A new look at microbial leaching patterns on sulfide minerals

Leaching patterns on sulfide minerals were investigated by high-resolution scanning electron microscopy (SEM). Our goal was to evaluate the relative contributions of inorganic surface reactions and reactions localized by attached cells: to surface morphology evolution. Experiments utilized pyrite (FeS2), marcasite (FeS2) and arsenopyrite (FeAsS), and two iron-oxidizing prokaryotes in order to determine the importance of cell typo, crystal structure, and mineral dissolution rate in microbially induced pit formation. Pyrite surfaces were reacted with the iron-oxidizing bactulium Acidithiobacillus ferrooxidans (at 25 degreesC), the iron-oxidizing archoeon 'Ferroplasma acidarmanus' (at 37 degreesC), and abiotically in the presence of Fe3+ ions. In ail three experiments, discrete bacillus-sized (1-2 mum) and -shaped (elliptical) pits developed on pyrite surfaces within 1 week; of reaction. Results show that attaching cells are not necessary fur pit formation on pyrite. Marcasite and arsenopyrite surfaces were reacted with A. ferrooxidans , (at 25 degreesC) and 'F. acidaranus' (at 37 degreesC). Cell-sized and cell-shaped dissolution pits were not observed oil marcasite or arsenopylite at any point during reaction with A. ferrooxidans, or on marcasite surfaces reacted with 'F. acidarmanus'. However, individual 'F. acidarmanus' cells were found within individual shallow (< 0.5 <mu>m deep) pits. The size and shape (round rather than elliptical) of the pits conformed closely to the shape of F. acidarmanus (cells) pits on arsenopyrite. We infer these pits to be cell-induced. We attribute the formation of pits readily detectable (by SEM) to thr higher rt activity of arsenopyrite compared to pyrite and marcasite under the conditions the experiment was conducted. These pits contributed little to the overall surface topographical evolution, and most likely did nor significantly increase surface area during reaction. Our results suggest that overall sulfide mineral dissolution may be dominated by surface reactions with Fe3+ rather than by reactions: at the cell-mineral interface. (C) 2001 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.