Changes in Ni Binding to and Uptake By Mycogenic Manganese Oxides with Aging.

Manganese (Mn) oxide minerals exert substantial controls on the fate of contaminants and bioavailability of micronutrients through adsorption, coprecipitation, and redox reactions. Mn oxides have a strong affinity for many trace metals, including Ni, which is an important micronutrient but also a water and soil contaminant at higher concentrations. Mn oxide formation in modern day natural and metal contaminated systems is largely considered to be controlled by microbial (bacterial as well as fungal) processes. Biomineralized Mn oxides are usually poorly crystalline phyllomanganates: Mn(IV/III) (hydr)oxide minerals with sheet structures and variable Mn(III) and Mn(IV) vacancy contents. Trace metal uptake by phyllomanganates usually occurs via adsorption above or incorporation into Mn(IV) vacancies or adsorption to sheet edges. In abiotic systems, Ni binding modes are affected by the timing of Ni addition. For example, coprecipitating Ni with abiotic phyllomanganates results in both incorporated and adsorbed Ni while Ni added to a preformed phyllomanganate is primarily adsorbed; these distributions are generally stable upon aging (barring changes to environmental conditions). Here we explore the partitioning and stability of solid-associated Ni in mycogenic phyllomanganates produced by the Mn-oxidizing fungi Stagonospora sp. SRC1lsM3a at circumneutral pH when added concurrently with Mn(II) during the initial Mn oxidation and when added after Mn oxidation is complete. We find that when Ni is coprecipitated, increasing Ni decreases overall Mn oxide production (an effect that is not attributable to changes in biomass production). Unlike with abiotic phyllomanganates, we find there is little to no difference in the partitioning of solid-associated Ni when coprecipitated with or added after phyllomanganate formation. We also observed that aging alters Ni partitioning, leading to increased incorporated Ni. These results demonstrate that trace metals associated with phyllomanganates behave differently in fungal and abiotic systems.