Majumder, and Wall reported in 2017 that uranium bio-transformations are the many and varying types of interactions that microbes can have with uranium encountered in their environment. In this review, bio-transformations, including reduction, oxidation, respiration, sorption, mineralization, accumulation, precipitation, biomarkers, and sensors are defined and discussed by the authors.. Consensus and divergences are noted in bioavailability, mechanism of uranium reduction, environment, metabolism and the type of organism.
The breadth of organisms with characterized bio-trans formations is also cataloged and discussed. We further debate if uranium biotransformations provide bio-protection or bio-benefit to the microbe and highlight the need for more work in the field to understand if microbes use uranium reduction for energy gain and growth, as having the ability is separate from exercising it. The authors focused on the fundamental drivers for these processes with an additional exposition of the essential contribution of inorganic chemistry techniques to the molecular characterization of these biological processes.
Contributions of Inorganic Chemistry to the Study of Uranium Bio-Transformations
The authors concluded that while perhaps not widely recognized, standard inorganic chemistry techniques and their advancement have been integral to the characterization and determination of the uranium bio-transformations on a molecular level. The following techniques have been used extensively in the references of this review paper:
- Mossbauer spectroscopy,
- X-ray Absorption Spectroscopy (XAS),
- Extended X-ray Absorption Fine Structure (EXAFS),
- X-ray Absorption near Edge Structure (XANES),
- X-ray Powder Diffraction (XRD),
- X-ray Photoelectron Spectroscopy (XPS),
- Transmission Electron Microscopy (TEM),
- Atomic Force Microscopy (AFM),
- Density Functional Theory calculations (DFT) and
- Laser Induced Fluorescence Spectroscopy.
These techniques determine the chemical species of the uranium in the solid mineral, soluble complexes, complexed and ligated to surfaces, mechanism intermediates, oxidation state and more. They have revealed how the important chemical speciation is to the biological process. As with any chemical reaction, certain ligands or chemical forms are highly favorable for reaction and others are inhibitory or energetically inaccessible. Knowledge of these chemical details from inorganic studies have transformed how microbiologists think about uranium bio-transformations and the mechanisms by which they happen.
Chemistry or Biology?
Even though uranium is a highly toxic metal, microbes have developed the ability to transform uranium in a number of ways. Microbes reduce, oxidize, possibly respire, mineralize, precipitate, and accumulate uranium. Humans take advantage of the reactions to stimulate remediation of contaminated sites, to sense uranium in the environment, to determine the biological activity in the early earth, or to synthesize nanoparticles and wires. The variety of interactions with uranium leaves many areas of research still open: characterization of more reduction mechanisms, identifying more organisms capable of these processes, understanding uranium-reducing community dynamics on a molecular scale, determining if microbes respire uranium, determining the biological benefit to the microbes that do uranium bio-transformations, and determining if and how microbes survive with accumulated and precipitated uranium in and on cells.
Underlying all of it is the classical question in Earth’s processes—chemistry or biology or an intriguing combination?
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