Microbial U Isotope Fractionation Depends on the U(VI) Reduction Rate

Mechanisms of U enrichment and helium generation potential in marine black shales following U isotope-constrained Neoproterozoic Oxidation Event

Following the NOE, the early Cambrian witnessed the global deposition of marine black shales with high U concentrations. This study analyzes the Lower Cambrian Yuertusi Formation in the Tarim Basin, China, focusing on U isotopes to elucidate U enrichment mechanisms in black shales and their potential for helium generation. In wells XK-1, LT-1, and LT-3, the average U concentrations in the Yuertusi Formation black shale are 41.7 ppm, 29.21 ppm, and 275.28 ppm, respectively. U enrichment in black shales is jointly controlled by continental weathering, paleoproductivity, oceanic oxidation, and organic matter. A synchronous increase in global atmospheric oxygen levels and weathering processes, leading to the continuous weathering of land rocks rich in U and nutrient elements, which were then transported to the ocean by rivers, laying the foundation for U enrichment in black shales and the accumulation of organic matter. The δ238U values of the Yuertusi Formation range from -0.44 ‰ to 0.37 ‰, showing two phases of first positive and then negative drift in δ238U values, reflecting a process where the area of oceanic oxidation experienced an expansion followed by contraction. During the expansion of the oceanic oxidation area, the paleoproductivity and U concentration in the oceanic oxidation layer increased, allowing soluble U elements to accumulate in black shales through reduction and organic matter adsorption in deep water anoxic environments. Conversely, during the contraction of the oceanic oxidation area, the U concentration in the oceanic oxidation layer decreased, resulting in significantly lower U concentration in the deposited dolostones or limestones compared to black shales. The early Cambrian black shales enriched with U can serve as effective helium source rocks, with an estimated cumulative release of approximately 1382 × 108 m3 of helium gas. The insights gained from this study are significant for understanding the redox state of the ocean following the NOE and for guiding the exploration of ultra-deep helium gas.

Electron correlation effects on uranium isotope fractionation in U(VI)-U(VI) and U(IV)-U(VI) equilibrium isotopic exchange systems

Uranium isotope fractionation has been extensively investigated in the fields of nuclear engineering and geochemical studies, yet the underlying mechanisms remain unclear. This study assessed isotope fractionations in U(VI)-U(VI) and U(IV)-U(VI) systems by employing various relativistic electron correlation methods to explore the effect of electron correlation and to realize accurate calculations of isotope fractionation coefficients (ε). The nuclear volume term, ln Knv, the major term in ε, was estimated using the exact two-component relativistic Hamiltonian in conjunction with either HF, DFT(B3LYP), MP2, CCSD, CCSD(T), FSCCSD, CASPT2, or RASPT2 approaches for small molecular models with high symmetry. In contrast, chemical species studied in prior experimental work had moderate sizes and were asymmetrical. In such cases, electron correlation calculations other than DFT calculations were not possible and so only the HF and B3LYP approaches were employed. For closed-shell U(VI)-U(VI) systems, the MP2, CCSD and CCSD(T) methods yielded similar ln Knv values that were intermediate between those for the HF and B3LYP methods. Comparisons with experimental results for U(VI)-U(VI) systems showed that the B3LYP calculations gave results closer to the experimental data than the HF calculations. Because of the open-shell structure of U(IV), multireference methods involving the FSCCSD, CASPT2 and RASPT2 techniques were used for U(IV)-U(VI) systems, but these calculations exhibited instability. The average-of-configuration HF method showed better agreement with the experimental ε values for U(IV)-U(VI) systems than the B3LYP method. Overall, electron correlation improved the description of ε for the U(VI)-U(VI) systems but challenges remain with regard to open-shell U(IV) calculations because an energy accuracy of 10-6-10-7Eh is required for ln Knv calculations.

