Manganese Oxide Biomineralization by Spores of the Marine Bacillus sp. Strain SG-1

Biogenic Mn oxides are ubiquitous in natural waters, have high sorptive capacities for metal ions, and oxidize organic and inorganic substances such as aromatic hydrocarbons, Cr(III), and hydrogen sulfide. In this fashion, Mn(II)-oxidizing bacteria impact the biogeochemical cycling of essential nutrients and toxic trace constituents of natural waters. In spite of their importance, the molecular mechanisms, intermediates, and products of Mn oxide biomineralization are poorly understood. Similarly, the relationship between biotic and abiotic Mn oxidation mechanisms is not well documented. We have studied Mn oxide biomineralization by spores of the marine Bacillus sp. strain SG-1 as functions of reaction time (10 min to 77 d), Mn(II) concentration (0.01 to 1 mM), major ion composition (50 mM NaCl to sea water), O2 partial pressure, and temperature. SG-1 spores are an ideal subject because they are dormant, Mn-oxidation is not inactivated by the x-rays utilized, and they previously have been extensively studied. Reaction products and Mn oxidation state evolution were directly observed in order to infer mechanisms and phase dominance. To obtain this information, a combination of Mn(II) uptake measurements, K-edge x-ray absorption spectroscopy (XAS), L-edge scanning transmission x-ray microspectroscopy (STXM, 60 nm nominal spot size), and synchrotron-based x-ray diffraction measurements were performed. All samples were measured under fully hydrated conditions to prevent dehydration of reaction products. This set of techniques provides chemical and structural information on Mn in amorphous and crystalline states in the samples. Mn oxide biomineralization products were sensitive to [Mn(II)]. At 0.01 mM [Mn(II)], biogenic Mn oxides were found to contain highly oxidized Mn (80-85% Mn(IV)) as observed after 48 hr. reaction. The dominant phase is identified as an amorphous Mn(IV) oxide similar to d-MnO2. K-edge XAS measurements suggest this phase forms within minutes of reaction onset and dominates after 6 hr. reaction, without detection of Mn(III). In contrast, L-edge STXM suggests that Mn(III) is important throughout the 6 - 48 hr interval and Mn(IV) is a minor component. At [Mn(II)] = 1 mM, crystalline b-MnOOH was observed as a dominant abiotic reaction product, likely formed on biogenic am. MnO2 surfaces. We will discuss important aspects of K- and L-edge spectroscopy and their application on microscopic scales and demonstrate how these aspects account for each set of observations. Implications for reaction mechanisms will be presented.