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Artificial photosynthetic systems can store solar energy and chemically reduce CO₂. We developed a hybrid water splitting–biosynthetic system based on a biocompatible Earth-abundant inorganic catalyst system to split water into molecular hydrogen and oxygen (H₂ and O₂) at low driving voltages. When grown in contact with these catalysts, Ralstonia eutropha consumed the produced H₂ to synthesize biomass and fuels or chemical products from low CO₂ concentration in the presence of O₂. This scalable system has a CO₂ reduction energy efficiency of ~50% when producing bacterial biomass and liquid fusel alcohols, scrubbing 180 grams of CO₂ per kilowatt-hour of electricity. Coupling this hybrid device to existing photovoltaic systems would yield a CO₂ reduction energy efficiency of ~10%, exceeding that of natural photosynthetic systems.

We demonstrate the synthesis of NH₃ from N₂ and H₂O at ambient conditions in a single reactor by coupling hydrogen generation from catalytic water splitting to a H₂-oxidizing bacterium Xanthobacter autotrophicus, which performs N₂ and CO₂ reduction to solid biomass. Living cells of X. autotrophicus may be directly applied as a biofertilizer to improve growth of radishes, a model crop plant, by up to ∼1,440% in terms of storage root mass. The NH₃ generated from nitrogenase (N₂ase) in X. autotrophicus can be diverted from biomass formation to an extracellular ammonia production with the addition of a glutamate synthetase inhibitor. The N₂ reduction reaction proceeds at a low driving force with a turnover number of 9 × 10⁹ cell−1 and turnover frequency of 1.9 × 10⁴ s−1·cell−1 without the use of sacrificial chemical reagents or carbon feedstocks other than CO₂. This approach can be powered by renewable electricity, enabling the sustainable and selective production of ammonia and biofertilizers in a distributed manner.

Photosynthesis fixes CO 2 from the air by using sunlight. Industrial mimics of photosynthesis seek to convert CO 2 directly into biomass, fuels, or other useful products. Improving on a previous artificial photosynthesis design, Liu et al. combined the hydrogen-oxidizing bacterium Raistonia eutropha with a cobalt-phosphorus water-splitting catalyst. This biocompatible self-healing electrode circumvented the toxicity challenges of previous designs and allowed it to operate aerobically. When combined with solar photovoltaic cells, solar-to-chemical conversion rates should become nearly an order of magnitude more efficient than natural photosynthesis. Science , this issue p. 1210 A biocompatible catalyst in an artificial photosynthesis system leads to efficient solar-to-fuel conversion. Artificial photosynthetic systems can store solar energy and chemically reduce CO 2 . We developed a hybrid water splitting–biosynthetic system based on a biocompatible Earth-abundant inorganic catalyst system to split water into molecular hydrogen and oxygen (H 2 and O 2 ) at low driving voltages. When grown in contact with these catalysts, Ralstonia eutropha consumed the produced H 2 to synthesize biomass and fuels or chemical products from low CO 2 concentration in the presence of O 2 . This scalable system has a CO 2 reduction energy efficiency of ~50% when producing bacterial biomass and liquid fusel alcohols, scrubbing 180 grams of CO 2 per kilowatt-hour of electricity. Coupling this hybrid device to existing photovoltaic systems would yield a CO 2 reduction energy efficiency of ~10%, exceeding that of natural photosynthetic systems.

