Oxidation of acetate through reactions of the citric acid cycle by Geobacter sulfurreducens in pure culture and in syntrophic coculture

Abstract.

Geobacter sulfurreducens strain PCA oxidized acetate to CO₂ via citric acid cycle reactions during growth with acetate plus fumarate in pure culture, and with acetate plus nitrate in coculture with Wolinella succinogenes. Acetate was activated by succinyl-CoA:acetate CoA-transferase and also via acetate kinase plus phosphotransacetylase. Citrate was formed by citrate synthase. Soluble isocitrate and malate dehydrogenases reduced NADP⁺ and NAD⁺, respectively. Oxidation of 2-oxoglutarate was measured as benzyl viologen reduction and was strictly CoA-dependent; a low activity was also observed with NADP⁺. Succinate dehydrogenase and fumarate reductase both were membrane-bound. Succinate oxidation was coupled to NADP⁺ reduction whereas fumarate reduction was coupled to NADPH and NADH oxidation. Coupling of succinate oxidation to NADP⁺ or cytochrome(s) reduction required an ATP-dependent reversed electron transport. Net ATP synthesis proceeded exclusively through electron transport phosphorylation. During fumarate reduction, both NADPH and NADH delivered reducing equivalents into the electron transport chain, which contained a menaquinone. Overall, acetate oxidation with fumarate proceeded through an open loop of citric acid cycle reactions, excluding succinate dehydrogenase, with fumarate reductase as the key reaction for electron delivery, whereas acetate oxidation in the syntrophic coculture required the complete citric acid cycle.