Percent-Level Copper Mineralisation Promoted by Copper-Tolerant Iron-Oxidising Bacteria in Circumneutral Mine Drainage.

Geomicrobiology of copper (Cu) and iron (Fe) in circumneutral-pH drainage from a Japanese Cu mine was investigated, with a focus on Fe(II)-oxidising microorganisms. Cu in the drainage was rapidly removed through Fe(II) oxidation and subsequent Fe(III) biomineralisation. The resulting Fe(III) biomineral-rich sediment contained up to ~2 wt% Cu, a percent-level high content. XAFS and XRD analyses directly demonstrated that Cu was predominantly associated with poorly crystalline Fe(III) oxyhydroxides. The microbial community in the drainage was dominated by chemolithotrophic Fe(II)-oxidising bacteria of family Gallionellaceae (33.8%), suggesting their key role in Cu and Fe biomineralisation. Using a custom-made medium, Gallionellaceae bacteria were enriched and a representative strain TK5 was isolated. 16S rRNA gene analysis revealed that TK5 was almost identical to the most abundant taxon at the site and likely represents a novel species of genus Sideroxyarcus. Physiological and genomic analyses confirmed its Fe(II) oxidation ability and a relatively high Cu(II) tolerance (up to 50 μM), which is about 20-fold higher than on-site Cu concentration. Strain TK5 is the first known Cu-tolerant Fe(II)-oxidising Gallionellaceae isolate. Batch experiments demonstrated that TK5 effectively promoted Cu deposition via Fe(III) biomineralisation. This study highlights the potential of Fe(II)-oxidising bacteria for Cu bioremediation and biomineralisation under circumneutral-pH conditions.