Iron minerals inhibit the growth of <i>Pseudomonas brassicacearum</i> J12 via a free-radical mechanism: implications for soil carbon storage

Abstract. Natural minerals in soil can inhibit the growth of bacteria that protect organic carbon from decay. However, the mechanism inhibiting the bacterial growth remains poorly understood. Here, using a series of cultivation experiments and biological, chemical and synchrotron-based spectral analyses, we showed that kaolinite, hematite, goethite and ferrihydrite had a significant inhibitory effect on the growth of the model bacteria Pseudomonas brassicacearum J12, which was more prominent with a concentration of 25 mg mL−1 than it was with either 10 or 5 mg mL−1. In contrast, montmorillonite promoted the growth of J12. Compared to Al-containing minerals, Fe(III)-containing minerals produced more hydroxyl radical (HO⚫) that has high efficiency for the inhibition of J12. Moreover, a significant positive correlation between HO⚫ radical and Fe(II) was found, suggesting that Fe(II) contributes to the generation of HO⚫. Furthermore, both micro X-ray fluorescence and X-ray photoelectron spectroscopies indicated that surface Fe(III) was reduced to Fe(II), which can produce HO⚫ through the well-known Fenton reaction series. Together, these findings indicate that the reduced surface Fe(II) derived from Fe(III)-containing minerals inhibits the growth of Pseudomonas brassicacearum J12 via a free-radical mechanism, which may serve as a ubiquitous mechanism between iron minerals and all of the heterotrophic bacteria in view of taxonomically and ecologically diverse heterotrophic bacteria from terrestrial environments as a vast source of superoxide.