Photovoltaic cells have considerable potential to satisfy future renewable-energy needs, but efficient and scalable methods of storing the intermittent electricity they produce are required for the large-scale implementation of solar energy. Current solar-to-fuels storage cycles based on water splitting produce hydrogen and oxygen, which are attractive fuels in principle but confront practical limitations from the current energy infrastructure that is based on liquid fuels. In this work, we report the development of a scalable, integrated bioelectrochemical system in which the bacterium Ralstonia eutropha is used to efficiently convert CO ₂, along with H ₂ and O ₂ produced from water splitting, into biomass and fusel alcohols. Water-splitting catalysis was performed using catalysts that are made of earth-abundant metals and enable low overpotential water splitting. In this integrated setup, equivalent solar-to-biomass yields of up to 3.2% of the thermodynamic maximum exceed that of most terrestrial plants. Moreover, engineering of R. eutropha enabled production of the fusel alcohol isopropanol at up to 216 mg/L, the highest bioelectrochemical fuel yield yet reported by >300%. This work demonstrates that catalysts of biotic and abiotic origin can be interfaced to achieve challenging chemical energy-to-fuels transformations. Significance Renewable-fuels generation has emphasized water splitting to produce hydrogen and oxygen. For accelerated technology adoption, bridging hydrogen to liquid fuels is critical to the translation of solar-driven water splitting to current energy infrastructures. One approach to establishing this connection is to use the hydrogen from water splitting to reduce carbon dioxide to generate liquid fuels via a biocatalyst. We describe the integration of water-splitting catalysts comprised of earth-abundant components to wild-type and engineered Ralstonia eutropha to generate biomass and isopropyl alcohol, respectively. We establish the parameters for bacterial growth conditions at low overpotentials and consequently achieve overall efficiencies that are comparable to or exceed natural systems.

Ralstonia eutropha H16 is a facultatively autotrophic hydrogen-oxidizing bacterium capable of producing polyhydroxybutyrate (PHB)-based bioplastics. As PHB’s physical properties may be improved by incorporation of medium-chain-length fatty acids (MCFAs), and MCFAs are valuable on their own as fuel and chemical intermediates, we engineered R. eutropha for MCFA production. Expression of UcFatB2 , a medium-chain-length-specific acyl-ACP thioesterase, resulted in production of 14 mg/L laurate in wild-type R. eutropha . Total fatty acid production (22 mg/L) could be increased up to 2.5-fold by knocking out PHB synthesis, a major sink for acetyl-CoA, or by knocking out the acyl-CoA ligase fadD3 , an entry point for fatty acids into β -oxidation. As Δ fadD3 mutants still consumed laurate, and because the R. eutropha genome is predicted to encode over 50 acyl-CoA ligases, we employed RNA-Seq to identify acyl-CoA ligases upregulated during growth on laurate. Knockouts of the three most highly upregulated acyl-CoA ligases increased fatty acid yield significantly, with one strain (Δ A2794 ) producing up to 62 mg/L free fatty acid. This study demonstrates that homologous β -oxidation systems can be rationally engineered to enhance fatty acid production, a strategy that may be employed to increase yield for a range of fuels, chemicals, and PHB derivatives in R. eutropha.

Plant litter represents a major basal resource in streams, where its decomposition is partly regulated by litter traits. Litter-trait variation may determine the latitudinal gradient in decomposition in streams, which is mainly microbial in the tropics and detritivore-mediated at high latitudes. However, this hypothesis remains untested, as we lack information on large-scale trait variation for riparian litter. Variation cannot easily be inferred from existing leaf-trait databases, since nutrient resorption can cause traits of litter and green leaves to diverge. Here we present the first global-scale assessment of riparian litter quality by determining latitudinal variation (spanning 107°) in litter traits (nutrient concentrations; physical and chemical defences) of 151 species from 24 regions and their relationships with environmental factors and phylogeny. We hypothesized that litter quality would increase with latitude (despite variation within regions) and traits would be correlated to produce ‘syndromes’ resulting from phylogeny and environmental variation. We found lower litter quality and higher nitrogen:phosphorus ratios in the tropics. Traits were linked but showed no phylogenetic signal, suggesting that syndromes were environmentally determined. Poorer litter quality and greater phosphorus limitation towards the equator may restrict detritivore-mediated decomposition, contributing to the predominance of microbial decomposers in tropical streams.

