Atomically dispersed hexavalent iridium oxide from MnO2 reduction for oxygen evolution catalysis.

Hexavalent iridium (IrVI) oxide is predicted to be more active and stable than any other iridium oxide for the oxygen evolution reaction in acid; however, its experimental realization remains challenging. In this work, we report the synthesis, characterization, and application of atomically dispersed IrVI oxide (IrVI-ado) for proton exchange membrane (PEM) water electrolysis. The IrVI-ado was synthesized by oxidatively substituting the ligands of potassium hexachloroiridate(IV) (K2IrCl6) with manganese oxide (MnO2). The mass-specific activity (1.7 × 105 amperes per gram of iridium) and turnover number (1.5 × 108) exceeded those of benchmark iridium oxides, and in situ x-ray analysis during PEM operations manifested the durability of IrVI at current densities up to 2.3 amperes per square centimeter. The high activity and stability of IrVI-ado showcase its promise as an anode material for PEM electrolysis.

Copper sulfide mineral performs non-enzymatic anaerobic ammonium oxidation through a hydrazine intermediate.

Anaerobic ammonium oxidation (anammox)-the biological process that activates ammonium with nitrite-is responsible for a significant fraction of N2 production in marine environments. Despite decades of biochemical research, however, no synthetic models capable of anammox have been identified. Here we report that a copper sulfide mineral replicates the entire biological anammox pathway catalysed by three metalloenzymes. We identified a copper-nitrosonium {CuNO}10 complex, formed by nitrite reduction, as the oxidant for ammonium oxidation that leads to heterolytic N-N bond formation from nitrite and ammonium. Similar to the biological process, N2 production was mediated by the highly reactive intermediate hydrazine, one of the most potent reductants in nature. We also found another pathway involving N-N bond heterocoupling for the formation of hybrid N2O, a potent greenhouse gas with a unique isotope composition. Our study represents a rare example of non-enzymatic anammox reaction that interconnects six redox states in the abiotic nitrogen cycle.

Rationalizing the Influence of the Binding Affinity on the Activity of Phosphoserine Phosphatases.

The Sabatier principle states that catalytic activity can be maximized when the substrate binding affinity is neither too strong nor too weak. Recent studies have shown that the activity of several hydrolases is maximized at intermediate values of the binding affinity (Michaelis-Menten constant: Km ). However, it remains unclear whether this concept of artificial catalysis is applicable to enzymes in general, especially for those which have evolved under different reaction environments. Herein, we show that the activity of phosphoserine phosphatase is also enhanced at an intermediate Km value of approximately 0.5 mM. Within our dataset, the variation of Km by three orders of magnitude accounted for a roughly 18-fold variation in the activity. Owing to the high phylogenetic and physiological diversity of our dataset, our results support the importance of optimizing Km for enzymes in general. On the other hand, a 77-fold variation in the activity was attributed to other physicochemical parameters, such as the Arrhenius prefactor of kcat , and could not be explained by the Sabatier principle. Therefore, while tuning the binding affinity according to the Sabatier principle is an important consideration, the Km value is only one of many physicochemical parameters which must be optimized to maximize enzymatic activity.

Gas fermentation combined with water electrolysis for production of polyhydroxyalkanoate copolymer from carbon dioxide by engineered Ralstonia eutropha.

A recycled-gas closed-circuit culture system was developed for safe autotrophic cultivation of a hydrogen-oxidizing, polyhydroxyalkanoate (PHA)-producing Ralstonia eutropha, using a non-combustible gas mixture with low-concentration of H2 supplied by water electrolysis. Automated feedback regulation of gas flow enabled input of H2, CO2, and O2 well balanced with the cellular demands, leading to constant gas composition throughout the cultivation. The engineered strain of R. eutropha produced 1.71 g/L of poly(3-hydroxybutyrate-co-12.5 mol% 3-hydroxyhexanoate) on a gas mixture of H2/CO2/O2/N2 = 4:12:7:77 vol% with a 69.2 wt% cellular content. Overexpression of can encoding cytosolic carbonic anhydrase increased the 3HHx fraction up to 19.6 mol%. The yields of biomass and PHA on input H2 were determined to be 72.9 % and 63.1 %, corresponding to 51.0 % and 44.2 % yield on electricity, respectively. The equivalent solar-to-biomass/PHA efficiencies were estimated to be 2.1-3.8 %, highlighting the high energy conversion capability of R. eutropha.

Aqueous breakdown of aspartate and glutamate to n-ω-amino acids on the parent bodies of carbonaceous chondrites and asteroid Ryugu.

