Iron reduction by diverse actinobacteria under oxic and pH-neutral conditions and the formation of secondary minerals

Microbial reduction of Fe(III) is an important biogeochemical process in anoxic or acidic environments. However, this process under oxic and pH-neutral conditions is understudied, and the iron reduction capability of the phylum Actinobacteria is poorly known. In this study, we investigated the capacity of diverse actinobacteria to reduce Fe(III) in the presence of oxygen and at pH 7. From 277 actinobacterial strains isolated from deep sea and terrestrial soil, 109 strains (39.40%) belonging to 17 genera showed the capacity of reducing FeCl3 under oxic and initially neutral pH conditions. Among the 109 strains, 49 spanning 13 genera also reduced ferric iron oxides under the same conditions. Four non-filamentous strains from four genera were selected for further analyses. All the four strains showed the capacity to reduce goethite under oxic and pH 7 and 8 conditions. Microbial fuel cell (MFC) experiments showed a current enhancement when the strains were cultured in the presence of goethite. Living actinobacterial cells and their metabolites played a key role in reducing Fe(III). The strains produced organic acids and siderophores, which likely promoted Fe(III) reduction due to altered reduction potentials of complexed Fe(III) and inhibition of Fe(II) oxidization by oxygen. SEM/EDS and XRD analyses detected vivianite in the culture of Kocuria oceani FXJ8.057 with goethite, where, unexpectedly, K. oceani FXJ8.057 produced nanowire-like structures. These results demonstrate that actinobacteria are able to reduce Fe(III) in both aqueous and solid forms under oxic and pH-neutral conditions. The mechanisms, however, still need further investigation.