The molecular geomicrobiology of bacterial manganese(II) oxidation

Manganese is the second most abundant transition metal found in the Earth's crust. It has a significant biological role as it is a cofactor of enzymes such as superoxide dismutase and is the key metal in the reaction center of photosystem II. In the environment, manganese is mostly found in three different oxidation states: II, III, and IV. Mn(II), primarily occurring as the soluble Mn²⁺ species, is the thermodynamically favored state at low pH and Eh while insoluble Mn(III) and Mn(IV) oxides are favored at high pH and Eh. Thus, studies of Mn in the environment have almost always employed this paradigm for defining different Mn phases based on operational definitions: Mn that passes through a 0.2 or 0.4 μm filter is defined as soluble Mn(II) while Mn that is trapped by the filter are the solid phase Mn(III,IV) oxides. Soluble Mn species other than Mn(II) were thought not to be important because Mn(III) ions are not stable in solution and rapidly disproportionate to Mn(II) and Mn(IV). However, recent work on the mechanism of bacterial Mn(II) oxidation has demonstrated that Mn(III) occurs as an intermediate in the oxidation of Mn(II) to Mn(IV) oxides (Webb et al. 2005b; Parker et al. 2007; Anderson et al. 2009b) and that a variety of inorganic and organic ligands can complex Mn(III) and render it relatively stable in solution. In this article we review these new insights into the molecular mechanism of bacterial Mn(II) oxidation and recent advances in our understanding of Mn(II) oxidation in the environment. The study of the importance of Mn in the environment needs to employ the new paradigm for Mn cycling which takes into account the role of soluble Mn(III) species.