Amino acids in carbonaceous chondrites may have seeded the origin of life on Earth and possibly elsewhere. Recently, the return samples from a C-type asteroid Ryugu were found to contain amino acids with a similar distribution to Ivuna-type CI chondrites, suggesting the potential of amino acid abundances as molecular descriptors of parent body geochemistry. However, the chemical mechanisms responsible for the amino acid distributions remain to be elucidated particularly at low temperatures (<50°C). Here, we report that two representative proteinogenic amino acids, aspartic acid and glutamic acid, decompose to ß-alanine and γ-aminobutyric acid, respectively, under simulated geoelectrochemical conditions at 25°C. This low-temperature conversion provides a plausible explanation for the enrichment of these two n-ω-amino acids compared to their precursors in heavily aqueously altered CI chondrites and Ryugu's return samples. The results suggest that these heavily aqueously altered samples originated from the water-rich mantle of their water/rock differentiated parent planetesimals where protein α-amino acids were decomposed.

Thermodynamic principle to enhance enzymatic activity using the substrate affinity.

Understanding how to tune enzymatic activity is important not only for biotechnological applications, but also to elucidate the basic principles guiding the design and optimization of biological systems in nature. So far, the Michaelis-Menten equation has provided a fundamental framework of enzymatic activity. However, there is still no concrete guideline on how the parameters should be optimized towards higher activity. Here, we demonstrate that tuning the Michaelis-Menten constant ([Formula: see text]) to the substrate concentration ([Formula: see text]) enhances enzymatic activity. This guideline ([Formula: see text]) was obtained mathematically by assuming that thermodynamically favorable reactions have higher rate constants, and that the total driving force is fixed. Due to the generality of these thermodynamic considerations, we propose [Formula: see text] as a general concept to enhance enzymatic activity. Our bioinformatic analysis reveals that the [Formula: see text] and in vivo substrate concentrations are consistent across a dataset of approximately 1000 enzymes, suggesting that even natural selection follows the principle [Formula: see text].

In situ electrosynthetic bacterial growth using electricity generated by a deep-sea hydrothermal vent.

Electroautotrophic microorganisms have attracted great attention since they exhibit a new type of primary production. Here, in situ electrochemical cultivation was conducted using the naturally occurring electromotive forces at a deep-sea hydrothermal vent. The voltage and current generation originating from the resulting microbial activity was observed for 12 days of deployment, with fluctuation in response to tidal cycles. A novel bacterium belonging to the genus Thiomicrorhabdus dominated the microbial community specifically enriched on the cathode. Metagenomic analysis provided the draft genome of the bacterium and the gene repertoire indicated that the bacterium has the potential for thio-autotrophic growth, which is a typical physiological feature of the members of the genus, while the bacterium had a unique gene cluster encoding multi-heme cytochrome c proteins responsible for extracellular electron transfer. Herein, we propose this bacterium as a new species, specifically enriched during electricity generation, as 'Candidatus Thiomicrorhabdus electrophagus'. This finding suggests the natural occurrence of electrosynthetic microbial populations using the geoelectricity in deep-sea hydrothermal environments.

Tracing and regulating redox homeostasis of model benthic ecosystems for sustainable aquaculture in coastal environments.

Aquaculture in coastal environments has an increasingly important role in the world's food supply; however, the accumulation of organic compounds on seafloors due to overfeeding adversely affects benthic ecosystems. To assess the ecological resilience of aquafarms to nutrient influx, we investigated the redox homeostasis of benthic ecosystems using a marine oligochaete as a model benthic organism in aquaculture fields. Real-time monitoring of the redox potential of a model benthic ecosystem constructed in an electrochemical reactor allowed evaluation of the homeostatic response of the system to nutrient addition. Although the detrimental effects of overfeeding were confirmed by irreversible potential changes in the sediment, redox homeostasis was reinforced through a cooperative relationship between oligochaetes and sediment microorganisms. Specifically, the oligochaetes exhibited reversible changes in metabolism and body position in response to dynamic changes in the sediment potential between -300 and 500 mV, thereby promoting the decomposition of organic compounds. The potential-dependent changes in metabolism and body position were reproduced by artificially manipulating the sediment potential in electrochemical reactors. Given the importance of benthic animals in sustaining coastal ecosystems, the electrochemical monitoring and physiologic regulation of marine oligochaetes could offer an intriguing approach toward sustainable aquaculture.

Nitrite as a causal factor for nitrate-dependent anaerobic corrosion of metallic iron induced by Prolixibacter strains.

