Continuous Time-Resolved X-Ray Diffraction of the Biological Reduction of Mn Oxides and Secondary Phase Precipitation

Manganese oxides are found in many natural environments, from deep oceans to shallow soils. The Mn oxides birnessite [(Na,Ca,Mn²⁺)Mn₇O₁₄•2.8H₂O)] and todorokite [(Ca,Na,K)_0.3-0.5(Mn⁴⁺,Mn³⁺,Mg)₆O₁₂•3-4.5H₂O] have open structures that promote rapid cation exchange and sorption. Birnessite is a layered Mn oxide composed of edge-sharing MnO₆ octahedra and todorokite is a tunnel structure composed of triple chains of edge-sharing octahedra. These minerals have been shown to be produced and dissolved by microorganisms in soils. Understanding the mechanisms by which Mn oxide structures change as a result of bacterial activity is necessary for models of the cycling of sorbed or exchanged elements.

The most robust method for determining crystal structures is X-ray diffraction, which is lethal to living organisms in continuous time-resolved experiments. However, recent research has shown that isolated total membrane (TM) fractions from the facultative anaerobe Shewanella oneidensis can effectively reduce Mn oxides by direct contact. We have developed a method that allows continuous monitoring of crystal structure changes in Mn oxides as a result of the reductive dissolution by bacteria. We have documented significant structural transformations as well as the precipitation of two secondary phases, rhodochrosite (MnCO₃) and hausmannite (Mn³⁺Mn⁴⁺₂O₄).

The biological reduction of birnessite is characterized by a significant contraction in the unit cell volume, due primarily to a decrease in the interlayer region. This change is likely the result of Mn(IV) reduction to Mn(III) in the octahedral sheets prior to dissolution. Our experiments also show that birnessite reduction can occur rapidly in the presence of high concentrations of TM and can be completely replaced by rhodochrosite and hausmannite in three days.