Manganese mineral formation by bacterial spores of the marine Bacillus, strain SG-1: Evidence for the direct oxidation of Mn(II) to Mn(IV)

Abstract

The spores of a marine Bacillus bacterium, strain SG-1, are able to oxidize Mn (H) over a wide range of temperatures (0–80°C) and Mn (II) concentrations (<I nM to >25 mM), in both low ionic strength N- (2-hydroxyethyl) piperazine- N′-ethanesulfonic acid (HEPES) buffer (HB) and in HEPES-buffered seawater (SW). Using SG-1 spores as a catalyst for manganese mineral formation, and by varying the temperature and Mn (II) concentration at pH 7.4–8.0, a variety of manganese oxide and manganate minerals were formed under environmentally relevant conditions in HB and SW. In general, mixed phases of lower valence state minerals (hausmannite, Mn 30 4; feitknechtite, βMnOOH; and manganite, γMnOOH) formed in HB and SW at high Mn (II) concentrations (10 mM initial), or at high temperatures (70°C), by two weeks. βMnOOH was favored at low temperatures (3°C) and Mn 3O 4 at higher temperatures (55–70°C). After 1 year of aging, yMnOOH became the dominant or only mineral present at 25 and 55°C. At lower Mn (II) concentrations (initial concentrations ≤100 μM in HB and ≤1 MM in SW), Mn(IV) minerals precipitated. In HB the Mn(IV) minerals most often resembled sodium buserite, evidenced by collapse of a 10 to 7 Å phase with air drying at room temperature. In SW both buserite and Mg-rich noncollapsible 10 Å manganates were formed. The Mg-rich 10 Å manganates did not collapse to 7 Å even with baking at 100°C. The oxidation state of the minerals were generally higher in SW (as high as 3.7) than in HB (3.2). Mn (IV) minerals also formed at higher Mn (II) concentrations in SW than in HB. These observed differences between SW and HB may have resulted from differences in the chemical milieu, or because of the marine adapted physiology of the bacterial spores. Under a variety of conditions (HB and SW, 3–55δC) Mn (IV) mineral formation often occurred at pH and Mn (II) concentrations too high to be favorable for the disproportionation of Mn 30 4, or βMnOOH to Mn (IV). The results strongly suggest direct oxidation of Mn(II) to Mn(IV) by SG-1 spores without lower valence intermediates. Considering the environmental relevance of these experiments, direct oxidation of Mn (II) to Mn (IV) by microbes is probably a common process in natural environments.