Mechanism of uranium reduction and immobilization in Desulfovibrio vulgaris biofilms

The prevalent formation of noncrystalline U(IV) species in the subsurface and their enhanced susceptibility to reoxidation and remobilization, as compared to crystalline uraninite, raise concerns about the long-term sustainability of the bioremediation of U-contaminated sites. The main goal of this study was to resolve the remaining uncertainty concerning the formation mechanism of noncrystalline U(IV) in the environment. Controlled laboratory biofilm systems (biotic, abiotic, and mixed biotic abiotic) were probed using a combination of U isotope fractionation and X-ray absorption spectroscopy (XAS). Regardless of the mechanism of U reduction, the presence of a biofilm resulted in the formation of noncrystalline U(IV). Our results also show that biotic U reduction is the most effective way to immobilize and reduce U. However, the mixed biotic abiotic system resembled more closely an abiotic system: (i) the U(IV) solid phase lacked a typically biotic isotope signature and (ii) elemental sulfur was detected, which indicates the oxidation of sulfide coupled to U(VI) reduction. The predominance of abiotic U reduction in our systems is due to the lack of available aqueous U(VI) species for direct enzymatic reduction. In contrast, in cases where bicarbonate is present at a higher concentration, aqueous U(VI) species dominate, allowing biotic U reduction to outcompete the abiotic processes

Uranium Isotopes Fingerprint Biotic Reduction

Knowledge of paleo-redox conditions in the Earth's history provides a window into events that shaped the evolution of life on our planet. The role of microbial activity in paleo-redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record. The ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo-history and to correlate specific biogeochemical events with the corresponding microbial metabolism. Here, we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium (U), i.e., the accumulation of the heavy isotope in the U(IV) phase, is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments. Thus, isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought-after tool to probe for biotic processes. Because uranium is a common element in the Earth's crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of U(VI), this tool is applicable to a multiplicity of geological epochs and terrestrial environments. The findings of this study indicate that biological activity contributed to the formation of many authigenic U deposits, including sandstone U deposits of various ages, as well as modern, Cretaceous, and Archean black shales. Additionally, engineered bioremediation activities also exhibit a biotic signature, suggesting that, although multiple pathways may be involved in the reduction, direct enzymatic reduction contributes substantially to the immobilization of uranium

Uranium Isotopes Fingerprint Biotic Reduction

Knowledge of paleo-redox conditions in the Earth's history provides a window into events that shaped the evolution of life on our planet. The role of microbial activity in paleo-redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record. The ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo-history and to correlate specific biogeochemical events with the corresponding microbial metabolism. Here, we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium (U), i.e., the accumulation of the heavy isotope in the U(IV) phase, is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments. Thus, isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought-after tool to probe for biotic processes. Because uranium is a common element in the Earth's crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of U(VI), this tool is applicable to a multiplicity of geological epochs and terrestrial environments. The findings of this study indicate that biological activity contributed to the formation of many authigenic U deposits, including sandstone U deposits of various ages, as well as modern, Cretaceous, and Archean black shales. Additionally, engineered bioremediation activities also exhibit a biotic signature, suggesting that, although multiple pathways may be involved in the reduction, direct enzymatic reduction contributes substantially to the immobilization of uranium

Report and preliminary results of SONNE cruise SO175, Miami - Bremerhaven, 12.11 - 30.12.2003 : (GAP, Gibraltar Arc Processes)

Expedition SO175 using FS Sonne aimed for a multidisciplinerary geoscientific approach with an international group of researchers. Methods covered the entire span from geophysical data acquisition (seafloor mapping, echography, seismic reflection), sediment coring at sites of active fluid venting, in situ heat flow measurements across the entire length of the Gibraltar thrust wedge, the deformation front, landslide bodies, and mud volcanoes, and finally the deployment of a long-term pore pressure probe. Video-supported operations helped to identify fluid vent sites, regions with tectonic activity, and other attractive high priority targets. Qualitative and quantitative examinations took place on board and are continued on land with respect to pore pressure variation, geomicrobiology, sediment- and fluid mobilization, geochemical processes, faunal assemblages (e.g. cold water corals), and gas hydrates (flammable methane-ice-crystals). Main focus of the expedition has been a better understanding of interaction between dynamic processes in a seismically active region region with slow plate convergence. In the context of earthquake nucleation and subduction zone processes, the SO175 research programme had a variety of goals, such as: • To test the frictional behaviour of the abyssal plain sediments. • To explore the temperature field of the 1755 thrust earthquake event via heat flow measurements. • To assess the role of fluid venting and gas hydrate processes control slope stability and mud volcanic activity along the Iberian continental margin. • To measure isotope geochemistry of pore waters and carbonates of deep fluids. • To quantify microbial activity in Gibraltar wedge sediments. • To test whether microseismicity in the area corresponds to in situ pore pressure changes. • To find out if enhanced heat flow max be indicative of active subduction. Initial tentative results during the cruise suggest that there is a component of active thrusting at the base of the wedge, as attested by heat flow data. Based on mostly geochemical evidence, mud volcanism was found less active than previously assumed. Highlights from post-cruise research include the successful deployment of the long-term station and high frictional resistance of all incoming sediment on the three abyssal plains

