Fungal transformation of natural and synthetic cobalt-bearing manganese oxides and implications for cobalt biogeochemistry

Manganese oxide minerals can become enriched in a variety of metals through adsorption and redox processes, and this forms the basis for a close geochemical relationship between Mn oxide phases and Co. Since oxalate-producing fungi can effect geochemical transformation of Mn oxides, an understanding of the fate of Co during such processes could provide new insights on the geochemical behaviour of Co. In this work, the transformation of Mn oxides by Aspergillus niger was investigated using a Co-bearing manganiferous laterite, and a synthetic Co-doped birnessite. A. niger could transform laterite in both fragmented and powder forms, resulting in formation of biomineral crusts that were composed of Mn oxalates hosting Co, Ni and, in transformed laterite fragments, Mg. Total transformation of Co-doped birnessite resulted in precipitation of Co-bearing Mn oxalate. Fungal transformation of the Mn oxide phases included Mn(III,IV) reduction by oxalate, and may also have involved reduction of Co(III) to Co(II). These findings demonstrate that oxalate-producing fungi can influence Co speciation in Mn oxides, with implications for other hosted metals including Al and Fe. This work also provides further understanding of the roles of fungi as geoactive agents which can inform potential applications in metal bioremediation, recycling and biorecovery.