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

In aerobic environments microorganisms are faced with a discrepancy of similar 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 contrast to facultative anaerobic environments, where dissimilatory iron-reducing bacteria (DIRB) are often abundant, few studies have detailed microbial interactions with Fe(III) (hydr) oxides in aerobic environments. To better understand acquisition of Fe from Fe(III) (hydr)oxides, we investigated the production of siderophore and Fe(III) reduction by a strict aerobe in the presence of synthetic hematite as a source of Fe. Pseudomonas mendocina grew best when Fe was supplied as FeEDTA (similar to1.8 x 10(8) colony-forming units [CFU] ml(-1)), grew abundantly when Fe was supplied as hematite (similar 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 siderophore was produced per cell when Fe was supplied as FeEDTA and more siderophore was produced in the hematite flasks than in the controls. Thus, growth of P. mendocina and the production of siderophore in the presence of hematite 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 supplied as hematite or as FeEDTA, but much more when the cells were cultured under extreme Fe deprivation. In fact, 16 times as much Fe reduction occurred in the controls as in the presence of either of the FeEDTA or hematite amendments. Our results, which contravene the long-held assumptions that Fe acquisition was facilitated solely by siderophores, provides a new perspective regarding microbial interactions with Fe-bearing minerals.