Phagosomal signalling of the C-type lectin receptor Dectin-1 is terminated by intramembrane proteolysis

Dectin-1 is a critical component of the innate sensing repertoire which is involved in pattern based recognition of fungal pathogens. Here the authors show that intramembrane proteolysis is involved in the regulation of the antifungal host response by termination of the phagosomal signalling of Dectin-1. Sensing of pathogens by pattern recognition receptors (PRR) is critical to initiate protective host defence reactions. However, activation of the immune system has to be carefully titrated to avoid tissue damage necessitating mechanisms to control and terminate PRR signalling. Dectin-1 is a PRR for fungal beta-glucans on immune cells that is rapidly internalised after ligand-binding. Here, we demonstrate that pathogen recognition by the Dectin-1a isoform results in the formation of a stable receptor fragment devoid of the ligand binding domain. This fragment persists in phagosomal membranes and contributes to signal transduction which is terminated by the intramembrane proteases Signal Peptide Peptidase-like (SPPL) 2a and 2b. Consequently, immune cells lacking SPPL2b demonstrate increased anti-fungal ROS production, killing capacity and cytokine responses. The identified mechanism allows to uncouple the PRR signalling response from delivery of the pathogen to degradative compartments and identifies intramembrane proteases as part of a regulatory circuit to control anti-fungal immune responses

Phagosomal signalling of the C-type lectin receptor Dectin-1 is terminated by intramembrane proteolysis

Sensing of pathogens by pattern recognition receptors (PRR) is critical to initiate protective host defence reactions. However, activation of the immune system has to be carefully titrated to avoid tissue damage necessitating mechanisms to control and terminate PRR signalling. Dectin-1 is a PRR for fungal β-glucans on immune cells that is rapidly internalised after ligand-binding. Here, we demonstrate that pathogen recognition by the Dectin-1a isoform results in the formation of a stable receptor fragment devoid of the ligand binding domain. This fragment persists in phagosomal membranes and contributes to signal transduction which is terminated by the intramembrane proteases Signal Peptide Peptidase-like (SPPL) 2a and 2b. Consequently, immune cells lacking SPPL2b demonstrate increased anti-fungal ROS production, killing capacity and cytokine responses. The identified mechanism allows to uncouple the PRR signalling response from delivery of the pathogen to degradative compartments and identifies intramembrane proteases as part of a regulatory circuit to control anti-fungal immune responses

Sulfidity controls molybdenum isotope fractionation into euxinic sediments: Evidence from the modern Black Sea

Molybdenum (Mo) isotope fractionation has recently been introduced as a new proxy in oceanography and biogeochemistry. It is therefore fundamental to understand the processes controlling Mo partitioning into modern marine environments. This study identifies the availability of dissolved sulfide as the dominant control on overall Mo removal from the water column in euxinic systems. Mo isotopic composition of surface sediments from different localities of the Black Sea demonstrates complete fixation of Mo only below 400 m water depth, above a critical concentration of 11 μmol l−1 aqueous hydrogen sulfide in the bottom water. The Mo isotopic composition of these sediments reflects the homogeneous seawater isotopic composition of 2.3‰. In contrast, significant Mo isotope fractionation into less euxinic sediments is evident at shallower depths in the Black Sea, as well as in temporarily euxinic deeps of the Baltic Sea, consistent with the observed lower maximum sulfide concentrations in the respective water columns. Therefore, Mo isotope signatures in the modern Black Sea constrain the processes responsible for global Mo removal from the ocean by euxinic sediments. Furthermore, models of past ocean anoxia reconstruction have to consider that the seawater Mo isotopic composition is not per se archived in euxinic sediments

Barium isotope fractionation during experimental formation of the double carbonate BaMn[CO₃]₂ at ambient temperature

In this study, we present the first experimental results for stable barium (Ba) isotope (¹³⁷Ba/¹³⁴Ba) fractionation during low-temperature formation of the anhydrous double carbonate BaMn[CO₃]₂. This investigation is part of an ongoing work on Ba fractionation in the natural barium cycle. Precipitation at a temperature of 21±1°C leads to an enrichment of the lighter Ba isotope described by an enrichment factor of−0.11±0.06‰ in the double carbonate than in an aqueous barium-manganese(II) chloride/sodium bicarbonate solution, which is within the range of previous reports for synthetic pure BaCO ₃ (witherite) formation