Siderophore production by an aerobic Pseudomonas mendocina bacterium in the presence of kaolinite

The purpose of this study was to quantify siderophore production by the aerobic bacterium, Pseudomonas mendocina, under Fe-limited conditions as a function of Fe source: supplied in dissolved form (as 30 μM Fe-EDTA), as natural Fe-containing kaolinite, and a no-Fe-added control. Siderophores are Fe(III)-specific organic ligands produced by microorganisms under conditions of Fe stress. Siderophore production was compared with previous experiments wherein Fe was supplied as hematite [Geomicrobiol. J. 17 (2000) 1]. Microbial growth increased in the order: no-added-Fe control<kaolinite<Fe-EDTA. Production of siderophore on a per cell basis decreased in the order: no-Fe-added control>kaolinite>hematite≥Fe-EDTA. Thus, the bacterium was less Fe stressed in the presence of kaolinite than in the no-added-Fe control, confirming that kaolinite serves as a source of Fe to the bacterium. Although more siderophore per cell was produced in the most stressed system (no-Fe-added), more total siderophore was produced in the least stressed system (Fe-EDTA). This is due to the presence of a larger total population size in the least stressed system. Hence, the ability of a microbial population to produce siderophores as a means to access mineral-bound Fe is a complex function of both Fe stress and population size.