Pentavalent U Reactivity Impacts U Isotopic Fractionation during Reduction by Magnetite

Meaningful interpretation of U isotope measurements relies on unraveling the impact of reduction mechanisms on the isotopic fractionation. Here, the isotope fractionation of hexavalent U [U(VI)] was investigated during its reductive mineralization by magnetite to intermediate pentavalent U [U(V)] and ultimately tetravalent U [U(IV)]. As the reaction proceeded, the remaining aqueous phase U [containing U(VI) and U(V)] systematically carried light isotopes, whereas in the bicarbonate-extracted solution [containing U(VI) and U(V)], the δ238U values varied, especially when C/C0 approached 0. This variation was interpreted as reflecting the variable relative contribution of unreduced U(VI) (δ238U < 0‰) and bicarbonate-extractable U(V) (δ238U > 0‰). The solid remaining after bicarbonate extraction included unextractable U(V) and U(IV), for which the δ238U values consistently followed the same trend that started at 0.3-0.5‰ and decreased to ∼0‰. The impact of PIPES buffer on isotopic fractionation was attributed to the variable abundance of U(V) in the aqueous phase. A few extremely heavy bicarbonate-extracted δ238U values were due to mass-dependent fractionation resulting from several hypothesized mechanisms. The results suggest the preferential accumulation of the heavy isotope in the reduced species and the significant influence of U(V) on the overall isotopic fractionation, providing insight into the U isotope fractionation behavior during its abiotic reduction process.

Mercury Isotope Fractionation during Dark Abiotic Reduction of Hg(II) by Dissolved, Surface-Bound, and Structural Fe(II)

Stable mercury (Hg) isotope ratios are an emerging tracer for biogeochemical transformations in environmental systems, but their application requires knowledge of isotopic enrichment factors for individual processes. We investigated Hg isotope fractionation during dark, abiotic reduction of Hg(II) by dissolved iron(Fe)(II), magnetite, and Fe(II) sorbed to boehmite or goethite by analyzing both the reactants and products of laboratory experiments. For homogeneous reduction of Hg(II) by dissolved Fe(II) in continuously purged reactors, the results followed a Rayleigh distillation model with enrichment factors of -2.20 ± 0.16‰ (ε202Hg) and 0.21 ± 0.02‰ (E199Hg). In closed system experiments, allowing reequilibration, the initial kinetic fractionation was overprinted by isotope exchange and followed a linear equilibrium model with -2.44 ± 0.17‰ (ε202Hg) and 0.34 ± 0.02‰ (E199Hg). Heterogeneous Hg(II) reduction by magnetite caused a smaller isotopic fractionation (-1.38 ± 0.07 and 0.13 ± 0.01‰), whereas the extent of isotopic fractionation of the sorbed Fe(II) experiments was similar to the kinetic homogeneous case. Small mass-independent fractionation of even-mass Hg isotopes with 0.02 ± 0.003‰ (E200Hg) and ≈ -0.02 ± 0.01‰ (E204Hg) was consistent with theoretical predictions for the nuclear volume effect. This study contributes significantly to the database of Hg isotope enrichment factors for specific processes. Our findings show that Hg(II) reduction by dissolved Fe(II) in open systems results in a kinetic MDF with a larger ε compared to other abiotic reduction pathways, and combining MDF with the observed MIF allows the distinction from photochemical or microbial Hg(II) reduction pathways.

What Controls the Sulfur Isotope Fractionation during Dissimilatory Sulfate Reduction?

