Biological Mn(II) oxidation under organic substrate-limited conditions and its application in mine drainage remediation

Manganese (Mn)-oxidizing bacteria are considered promising tools for remediation of Mn-containing mine drainage. However, whether such microorganisms function in an environment rich in metals and poor in organic substrate remains largely unexplored. In repeated batch experiments, we found that suspensions of mine drainage sediment showed heat-labile, azide-sensitive, and Mn(II) concentration-dependent removal of dissolved Mn(II), consistent with the contribution of biological activity to the Mn(II) oxidation. In limestone-packed bioreactors, most Mn(II) and zinc (Zn) (II) ions added at 20 or 60 mg L−1 and 6 mg L−1, respectively, were removed at hydraulic retention times ranging from 0.5 days to 3.0 days. The Mn oxide deposits in the reactors showed network structures of nanosheets, which structurally incorporated most parts of Mn and Zn to form a Zn-bearing Mn(IV) oxide mineral resembling woodruffite. The Mn deposit suspensions incubated with 13C-labeled bicarbonate assimilated the inorganic carbon and this process enhanced the Mn(II) oxidation. The preferential growth of oligotrophs or methylotrophs might provide organic carbon within Mn(II)-oxidizing microbial communities. This study first demonstrates the important role of inorganic carbon assimilation in heterotrophic Mn(II)-oxidizing community maintained under metal-rich, organic substrate-limited conditions and reinforces the advantage of use of biological Mn(II) oxidation for mine drainage remediation.