ABSTRACT

Manganese (Mn) is a biologically vital element, supporting life through its use by many enzymes, including those that produce oxygen in plants and that defend many living organisms against reactive oxygen species. Thus, the manganese cycle?the interconversion between bioavailable Mn ions (Mn(II)) and insoluble Mn oxide minerals (MnO2)?is globally important. Microorganisms play a significant part in driving the manganese cycle: some bacteria use MnO2 for respiration, in the process converting the mineral to dissolved Mn(II), while other bacteria can oxidize Mn(II), forming the MnO2 deposits that can be found in many environments. The latter process, biomineralization, is a less-understood field in manganese biogeochemistry. With this award, Dr. Bradley Tebo and Dr. Thomas Spiro will develop a comprehensive picture of bacterial manganese oxide biomineralization, which is essential for understanding how MnO2 are processed in nature, and how this insight might be applied to the burgeoning uses of MnO2 minerals in environmental remediation and bioenergy production. The project will enhance the training of the next generation of scientists and embrace outreach activities, including mentoring students and programs that seek to attract underserved/underrepresented middle and high school students to science majors. A set of artistic illustrations to communicate the project to a broader audience will be created and made available through various channels.

In many Mn(II)-oxidizing bacteria, multicopper oxidase (MCO) enzymes have been implicated to be the catalysts for Mn(II) oxidation. In Mn(II)-oxidizing Bacillus species, dormant spores oxidize Mn(II) and form MnO2 minerals, catalyzed by MCOs residing in the exosporium?a complex structure that surrounds the spores. However, the molecular mechanism of MnO2 production remains to be elucidated. With the first purified bacterial manganese oxidizing complex, Mnx, and a collection of manganese-oxidizing bacteria, this project will reveal how bacteria control the formation of MnO2 nanoparticles. Specifically, investigators will characterize how the protein guides the formation of mineral units, how they are expelled into the solution and further grow to form the mineral found in nature, and how complexities of natural environment?biological matter of whole cells, complexing agents, and dissolved iron?affect the final biomineral. The project is highly leveraged through a collaborative and integrated approach of multiple state-of-the-art techniques, including cryoEM, SAXS, EXAFS, liquid-cell TEM, and computational methods to offer a unique molecular-level view of manganese oxide biomineralization. Additionally, the project will expand the use of state-of-the-art microscopic techniques, primarily used in biomedical and materials science research, to address questions of geochemical significance.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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