ENERGETICS OF METHANOGENIC BENZOATE DEGRADATION BY SYNTROPHUS-GENTIANAE IN SYNTROPHIC COCULTURE

Growing cocultures of Syntrophus gentianae with Methanospirillum hunga tei 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. Th ey corresponded to a final energy situation in the range -31.8 to -45. 8 kJ mol(-1) for the fermenting bacterium. Addition of a H-2-oxidizing sulfate reducer to the methanogenic coculture inhibited by bromoethan esulfonate (BES) resulted in benzoate degradation to below the limit o f benzoate detection (10 mu M). Accumulated acetate proved to be therm odynamically inhibitory; removal of acetate by Methanosaeta concilii i n methanogenic or molybdate-inhibited sulfate-reducing cocultures led to degradation of residual benzoate with a final Delta G' of -45.8 kJ mol(-1). In methanogenic cocultures, the residual Gibbs free energy (D elta G') available for the fermenting bacterium at the end of benzoate degradation correlated with the concentration of acetate built up dur ing the course of benzoate degradation; higher concentrations led to m ore positive values for Delta G', Addition of different concentrations of propionate resulted in different values for Delta G' when benzoate degradation had ceased; higher concentrations led to more positive va lues for Delta G'. Addition of acetate or propionate to benzoate-degra ding cocultures also lowered the rate of benzoate degradation. The pro tonophore carbonylcyanide chlorophenyl hydrazone (CCCP) facilitated fu rther benzoate degradation in methanogenic BES-inhibited cocultures un til a Delta G' of -31 kJ mol(-1) was reached. We conclude that the min imum energy required for growth and energy conservation of the benzoat e-fermenting bacterium S, gentianae is approximately -45 kJ (mel benzo ate)(-1), equivalent to two-thirds of an ATP unit, Both hydrogen and a cetate inhibit benzoate degradation thermodynamically, and acetate als o partly uncouples substrate degradation from energy conservation.