Production of aerobactin by microorganisms from a compost enrichment culture and soybean utilization

Abstract

Fe chelators produced by compost microorganisms in C enrichment culture can efficiently improve Fe nutrition of soybean and oat under slightly alkaline conditions. Little information is available, however, concerning the identity of these chelators. A study was conducted to identify the Fe chelators and the microorganisms responsible for their production and to evaluate the mechanism of Fe uptake by soybean of these Fe chelates. One Fe chelator predominated in the culture solution and, after purification using chromatography, it was identified by nuclear magnetic resonance and mass spectrometry as the siderophore, aerobactin. A strain of Citrobacter diversus, which produced a high concentration of aerobactin, was isolated and purified from the culture. Bathophenanthrolinedisulfonic acid (BPDS), an Fe²⁺ trap, did not decrease soybean uptake of Fe of which more than 90% was in the form of an Fe‐aerobactin complex. Reduction of Fe as Fe‐aerobactin by soybean roots was not detected. Iron uptake was, however, significantly (P∼ 0.05) inhibited by NaN₃, a respiratory inhibitor. These results suggest that the Fe may have been taken up by soybean as an Fe‐aerobactin complex through an active transport mechanism.