Microbially influenced corrosion (MIC) may contribute significantly to overall corrosion risks, especially in the gas and petroleum industries. In this study, we isolated four Prolixibacter strains, which belong to the phylum Bacteroidetes, and examined their nitrate respiration- and Fe0 -corroding activities, together with two previously isolated Prolixibacter strains. Four of the six Prolixibacter strains reduced nitrate under anaerobic conditions, while the other two strains did not. The anaerobic growth of the four nitrate-reducing strains was enhanced by nitrate, which was not observed in the two strains unable to reduce nitrate. When the nitrate-reducing strains were grown anaerobically in the presence of Fe0 or carbon steel, the corrosion of the materials was enhanced by more than 20-fold compared to that in aseptic controls. This enhancement was not observed in cultures of the strains unable to reduce nitrate. The oxidation of Fe0 in the anaerobic cultures of nitrate-reducing strains occurred concomitantly with the formation of nitrite. Since nitrite chemically oxidized Fe0 under anaerobic and aseptic conditions, the corrosion of Fe0 - and carbon steel by the nitrate-reducing Prolixibacter strains was deduced to be mainly enhanced via the biological reduction of nitrate to nitrite, followed by the chemical oxidation of Fe0 to Fe2+ and Fe3+ coupled to the reduction of nitrite.

Electrochemically induced metal- vs. ligand-based redox changes in mackinawite: identification of a Fe3+- and polysulfide-containing intermediate.

Under anaerobic conditions, ferrous iron reacts with sulfide producing FeS, which can then undergo a temperature, redox potential, and pH dependent maturation process resulting in the formation of oxidized mineral phases, such as greigite or pyrite. A greater understanding of this maturation process holds promise for the development of iron-sulfide catalysts, which are known to promote diverse chemical reactions, such as H+, CO2 and NO3- reduction processes. Hampering the full realization of the catalytic potential of FeS, however, is an incomplete knowledge of the molecular and redox processess ocurring between mineral and nanoparticulate phases. Here, we investigated the chemical properties of iron-sulfide by cyclic voltammetry, Raman and X-ray absorption spectroscopic techniques. Tracing oxidative maturation pathways by varying electrode potential, nanoparticulate n(Fe2+S2-)(s) was found to oxidize to a Fe3+ containing FeS phase at -0.5 V vs. Ag/AgCl (pH = 7). In a subsequent oxidation, polysulfides are proposed to give a material that is composed of Fe2+, Fe3+, S2- and polysulfide (Sn2-) species, with its composition described as Fe2+1-3xFe3+2xS2-1-y(Sn2-)y. Thermodynamic properties of model compounds calculated by density functional theory indicate that ligand oxidation occurs in conjunction with structural rearrangements, whereas metal oxidation may occur prior to structural rearrangement. These findings together point to the existence of a metastable FeS phase located at the junction of a metal-based oxidation path between FeS and greigite (Fe2+Fe3+2S2-4) and a ligand-based oxidation path between FeS and pyrite (Fe2+(S2)2-).

In situ FTIR study of CO2 reduction on inorganic analogues of carbon monoxide dehydrogenase.

The CO2-to-CO reduction by carbon monoxide dehydrogenase (CODH) with a [NiFe4S4] cluster is considered to be the oldest pathway of biological carbon fixation and therefore may have been involved in the origin of life. Although previous studies have investigated CO2 reduction by Fe and Ni sulfides to identify the prebiotic origin of the [NiFe4S4] cluster, the reaction mechanism remains largely elusive. Herein, we applied in situ electrochemical ATR-FTIR spectroscopy to probe the reaction intermediates of greigite (Fe3S4) and violarite (FeNi2S4). Intermediate species assignable to surface-bound CO2 and formyl groups were found to be stabilized in the presence of Ni, lending insight into its role in enhancing the multistep CO2 reduction process.

Thioester synthesis through geoelectrochemical CO2 fixation on Ni sulfides.

A prevailing scenario of the origin of life postulates thioesters as key intermediates in protometabolism, but there is no experimental support for the prebiotic CO2 fixation routes to thioesters. Here we demonstrate that, under a simulated geoelectrochemical condition in primordial ocean hydrothermal systems (-0.6 to -1.0 V versus the standard hydrogen electrode), nickel sulfide (NiS) gradually reduces to Ni0, while accumulating surface-bound carbon monoxide (CO) due to CO2 electroreduction. The resultant partially reduced NiS realizes thioester (S-methyl thioacetate) formation from CO and methanethiol even at room temperature and neutral pH with the yield up to 35% based on CO. This thioester formation is not inhibited, or even improved, by 50:50 coprecipitation of NiS with FeS or CoS (the maximum yields; 27 or 56%, respectively). Such a simple thioester synthesis likely occurred in Hadean deep-sea vent environments, setting a stage for the autotrophic origin of life.

