To examine the pathways that form Mn
(III) and Mn
(IV) in the Mn
(II)-oxidizing bacterial strains
Pseudomonas putida GB-1 and MnB1, and to test whether the siderophore pyoverdine (PVD) inhibits Mn
(IV)O
2 formation, cultures were subjected to various protocols at known concentrations of iron and PVD. Depending on growth conditions,
P. putida produced one of two oxidized Mn species - either soluble PVD-Mn
(III) complex or insoluble Mn
(IV)O
2 minerals - but not both simultaneously. PVD-Mn
(III) was present, and MnO
2 precipitation was inhibited, both in iron-limited cultures that had synthesized 26-50 μM PVD and in iron-replete (non-PVD-producing) cultures that were supplemented with 10-550 μM purified PVD. PVD-Mn
(III) arose by predominantly ligand-mediated air oxidation of Mn
(II) in the presence of PVD, based on the following evidence: (a) yields and rates of this reaction were similar in sterile media and in cultures, and (b) GB-1 mutants deficient in enzymatic Mn oxidation produced PVD-Mn
(III) as efficiently as wild type. Only wild type, however, could degrade PVD-Mn
(III), a process linked to the production of both MnO
2 and an altered PVD with absorbance and fluorescence spectra markedly different from those of either PVD or PVD-Mn
(III). Two conditions, the presence of bioavailable iron and the absence of PVD at concentrations exceeding those of Mn, both had to be satisfied for MnO
2 to appear. These results suggest that
P. putida cultures produce soluble Mn
(III) or MnO
2 by different and mutually inhibitory pathways: enzymatic catalysis yielding MnO
2 under iron sufficiency or PVD-promoted oxidation yielding PVD-Mn
(III) under iron limitation. Since PVD-producing
Pseudomonas species are environmentally prevalent Mn oxidizers, these data predict influences of iron (via PVD-Mn
(III) versus MnO
2) on the global oxidation/reduction cycling of various pollutants, recalcitrant organic matter, and elements such as C, S, N, Cr, U, and Mn.
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