Production of lactic and acetic acids by Bacillus sp. ZM20 and Bacillus cereus following exposure to zinc oxide: A possible mechanism for Zn solubilization

Zinc (Zn) occurs in high concentrations in heavy metal polluted ecosystems which are colonized by bacterial populations. Some members of these communities are attractive candidates for phytoremediation of Zn-contaminated soils as they are able to convert insoluble forms of Zn into soluble forms, making Zn available to plants as a micronutrient. However, mechanisms of bacterial Zn solubilization and homeostasis are often not well understood. In the present study, two Gram positive rhizosphere strains, Bacillus sp. ZM20 and Bacillus subtilis ZM63, which were shown previously to solubilize phosphate and produce indole acetic acid plus siderophores, and two laboratory strains selected as controls (Bacillus cereus and Paenibacillus polymyxa), were tested for their ability to solubilize zinc oxide (ZnO). All of the strains tested could solubilize Zn to some degree, as they produced acids that lowered the medium pH. Two strains, Bacillus sp. ZM20 and B. cereus, were more Zn and acid resistant. When cultured aerobically in a minimum salt medium with glucose as carbon source plus 0.1% (w/v) Zn as ZnO, Bacillus sp. ZM20 and B. cereus both produced significantly higher concentrations of lactic and acetic acids than detected in control broths lacking Zn, with formic, citric, succinic, isobutyric and isovaleric acids detected as minor end-products. In in vitro tests, mixtures and individual acids at concentrations seen in broth cultures were shown to solubilize ZnO. However, the less acid- and Zn-resistant strains showed lower Zn solubility efficiency at higher concentrations of ZnO, as their growth was impaired, and higher concentrations of acids were not induced to the same extent by culture in ZnO. Induction of lactic, acetic, succinic, formic, isobutyric and isovaleric acids during culture correlated with ZnO solubilization, which has not been shown before for Gram positive species.