Atomic-scale evidence for highly selective electrocatalytic N-N coupling on metallic MoS2.

Molybdenum sulfide (MoS2) is the most widely studied transition-metal dichalcogenide (TMDs) and phase engineering can markedly improve its electrocatalytic activity. However, the selectivity toward desired products remains poorly explored, limiting its application in complex chemical reactions. Here we report how phase engineering of MoS2 significantly improves the selectivity for nitrite reduction to nitrous oxide, a critical process in biological denitrification, using continuous-wave and pulsed electron paramagnetic resonance spectroscopy. We reveal that metallic 1T-MoS2 has a protonation site with a pKa of ∼5.5, where the proton is located ∼3.26 Šfrom redox-active Mo site. This protonation site is unique to 1T-MoS2 and induces sequential proton-electron transfer which inhibits ammonium formation while promoting nitrous oxide production, as confirmed by the pH-dependent selectivity and deuterium kinetic isotope effect. This is atomic-scale evidence of phase-dependent selectivity on MoS2, expanding the application of TMDs to selective electrocatalysis.

CO2 reduction driven by a pH gradient.

All life on Earth is built of organic molecules, so the primordial sources of reduced carbon remain a major open question in studies of the origin of life. A variant of the alkaline-hydrothermal-vent theory for life's emergence suggests that organics could have been produced by the reduction of CO2 via H2 oxidation, facilitated by geologically sustained pH gradients. The process would be an abiotic analog-and proposed evolutionary predecessor-of the Wood-Ljungdahl acetyl-CoA pathway of modern archaea and bacteria. The first energetic bottleneck of the pathway involves the endergonic reduction of CO2 with H2 to formate (HCOO-), which has proven elusive in mild abiotic settings. Here we show the reduction of CO2 with H2 at room temperature under moderate pressures (1.5 bar), driven by microfluidic pH gradients across inorganic Fe(Ni)S precipitates. Isotopic labeling with 13C confirmed formate production. Separately, deuterium (2H) labeling indicated that electron transfer to CO2 does not occur via direct hydrogenation with H2 but instead, freshly deposited Fe(Ni)S precipitates appear to facilitate electron transfer in an electrochemical-cell mechanism with two distinct half-reactions. Decreasing the pH gradient significantly, removing H2, or eliminating the precipitate yielded no detectable product. Our work demonstrates the feasibility of spatially separated yet electrically coupled geochemical reactions as drivers of otherwise endergonic processes. Beyond corroborating the ability of early-Earth alkaline hydrothermal systems to couple carbon reduction to hydrogen oxidation through biologically relevant mechanisms, these results may also be of significance for industrial and environmental applications, where other redox reactions could be facilitated using similarly mild approaches.

Multi-omics analysis on an agroecosystem reveals the significant role of organic nitrogen to increase agricultural crop yield.

Both inorganic fertilizer inputs and crop yields have increased globally, with the concurrent increase in the pollution of water bodies due to nitrogen leaching from soils. Designing agroecosystems that are environmentally friendly is urgently required. Since agroecosystems are highly complex and consist of entangled webs of interactions between plants, microbes, and soils, identifying critical components in crop production remain elusive. To understand the network structure in agroecosystems engineered by several farming methods, including environmentally friendly soil solarization, we utilized a multiomics approach on a field planted with Brassica rapa We found that the soil solarization increased plant shoot biomass irrespective of the type of fertilizer applied. Our multiomics and integrated informatics revealed complex interactions in the agroecosystem showing multiple network modules represented by plant traits heterogeneously associated with soil metabolites, minerals, and microbes. Unexpectedly, we identified soil organic nitrogen induced by soil solarization as one of the key components to increase crop yield. A germ-free plant in vitro assay and a pot experiment using arable soils confirmed that specific organic nitrogen, namely alanine and choline, directly increased plant biomass by acting as a nitrogen source and a biologically active compound. Thus, our study provides evidence at the agroecosystem level that organic nitrogen plays a key role in plant growth.

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media.