Ralstonia eutropha H16 is a facultatively autotrophic hydrogen-oxidizing bacterium capable of producing polyhydroxybutyrate (PHB)-based bioplastics. As PHB's physical properties may be improved by incorporation of medium-chain-length fatty acids (MCFAs), and MCFAs are valuable on their own as fuel and chemical intermediates, we engineered R. eutropha for MCFA production. Expression of UcFatB2, a medium-chain-length-specific acyl-ACP thioesterase, resulted in production of 14 mg/L laurate in wild-type R. eutropha. Total fatty acid production (22 mg/L) could be increased up to 2.5-fold by knocking out PHB synthesis, a major sink for acetyl-CoA, or by knocking out the acyl-CoA ligase fadD3, an entry point for fatty acids into [beta]-oxidation. As [DELTA]fadD3 mutants still consumed laurate, and because the R. eutropha genome is predicted to encode over 50 acyl-CoA ligases, we employed RNA-Seq to identify acyl-CoA ligases upregulated during growth on laurate. Knockouts of the three most highly upregulated acyl-CoA ligases increased fatty acid yield significantly, with one strain ([DELTA]A2794) producing up to 62 mg/L free fatty acid. This study demonstrates that homologous [beta]-oxidation systems can be rationally engineered to enhance fatty acid production, a strategy that may be employed to increase yield for a range of fuels, chemicals, and PHB derivatives in R. eutropha.

The relationship between detritivore diversity and decomposition can provide information on how biogeochemical cycles are affected by ongoing rates of extinction, but such evidence has come mostly from local studies and microcosm experiments. We conducted a globally distributed experiment (38 streams across 23 countries in 6 continents) using standardised methods to test the hypothesis that detritivore diversity enhances litter decomposition in streams, to establish the role of other characteristics of detritivore assemblages (abundance, biomass and body size), and to determine how patterns vary across realms, biomes and climates. We observed a positive relationship between diversity and decomposition, strongest in tropical areas, and a key role of abundance and biomass at higher latitudes. Our results suggest that litter decomposition might be altered by detritivore extinctions, particularly in tropical areas, where detritivore diversity is already relatively low and some environmental stressors particularly prevalent. It is unclear whether stream detritivore diversity enhances decomposition across climates. Here the authors manipulate litter diversity and examine detritivore assemblages in a globally distributed stream litterbag experiment, finding a positive diversity-decomposition relationship stronger in tropical streams, where detritivore diversity is lower.

We present the discoveries of KELT-25b (TIC 65412605, TOI-626.01) and KELT-26b (TIC 160708862, TOI-1337.01), two transiting companions orbiting relatively bright, early A-stars. The transit signals were initially detected by the KELT survey, and subsequently confirmed by \\textit{TESS} photometry. KELT-25b is on a 4.40-day orbit around the V = 9.66 star CD-24 5016 (\\(T_{\\rm eff} = 8280^{+440}_{-180}\\) K, \\(M_{\\star}\\) = \\(2.18^{+0.12}_{-0.11}\\) \\(M_{\\odot}\\)), while KELT-26b is on a 3.34-day orbit around the V = 9.95 star HD 134004 (\\(T_{\\rm eff}\\) =\\(8640^{+500}_{-240}\\) K, \\(M_{\\star}\\) = \\(1.93^{+0.14}_{-0.16}\\) \\(M_{\\odot}\\)), which is likely an Am star. We have confirmed the sub-stellar nature of both companions through detailed characterization of each system using ground-based and \\textit{TESS} photometry, radial velocity measurements, Doppler Tomography, and high-resolution imaging. For KELT-25, we determine a companion radius of \\(R_{\\rm P}\\) = \\(1.64^{+0.039}_{-0.043}\\) \\(R_{\\rm J}\\), and a 3-sigma upper limit on the companion's mass of \\(\\sim64~M_{\\rm J}\\). For KELT-26b, we infer a planetary mass and radius of \\(M_{\\rm P}\\) = \\(1.41^{+0.43}_{-0.51}\\) \\(M_{\\rm J}\\) and \\(R_{\\rm P}\\) = \\(1.940^{+0.060}_{-0.058}\\) \\(R_{\\rm J}\\). From Doppler Tomographic observations, we find KELT-26b to reside in a highly misaligned orbit. This conclusion is weakly corroborated by a subtle asymmetry in the transit light curve from the \\textit{TESS} data. KELT-25b appears to be in a well-aligned, prograde orbit, and the system is likely a member of a cluster or moving group.