Cobalt(II) sequestration on fungal biogenic manganese oxide enhanced by manganese(II) oxidase activity

We examined the ability of biogenic manganese oxide (BMO) formed in the cultures of a Mn(II) oxidizing fungus, Acremonium strictum strain KR21-2, to sequester Co(II) and found that the newly formed BMO effectively sequestered Co(II) under aerobic conditions with virtually no release of Mn(II). Under anaerobic conditions, smaller amounts of Co(II) were sequestered and a significant amount of Mn(II) was released. Similar trends were observed when the BMOs were poisoned with 50 mM NaN₃ or heated at 85 °C for 1 h. X-ray absorption near-edge structure spectroscopy and two-step extraction confirmed that oxidation of Co(II) to Co(III) occurs with BMOs with higher oxidation efficiency under aerobic conditions. These results demonstrate that BMOs can reoxidize Mn(II) through the Mn(II) oxidase associated with the BMO phase and can subsequently provide a new reaction site for Co sequestration. The ability of BMO to sequester Co(II) was also found to be long lasting in 20 mM 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid (HEPES) buffer (pH 7.0) containing no nutrients to maintain fungal growth, because sequential treatment of BMOs with the Co(II) solution every 24 h for at least 8 days led to Co(II) sequestration. In addition, Co accumulation in the solid phase was eventually 2.48-fold relative to the accumulation of Mn oxide (molar ratio). X-ray diffraction results suggest that the continuing Co(II) oxidation by newly formed BMOs results in the formation of heterogenite (β-CoOOH) aggregates. Assays using the concentrated Mn(II) oxidase crude solution showed that the preformed Mn oxide phase was important for further Mn(II) oxidation in coexisting Co(II). The fact that the coexisting Co(II) was less inhibitory to Mn(II) oxidation if the preformed Mn oxide phase was present suggests a possible electron path from Co(II) to the final electron acceptor O₂ through BMO and Mn(II) oxidase in BMO/enzyme aggregation. These results suggest that fungal BMOs supporting Mn(II) oxidase activity can serve as an effective Co(II) sequestering material, without the need for additional nutrients.