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Uranium isotope fractionation: a novel biosignature to identify microbial metabolism on early Earth

Periodic Reporting for period 2 - UNEARTH (Uranium isotope fractionation: a novel biosignature to identify microbial metabolism on early Earth)

Reporting period: 2019-03-01 to 2020-08-31

Uranium reduction is a natural process that occurs in the absence of oxygen and is catalyzed by microorganisms or by reduced aqueous and solid phase chemical species. Oxidized uranium (hexavalent U) is soluble in the presence of common water chemicals (e.g. carbonate) while reduced U (tetravalent U) is a highly insoluble chemical species.
Because of the change in physical state associated with U reduction, this transformation is an engineered strategy for the remediation of the uranium-contaminated subsurface (sediments, groundwater). Uranium includes two major isotopes, 238U and 235U. Based on theoretical considerations, these two isotopes are expected to exhibit distinct behaviors during uranium reduction. The heavy isotope (238U) is predicted to be preferentially reduced and, as a result, reduced U chemical species (thus, containing tetravalent U) are expected to contain more 238U than oxidized U (hexavalent U). Thus, when measuring the isotopic signature of a sediment or a rock, a heavy U isotopic signature (richer in 238U than background environmental samples) is interpreted as having been deposited in the absence of O2. This is because, in the absence of O2, U is expected to be reduced and sequestered in the solid phase. As a result, rocks are expected to preserve the heavy U associated with reduced U at the time of formation.
Such signatures in the rock record are used to pinpoint periods of time during which anoxia was prevalent. This is one of many strategies to reconstruct past redox conditions at the planetary scale (i.e. periods of time before the oxygen concentration reached the current value). For remediation, the isotopic signature of the remaining hexavalent U, that has not been reduced, is expected to become increasingly light (richer in 235U), as the heavy U (238U) is progressively depleted. Understanding the temporal evolution of the planet provides insight into how life evolved on Earth, revealing the conditions that led to the emergence of life. This is also useful for the exploration of other planets.
However, there remain fundamental questions about how to interpret U isotope fractionation. Data are emerging that show that, under specific circumstances, U isotope fractionation does not behave as predicted by theory. It is therefore of fundamental importance to unravel the mechanistic underpinnings of U isotopic fractionation in order to correctly interpret the data from the rock record and from engineered applications.
The UNEARTH project was designed to investigate the mechanistic basis for U isotopic fractionation for biological and abiotic reduction. It combines biology, geochemistry, isotope geochemistry, computational chemistry, spectroscopy, electron microscopy, and coordination chemistry to provide a fundamental understanding of this process.
The inter-disciplinary approach in this project has yielded several results. The first major result is the unveiling of the mechanism of U reduction by the reduced iron oxide mineral phase, magnetite. The new model evidences the binding of hexavalent uranium on the surface of magnetite, followed a one-electron transfer, that produces pentavalent and later, tetravalent U. Upon the reduction, tetravalent U nucleates into 1-2 nm UO2 nanoparticles, which self-organize into nanowires in which nanoparticles exhibit random orientation. These nanowire structures are transient and collapse and reorganize into clusters of UO2 nanoparticles with the same orientation. The impact of nanowire formation on isotope fraction is not well understood but will be investigated next.
The second major result is the identification of the biological reduction of pentavalent U to tetravalent as an enzymatic step when pentavalent U is stabilized by an organic ligand. Once again, this represents the first phase of the project, in which the mechanism of U reduction is explored in more detail than previously in order to link it to the associated isotope fractionation. The possibility of enzymatic reduction of pentavalent U suggests that, in organic-rich environments, the reduction of U may proceed in two enzymatic steps rather than a one-electron transfer followed by disproportionation.
We expect to be able to provide a comprehensive understanding of the mechanistic basis of U isotope fractionation and to pinpoint the factor that control the extent and the direction of fractionation. This will provide a sound scientific basis for the interpretation of U isotope fractionation in the rock record and in environmental applications.