Role of Microbially produced Siderophores in Increasing Availability of Micronutrient Cations in Soils

Siderophores are low molecular weight organic compounds secreted into the environment by bacteria and fungi in response to iron stress. Significant levels of siderophores have been found in the rhizosphere of crop plants; however, their potential significance in terms of mobilizing iron and other micronutrient cations has been overlooked. This research project studied the occurrence of siderophores in some Saskatchewan soils, and their importance in complexing and increasing availability of Cu, Fe, Mn, and Zn. Twenty four soils were surveyed for siderophore levels using a bioassay which employed the siderophore-requiring bacterium, Arthrobacter jlavescens JG-9. Hydroxamate type siderophores (HS) in the range of 2.1 to 27.3 x 10-9 M deferrioxamine methane sulfonate (DFOM) equivalents were detected in these soils. There was no simple correlation between HS levels and soil pH, % organic carbon, % clay, or DTPA-extractable Cu, Fe, Mn, and Zn. A total of 210 bacteria were isolated from the rhizosphere of wheat roots grown in three soils (Medstead, Smeaton, and Choiceland), which contained the highest HS levels. Bacteria were screened for siderophore production using the Arthobacterjlavescens JG-9 bioassay and the Chrome Azurol-S chemical method. Out of the 210 isolates, 29 were found to be producing siderophore. The 29 siderophore-producing bacteria and 9 plant growth-promoting rhizobacteria (PGPR) were further screened for the most efficient siderophore-producing bacteria. Three bacterial strains, CH 16, R III and KC 21, were selected and identified as Pseudomonas fluorescens. The efficient strains were grown in broth culture and their siderophores extracted and purified by Bio-Gel P2 filtration. The purified siderophores were used to study the mechanisms by which siderophores could increase availability of micronutrient cations. It was shown that siderophores produced by these bacterial strains were able to complex not only with Fe, but also with Cu. Another way by which these siderophores increased the availability of micronutrient cations was through dissolution of iron-hydroxide, thereby releasing specifically adsorbed and coprecipitated Cu, Mn, and Zn. The qualitative characterization of bacterial siderophores indicated that some components of these compounds are heat labile, and have maximum absorbance between 375 to 425 nm. The scanning of these siderophores in the infrared region indicated the presence of carboxyl and amino groups. My research shows that Saskatchewan soils contain siderophore levels that are high enough to increase the availability of Cu, Fe, Mn, and Zn. However, the ability of siderophore-producing bacteria to colonize plant roots under field conditions may be limited. Therefore, the use of siderophore-producing bacteria as seed inoculants might increase the level of siderophores in the rhizosphere and facilitate micronutrient nutrition of crops