The spores of a marine Bacillus bacterium, strain SG-1, are able to oxidize Mn(II) over a wide range of temperatures (0-80 degrees C) and Mn(II) concentrations (<1 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, Mn3O4; feitknechtite, beta MnOOH; and manganite, gamma MnOOH) formed in HB and SW at high Mn(II) concentrations (10 mM initial), or at high temperatures (70 degrees C), by two weeks. beta MnOOH was favored at low temperatures (3 degrees C) and Mn3O4 at higher temperatures (55-70 degrees C). After 1 year of aging, gamma MnOOH became the dominant or only mineral present at 25 and 55 degrees C. At lower Mn(II) concentrations (initial concentrations less than or equal to 100 mu M in HB and less than or equal to 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 Angstrom phase with air drying at room temperature. In SW both buserite and Mg-rich noncollapsible 10 Angstrom manganates were formed. The Mg-rich 10 Angstrom manganates did not collapse to 7 Angstrom even with baking at 100 degrees 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, 3-55 degrees C) Mn(IV) mineral formation often occurred at pH and Mn(II) concentrations too high to be favorable for the disproportionation of Mn3O4, or beta MnOOH 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.
