Siderophore production and iron reduction by Pseudomonas mendocina in response to iron deprivation

In aerobic environments microorganisms are faced with a discrepancy of simi lar to 10 orders of magnitude between the available Fe (similar to 10(-17) M) and their metabolic requirement for it (similar to 10(-7) M). In contras t to facultative anaerobic environments, where dissimilatory iron-reducing bacteria (DIRB) are often abundant, few studies have detailed microbial int eractions with Fe(III) (hydr) oxides in aerobic environments. To better und erstand acquisition of Fe from Fe(III) (hydr)oxides, we investigated the pr oduction of siderophore and Fe(III) reduction by a strict aerobe in the pre sence of synthetic hematite as a source of Fe. Pseudomonas mendocina grew b est when Fe was supplied as FeEDTA (similar to1.8 x 10(8) colony-forming un its [CFU] ml(-1)), grew abundantly when Fe was supplied as hematite (simila r to1.2 x 10(8) CFU ml(-1)), and grew poorly when Fe was withheld from the medium (similar to5.5 x 10(7) CFU ml(-1)). As expected, negligible sideroph ore was produced per cell when Fe was supplied as FeEDTA and more sideropho re was produced in the hematite flasks than in the controls. Thus, growth o f P. mendocina and the production of siderophore in the presence of hematit e present compelling evidence that siderophore was produced as a mechanism to acquire Fe from hematite. For the Fe reduction experiments, Fe reduction by components of the supernatant fluid was induced weakly when Fe was supp lied as hematite or as FeEDTA, but much more when the cells were cultured u nder extreme Fe deprivation. In fact, 16 times as much Fe reduction occurre d in the controls as in the presence of either of the FeEDTA or hematite am endments. Our results, which contravene the long-held assumptions that Fe a cquisition was facilitated solely by siderophores, provides a new perspecti ve regarding microbial interactions with Fe-bearing minerals.