to benzoate-degrading cocultures also lowered the rate of benzoate degradation. The protonophore carbonylcyanide chlorophenylhydrazone (CCCP) facilitated further benzoate degradation in methanogenic BES-inhibited cocultures until a δG’ of -31 kJ mol−1 was reache We conclude that the minimum energy required for growth and energy conservation of the benzoate-fermenting bacterium S. gentianae is approximately -45 kJ (mol benzoate)−1, equivalent to two-thirds of an ATP unit. Both hydrogen and acetate inhibit benzoate degradation thermodynamically, and acetate also partly uncouples substrate degradation from energy conservation.