Nitrate is a pervasive aquatic contaminant of global environmental concern. In nature, the most effective nitrate reduction reaction (NRR) is catalyzed by nitrate reductase enzymes at neutral pH, using a highly-conserved Mo center ligated mainly by oxo and thiolate groups. Mo-based NRR catalysts mostly function in organic solvents with a low water stability. Recently, an oxo-containing molybdenum sulfide nanoparticle that serves as an NRR catalyst at neutral pH was first reported. Herein, in a nanoparticle-catalyzed NRR system a pentavalent MoV (=O)S4 species, an enzyme mimetic, served as an active intermediate for the NRR. Potentiometric titration analysis revealed that a redox synergy among MoV -S, S radicals, and MoV (=O)S4 likely play a key role in stabilizing MoV (=O)S4 , showing the importance of secondary interactions in facilitating NRR. The first identification and characterization of an oxo- and thiolate-ligated Mo intermediates pave the way to the molecular design of efficient enzyme mimetic NRR catalysts in aqueous solution.

Shift of the Optimum Binding Energy at Higher Rates of Catalysis.

The binding energy between the catalyst and the reactant is considered to be the primary descriptor of catalytic activity. Therefore, identifying the optimum binding energy that would yield maximum activity is fundamentally important for the development of efficient catalysts. Here, we show analytically how the binding energy that maximizes the activity at large reaction rates, i.e., the operating conditions of catalysis, may deviate from the traditional understanding obtained near equilibrium. This shift can be on the order of 0.5 eV, which is easily sufficient for the optimum material to change. This binding energy shift is consistent with experimental observations in the literature, suggesting that a reinvestigation of materials previously considered to be inactive may be necessary.

Metals likely promoted protometabolism in early ocean alkaline hydrothermal systems.

One of the most plausible scenarios of the origin of life assumes the preceding prebiotic autotrophic metabolism in sulfide-rich hydrothermal vent environments. However, geochemical mechanisms to harness the reductive power provided by hydrothermal systems remain to be elucidated. Here, we show that, under a geoelectrochemical condition realizable in the early ocean hydrothermal systems, several metal sulfides (FeS, Ag2S, CuS, and PbS) undergo hour- to day-scale conversion to the corresponding metals at ≤-0.7 V (versus the standard hydrogen electrode). The electrochemically produced FeS-Fe0 assemblage promoted various reactions including certain steps in the reductive tricarboxylic acid cycle with efficiencies far superior to those due to pure FeS. The threshold potential is readily generated in the H2-rich alkaline hydrothermal systems that were probably ubiquitous on the Hadean seafloor. Thus, widespread metal production and metal-sustained primordial metabolism were likely to occur as a natural consequence of the active hydrothermal processes on the Hadean Earth.

Electrochemical characterization of manganese oxides as a water oxidation catalyst in proton exchange membrane electrolysers.

The performance of four polymorphs of manganese (Mn) dioxides as the catalyst for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolysers was examined. The comparison of the activity between Mn oxides/carbon (Mn/C), iridium oxide/carbon (Ir/C) and platinum/carbon (Pt/C) under the same condition in PEM electrolysers showed that the γ-MnO2/C exhibited a voltage efficiency for water electrolysis comparable to the case with Pt/C, while lower than the case with the benchmark Ir/C OER catalyst. The rapid decrease in the voltage efficiency was observed for a PEM electrolyser with the Mn/C, as indicated by the voltage shift from 1.7 to 1.9 V under the galvanostatic condition. The rapid deactivation was also observed when Pt/C was used, indicating that the instability of PEM electrolysis with Mn/C is probably due to the oxidative decomposition of carbon supports. The OER activity of the four types of Mn oxides was also evaluated at acidic pH in a three-electrode system. It was found that the OER activity trends of the Mn oxides evaluated in an acidic aqueous electrolyte were distinct from those in PEM electrolysers, demonstrating the importance of the evaluation of OER catalysts in a real device condition for future development of noble-metal-free PEM electrolysers.

Stable Potential Windows for Long-Term Electrocatalysis by Manganese Oxides Under Acidic Conditions.

Efficient, earth-abundant, and acid-stable catalysts for the oxygen evolution reaction (OER) are missing pieces for the production of hydrogen via water electrolysis. Here, we report how the limitations on the stability of 3d-metal materials can be overcome by the spectroscopic identification of stable potential windows in which the OER can be catalyzed efficiently while simultaneously suppressing deactivation pathways. We demonstrate the benefits of this approach using gamma manganese oxide (γ-MnO2 ), which shows no signs of deactivation even after 8000 h of electrolysis at a pH of 2. This stability is vastly superior to existing acid-stable 3d-metal OER catalysts, but cannot be realized if there is a deviation as small as 50-mV from the stable potential window. A stable voltage efficiency of over 70 % in a polymer-electrolyte membrane (PEM) electrolyzer further verifies the availability of this approach and showcases how materials previously perceived to be unstable may have potential application for water electrolysis in an acidic environment.