Oxalic acid production and aluminum tolerance in Pseudomonas fluorescens

13C NMR studies on intact cells from Al-stressed Pseudomonas fluorescens incubated with citric acid or Al-citrate yielded peaks at 158 and 166 ppm that were attributable to free and complexed oxalic acid, respectively. The presence of oxalic acid was further confirmed with the aid of oxalate oxidase. These peaks were not discernable in experiments performed with cells taken from control cultures. Enzymatic analyses of cell fractions showed the highest production of oxalic acid in the inner membrane fraction of Al-stressed cells incubated with glyoxylate. There was an eight-fold increase in the synthesis of oxalic acid in the inner membrane fraction from the Al-stressed cells compared to the control cells. Although oxalic acid production was observed when citrate, Al-citrate and isocitrate were utilized as substrates, the inner membrane fraction did not mediate the formation of oxalic acid from glycine/pyruvate, glycolic acid, oxaloacetate or ascorbate. These data suggest that the increased oxalic acid production in response to Al stress is effected via the oxidation of glyoxylate.