NATURE'S OWN NANOPARTICLES: BACTERIOGENIC MANGANESE OXIDES AS AGENTS OF BIOGEOCHEMICAL CHANGE

Manganese(IV) oxides are widely distributed in terrestrial and aquatic environments as discrete particles, as crusts or coatings on rocks and biological substrates, and as major components of marine and freshwater nodules. These dominantly layer-type MnO₂ minerals, comprising stacked nanosheets of edge-sharing MnO₆ octahedra studded with cation vacancies, engage in a host of reactions important to the life cycles of organisms and to the quality of soils and natural waters, prominent among them the sorption of metals and metalloids and the oxidation of a broad range of inorganic and organic species, including pollutants. These minerals also serve as terminal electron acceptors for anaerobic microbial respiration linked to the oxidation of fermentation products and other organic compounds found in natural and contaminated settings. Consensus exists that the Mn(IV) oxides found in weathering environments form primarily by the oxidation of soluble Mn(II), a process that is favorable thermodynamically in oxic or suboxic settings, but which exhibits very slow homogeneous reaction kinetics. The rates of homogeneous and bacterial oxidation of soluble Mn(II) are related in order of magnitude as 1:1000 under conditions representative of natural waters. This stunning kinetic advantage of biological catalysis has led to the widespread view that most if not all secondary Mn(IV) oxides formed in surficial environments are microbially-produced, mainly by bacteria.

Gordon Brown has pioneered an approach, termed molecular biogeochemistry, which, as will be illustrated in our presentation, is especially well-suited for probing the fundamental properties of this unique class of environmental nanoparticles, particularly molecular structure, electron-transfer reactions, and metal-binding capability. The distinguishing characteristic of this approach is the use of molecular-scale techniques to characterize natural samples exhibiting significant complexity and heterogeneity. Adaptive interplay between discipline (chemistry, microbiology, physics) and technique (wet chemistry, spectroscopy, molecular modeling) is an essential feature of both the questions posed and the science undertaken.