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

Periodic Reporting for period 4 - UNEARTH (Uranium isotope fractionation: a novel biosignature to identify microbial metabolism on early Earth)

Periodo di rendicontazione: 2022-03-01 al 2023-02-28

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.
At the end of the study, the findings are numerous and complex. The work has helped uncover the parameters that control isotopic fractionation of U. They are detailed below:
1. The impact of the flux of electrons (moles of electron per surface area per time) on isotope fractionation is critical. In order for uranium reduction to take place, electrons must be donated to the radionuclide and the rate at which those electrons are supplied is a determining factor in the fractionation. When there is a high flux of electrons, forward electron transfer from the cell to uranium is strongly favored, meaning that electron flow in the opposite direction (from uranium to cell) is unfavorable. In this case, there is limited uranium fractionation because this process is more prevalent when there is a back reaction. In contrast, when the cell is electron limited (corresponding to an environment with limited organic carbon, for example), the electrons can flow back into the cell, allowing re-equilibration and the increased expression of fractionation. For the community that use uranium isotopic fractionation to reconstruct conditions on early Earth, this means that the presence of abundant organic carbon would have dampened the extent of fractionation, while a low carbon environment would have favored that signature.

2. In the process of investigating abiotic uranium reduction (by minerals), we discovered that uranium forms nanoparticles consisting of mixed-valence uranium oxide particles, that self-assemble end-to-end to form nanowires. This is the mechanism of reduction of hexavalent to tetravalent uranium by iron oxides and some clays. Because these nanoparticles are very small (2 nanometers), it is extremely challenging to interrogate the nanowires and understand what controls their formation and ultimately their collapse. By using electron energy loss spectroscopy, we can show that the nanowires persist as long as they include pentavalent uranium but collapse once uranium is completely reduced to tetravalent uranium. The conclusion for modern and ancient environments is that, depending on the availability of electrons to reduce uranium, these nanowires containing pentavalent uranium may persist for extended periods of time, impacting the isotopic signature in (currently) unpredictable ways.

2. The isotopic fractionation of uranium was studied extensively in the mid 20th century by Bigeleisen and he identified a phenomenon that was specific to heavy elements (such as uranium) and named it the nuclear field shift. It was not well understood whether this process impacted isotopic fractionation during microbial uranium reduction. By using four uranium isotopes (233U, 235U, 236U, and 238U), we carried out an experiment that allows to separate the two major processes thought to be involved in isotopic fractionation: the nuclear field shift and vibrational effects. Combining the experimental data with ab initio calculations, it was possible to determine that while the vibrational effect behaves in the way calculated, the nuclear field shift does not. This result suggests that variations in the overall isotopic fractionation, as a function of environmental conditions are driven by the nuclear field shift.

4. To fully investigate uranium isotopic fractionation, it was necessary to unravel the mechanism of uranium reduction. For the biotic system, the paradigm that U(V) disproportionation is a major avenue for U reduction was challenged by evidencing the reduction of pentavalent uranium by cells and by purified protein. For the abiotic system, U(V) chemical stability as an incorporated impurity in magnetite and goethite was investigated and challenged by changing chemical conditions. In other words, iron oxides will not retain the U isotopic signature that represents their formation conditions if there is any change in the chemical environment due to substantial dissolution and reprecipitation undergone by these minerals.
With this work, we have been able to provide a comprehensive understanding of the mechanistic basis of U isotope fractionation and to pinpoint the factors that control the extent and the direction of fractionation. These are electron flux, complexation by ligands, and the variable expression of the nuclear field shift. These findings provide a sound scientific basis for the interpretation of U isotope fractionation in the rock record and in environmental applications.
Electron micrograph showing a uraninite nanowire (arrow) attached to magnetite. scale bar 5 nm