Roles of Siderophores, Oxalate, and Ascorbate in Mobilization of Iron from Hematite by the Aerobic Bacterium Pseudomonas mendocina

In aerobic, circumneutral environments, the essential element Fe occurs primarily in scarcely soluble mineral forms. We examined the independent and combined effects of a siderophore, a reductant (ascorbate), and a low-molecular-weight carboxylic acid (oxalate) on acquisition of Fe from the mineral hematite (alpha-Fe2O3) by the obligate aerobe Pseudomonas mendocina ymp. A site-directed Delta pmhA mutant that was not capable of producing functional siderophores (i.e., siderophore(-) phenotype) did not grow on hematite as the only Fe source. The concentration of an added exogenous siderophore (1 mu M desferrioxamine B [DFO-B]) needed to restore wild-type (WT)-like growth kinetics to the siderophore(-) strain was similar to 50-fold less than the concentration of the siderophore secreted by the WT organism grown under the same conditions. The roles of a reductant (ascorbate) and a simple carboxylic acid (oxalate) in the Fe acquisition process were examined in the presence and absence of the siderophore. Addition of ascorbate (50 mu M) alone restored the growth of the siderophore(-) culture to the WT levels. A higher concentration of oxalate (100 mu M) had little effect on the growth of a siderophore(-) culture; however, addition of 0.1 mu M DFO-B and 100 mu M oxalate restored the growth of the mutant to WT levels when the oxalate was prereacted with the hematite, demonstrating that a metabolizing culture benefits from a synergistic effect of DFO-B and oxalate.