Sulfate often behaves conservatively in the oxygenated environments but serves as an electron acceptor for microbial respiration in a wide range of natural and engineered systems where oxygen is depleted. As a ubiquitous anaerobic dissimilatory pathway, therefore, microbial reduction of sulfate to sulfide has been of continuing interest in the field of microbiology, ecology, biochemistry, and geochemistry. Stable isotopes of sulfur are an effective tool for tracking this catabolic process as microorganisms discriminate strongly against heavy isotopes when cleaving the sulfur-oxygen bond. Along with its high preservation potential in environmental archives, a wide variation in the sulfur isotope effects can provide insights into the physiology of sulfate reducing microorganisms across temporal and spatial barriers. A vast array of parameters, including phylogeny, temperature, respiration rate, and availability of sulfate, electron donor, and other essential nutrients, has been explored as a possible determinant of the magnitude of isotope fractionation, and there is now a broad consensus that the relative availability of sulfate and electron donors primarily controls the magnitude of fractionation. As the ratio shifts toward sulfate, the sulfur isotope fractionation increases. The results of conceptual models, centered on the reversibility of each enzymatic step in the dissimilatory sulfate reduction pathway, are in qualitative agreement with the observations, although the underlying intracellular mechanisms that translate the external stimuli into the isotopic phenotype remain largely unexplored experimentally. This minireview offers a snapshot of our current understanding of the sulfur isotope effects during dissimilatory sulfate reduction as well as their potential quantitative applications. It emphasizes the importance of sulfate respiration as a model system for the isotopic investigation of other respiratory pathways that utilize oxyanions as terminal electron acceptors.

Stable ocean redox during the main phase of the Great Ordovician Biodiversification Event

The Great Ordovician Biodiversification Event (GOBE) represents the greatest increase in marine animal biodiversity ever recorded. What caused this transformation is heavily debated. One hypothesis states that rising atmospheric oxygen levels drove the biodiversification based on the premise that animals require oxygen for their metabolism. Here, we present uranium isotope data from a Middle Ordovician marine carbonate succession that shows the steepest rise in generic richness occurred with global marine redox stability. Ocean oxygenation ensued later and could not have driven the biodiversification. Stable marine anoxic zones prevailed during the maximum increase in biodiversity (Dapingian-early Darriwilian) when the life expectancy of evolving genera greatly increased. Subsequently, unstable ocean redox conditions occurred together with a marine carbon cycle disturbance and a decrease in relative diversification rates. Therefore, we propose that oceanic redox stability was a factor in facilitating the establishment of more resilient ecosystems allowing marine animal life to radiate.

Bioaccumulation of uranium by Candida utilis: Investigated by water chemistry and biological effects

The bioaccumulation of hexavalent uranium (U(VI)) on Candida utilis (C. utilis) and its biological effects were investigated via batch and biologic techniques. The bioaccumulation mechanism of U(VI) and C. utilis were characterized by SEM, TEM, FT-IR and XPS. The batch results showed that C. utilis had a high adsorption capacity (41.15 mg/g wet cells at pH 5.0) and high equilibrium rate (~100% within 3.5 h). The analysis of intracellular hydrogen peroxides and malondialdehyde suggested that the growth of C. utilis was inhibited under different concentrations of U(VI) due to the abundant production of reactive oxide species. The activity of intracellular antioxidants (e.g., super oxide dismutase and glutathione) was significantly enhanced under U(VI) stress, indicating the anti-toxic effect of C. utilis cells under low U(VI) stress. These results indicated that C. utilis is an ideal biosorbent for removing radionuclides in environmental remediation.

Rapid and highly efficient genomic engineering with a novel iEditing device for programming versatile extracellular electron transfer of electroactive bacteria

The advances in synthetic biology bring exciting new opportunities to reprogram microorganisms with novel functionalities for environmental applications. For real-world applications, a genetic tool that enables genetic engineering in a stably genomic inherited manner is greatly desired. In this work, we design a novel genetic device for rapid and efficient genome engineering based on the intron-encoded homing-endonuclease empowered genome editing (iEditing). The iEditing device enables rapid and efficient genome engineering in Shewanella oneidensis MR-1, the representative strain of the electroactive bacteria group. Moreover, combining with the Red or RecET recombination system, the genome-editing efficiency was greatly improved, up to approximately 100%. Significantly, the iEditing device itself is eliminated simultaneously when genome editing occurs, thereby requiring no follow-up to remove the encoding system. Then, we develop a new extracellular electron transfer (EET) engineering strategy by programming the parallel EET systems to enhance versatile EET. The engineered strains exhibit sufficiently enhanced electron output and pollutant reduction ability. Furthermore, this device has demonstrated its great potential to be extended for genome editing in other important microbes. This work provides a useful and efficient tool for the rapid generation of synthetic microorganisms for various environmental applications.