Whole--cell and cell--extract experiments were performed to study the mechanism of oxalate metabolism in the acetogenic bacterium Moorella thermoacetica. In short--term, whole--cell assays, oxalate consumption was low unless cell suspensions were supplemented with CO2, KNO3, or Na2S2O3. Cell extracts catalyzed the oxalate--dependent reduction of benzyl viologen. Oxalate consumption occurred concomitant to benzyl viologen reduction; when benzyl viologen was omitted, oxalate was not appreciably consumed. Based on benzyl viologen reduction, specific activities of extracts averaged 0.6 μmol oxalate oxidized min−1 mg protein−1. Extracts also catalyzed the formate--dependent reduction of NADP+; however, oxalate--dependent reduction of NADP+ was negligible. Oxalate--or formate--dependent reduction of NAD+ was not observed. Addition of coenzyme A (CoA), acetyl--CoA, or succinyl--CoA to the assay had a minimal effect on the oxalate--dependent reduction of benzyl viologen. These results suggest that oxalate metabolism by M. thermoacetica requires a utilizable electron acceptor and that CoA--level intermediates are not involved. Introduction Information on the bacterial metabolism of oxalate ( − OOC--COO − ) has been obtained mostly through studies with Ralstonia oxalatica [1][2][3][4][5][6] and Oxalobacter formigenes [7][8][9][10][11]. With both of these bacteria, oxalate is first activated to oxalyl--coenzyme A (CoA) and then decarboxylated by oxalyl--CoA decarboxylase to CO2 and formyl--CoA; the CoA of formyl--CoA is transferred by formyl--CoA transferase to a new molecule of oxalate, and formate is produced [4][5][6][8][9][10]. With R. oxalatica, formate is oxidized by a NAD--dependent formate dehydrogenase, and this oxidation is coupled to ATP synthesis via electron transport phosphorylation with O2 as the terminal electron acceptor [5,[12][13][14]. With O. formigenes, the formate derived from oxalyl--CoA decarboxylation is not metabolized and is released as an end product [7,8]. Furthermore, O. formigenes lacks the ability to form ATP by substrate--level or electron transport phosphorylation. It is now known that a membrane--bound, oxalate-formate antiporter and a cytoplasmic oxalyl--CoA decarboxylase work together in O. formigenes to create a proton gradient (for ATP synthesis) by coupling the electrogenic exchange of oxalate and formate across the membrane with a proton--consuming decarboxylation reaction [7,15,16]. Lastly, in contrast to R. oxalatica and O. formigenes, Pseudomonas sp. OX--53 engages yet another mechanism for oxalate metabolism. This aerobic bacterium possesses an oxalate oxidase (oxalate:oxygen oxidoreductase) that directly couples oxalate oxidation to the reduction of oxygen, yielding CO2 and hydrogen peroxide [17].Moorella thermoacetica [18,19] is a thermophilic acetogenic bacterium that uses CO2 as a terminal electron acceptor and concomitantly synthesizes biomass and acetate via the acetyl--CoA or Wood-Ljungdahl pathway [20][21][22][23][24]. This anaerobe is the most metabolically div...

The microbial consumption of oxalate was examined under anaerobic conditions in soil suspensions at 15e20 _C. With soil (horizon Ah, pH 6.4) from a beech forest, microbial consumption of added oxalate (15 mM) began after 10 days, and oxalate was totally consumed by day 20. The presence of supplemental electron donors (acetate, glucose, vanillate, or hydrogen) or electron acceptors (nitrate or sulfate) did not significantly influence anaerobic oxalate consumption, whereas supplementation of soil suspensions with CO2/bicarbonate totally repressed oxalate consumption. Thus, CO2--, nitrate--or sulfate--respiring bacteria were apparently not active in the anaerobic consumption of oxalate in these soil suspensions. With soil (horizon Bt, pH 7) from a beech forest, oxalate consumption began after an approximate lag of 14 days, and oxalate was totally consumed by day 41. With both soils, acetate was the major aliphatic organic acid detected during oxalate consumption. Near pH--neutral soils from two additional forest field sites were also competent in anaerobic oxalate consumption. In contrast, anaerobic oxalate consumption was negligible in suspensions prepared with acidic soils (