Isolation and physiology of a manganese-reducing Bacillus polymyxa from an Oligocene silver-bearing ore and sediment with reference to Precambrian biogeochemistry

Samples from an Oligocene silver-bearing manganese ore mining site in Saguochi country, Colorado were screened for bacteria which solubilize manganese in the refractory ore thus making the silver more amenable to standard extraction techniques. Over 300 bacterial isolates were tested. Manganese reduction was detected by growing the organisms in a mineral salts medium supplemented with the insoluble MnO₂ (pyrolusite). Reduction of the black MnO₂ resulted in the formation of colorless solubilized Mn²⁺. One of the most efficient manganese reducers was a Bacillus polymyxa strain D1. This bacterium was isolated from the ore heap and from a zone 5 cm below the sediment surface of the inlet which drained ore heap leachate into a holding pond. Isolate D1 has a unique mode of manganese reduction as shown by the following combination of characteristics: (1) anaerobiosis is absolutely necessary; (2) direct surface contact with the mineral particle is required; (3) facultative fermentation occurs concomitantly with manganese reduction; and (4) Mn⁴⁺ does not substitute for O₂ at the end of the respiratory electron transport chain. Although the ore deposit is approximately 35-30 Ma old, D1 probably have first appeared at the site contemporaneous with mining activities as evidenced by its resistance to exogenous zinc used in the silver extractive process. If organisms such as D1 were present > 600 Ma ago they could have played a role in the Precambrian manganese cycle as they do in the Holocene. Encrustations of MnO₂ have been found in Precambrian sediments which are thought to be biogenic in origin. Solubilized reduced Mn²⁺ must also have been present to serve as a substrate. Anaerobic conditions alone are insufficient for the reduction of manganese dioxide, therefore, it is likely that manganese reduction was due, in part, to microbial activity. Solubilization of manganese occurs via microbial dissimilatory reduction as seen with isolate D1. Precipitation results from oxidation of manganese coupled to ATP synthesis as described by Ehrlich (1976, 1983).