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Hydrothermal dehydration is an attractive method for deoxygenation and upgrading of biofuels because it requires no reagents or catalysts other than superheated water. Although mono-alcohols cleanly deoxygenate via dehydration under many conditions, polyols such as those derived from saccharides and related structures are known to be recalcitrant with respect to dehydration. Here, we describe detailed mechanistic and kinetic studies of hydrothermal dehydration of 1,2- and 1,4-cyclohexanediols as model compounds to investigate how interactions between the hydroxyls can control the reaction. The diols generally dehydrate more slowly and have more complex reaction pathways than simple cyclohexanol. Although hydrogen bonding between hydroxyls is an important feature of the diol reactions, hydrogen bonding on its own does not explain the reduced reactivity. Rather, it is the way that hydrogen bonding influences the balance between the E1 and E2 elimination mechanisms. We also describe the reaction pathways and follow-up secondary reactions for the slower-dehydrating diols.

Water is necessary for all life on Earth. Water is so critical that organisms have developed strategies to survive in hyperarid environments. These regions with extremely low water availability are also unique analogs in which to study the physico-chemical conditions of extraterrestrial environments such as Mars. We have identified a daily, sustainable cycle of water vapor adsorption (WVA) and desorption that measurably affects soil water content (SWC) in the hyperarid region of the Atacama Desert in southern Perú. We pair field-based soil temperature and relative humidity soil profiles with laboratory simulations to provide evidence for a daily WVA cycle. Using our WVA model, we estimate that one adsorptive period-one night-increases SWC by 0.2-0.3 mg/g of soil (∼30 μm rainfall). We can plausibly rule out other water inputs during our field campaign that could account for this water input, and we provide evidence that this WVA cycle is driven by solar heating and maintained by atmospheric water vapor. The WVA may also serve to retain water from infrequent rain events in these soils. If the water provided by WVA in these soils is bioavailable, it could have significant implications for the microorganisms that are endemic to hyperarid environments.

Amide bonds are fundamental products in biochemistry, forming peptides critical to protein formation, but amide bonds are also detected in sterile environments and abiotic synthesis experiments. The abiotic formation of amide bonds may represent a prerequisite to the origin of life. Here we report thermodynamic models that predict optimal conditions for amide bond synthesis across geologically relevant ranges of temperature, pressure, and pH. We modeled acetamide formation from acetic acid and ammonia as a simple analog to peptide bond formation, and tested this model with hydrothermal experiments examining analogous reactions of amides including benzanilide and related structures. We also expanded predictions for optimizing diglycine formation, revealing that in addition to synthesis becoming more favorable at near-ambient pressures (Psat) with increasing temperatures, the strongest thermodynamic drive exists at extremely high pressures (>15,000 bar) and decreasing temperatures. Beyond implications for life’s origins, the reactants and products involved in simple amide formation reactions can potentially be used as geochemical tracers for planetary exploration of environments that may be habitable.

The formation of ketone products from carboxylic acids in the presence of minerals has not been considered in the interpretations of aqueous geochemistry, even though the formation of ketones is a ...

Measured abundances of organic compounds can reveal information about the environments in which they formed. Since organic compounds can be mobilized and released from geologic and planetary settings, they have the potential to provide insights into environments that are difficult to observe directly. To advance our understanding of these environments, this study identifies organic reactions that approach metastable equilibrium in experiments, so that future studies can predict geochemical conditions in remote settings (e.g., deep Earth, extraterrestrial bodies) by monitoring reaction ratios of compounds involved in similar organic reactions. At high temperatures organic redox reactions can equilibrate, which allows comparisons with thermodynamic properties to yield estimates of reaction conditions. However, redox reactions may equilibrate too slowly to be applicable to lower temperature systems. To explore metastable equilibria at lower temperatures, we studied substitution reactions that tend to be faster than redox reactions. In this study, we demonstrate that oxygen- and nitrogen-bearing organic compounds at hydrothermal conditions undergo a series of simultaneous substitution reactions that rapidly approach steady-state ratios indicative of metastable equilibrium. Four sets of aqueous experiments were performed at 250 °C and 40 bar, each beginning with a single organic reactant: benzyl alcohol, benzylamine, dibenzylamine, or tribenzylamine. All reactant solutions were prepared with identical bulk elemental compositions by adjusting the concentrations of the starting organic compounds and adding ammonia as needed. After 2 h at hydrothermal reaction conditions, all of the model compounds were detectable in all four sets of experiments, evidence for reversibility of reactions among the compounds. After 72 h, reaction ratios between the model compounds converged in all four sets of experiments, consistent with approaches toward metastable equilibrium. Reaction ratios for ether and aldehyde formation reactions were also observed to group within a relatively small range, but without a clear convergence pattern, suggesting other non-redox reactions may approach metastable equilibrium. The approach to metastable equilibrium among the initial organic reactants could be observed and quantified even in the presence of competing redox reactions whose mechanisms are less understood, including dibenzylimine and toluene formation, which did not appear to reach steady-states. These findings identify classes of organic compounds and reactions that can reflect the conditions at which they last equilibrated and should be targeted for analysis in natural systems.

Chemodenitrification – the non-enzymatic process of nitrite reduction – may be an important sink for fixed nitrogen in tropical peatlands. Rates and products of chemodenitrification are dependent on O2, pH, Fe2+ concentration, and organic matter composition, which are variable across peat soils. Assessing abiotic reaction pathways is difficult because sterilization and inhibition agents can alter the availability of reactants by changing iron speciation and organic matter composition. We compared six commonly used soil sterilization techniques – γ irradiation, chloroform, autoclaving, and the use of three different chemical inhibitors (mercury, zinc, and azide) – for their compatibility with chemodenitrification assays for tropical peatland soils (organic-rich, low-pH soil from the eastern Amazon). Out of the six techniques, γ irradiation resulted in soil treatments with the lowest cell viability and denitrification activity and the least effect on pH, iron speciation, and organic matter composition. Nitrite depletion rates in γ-irradiated soils were highly similar to untreated (live) soils, whereas other sterilization techniques showed deviations. Chemodenitrification was a dominant process of nitrite consumption in tropical peatland soils assayed in this study. Nitrous oxide (N2O) is one possible product of chemodenitrification reactions. Abiotic N2O production was low to moderate (3 %–16 % of converted nitrite), and different sterilization techniques lead to significant variations on production rates due to inherent processes or potential artifacts. Our work represents the first methodological basis for testing the abiotic denitrification and N2O production potential in tropical peatland soil.

Many experimental studies report rates of decomposition for amino acids under hydrothermal conditions without describing the kinetics of competing reactions or definitively characterizing individual reaction mechanisms. As a step toward comprehensive models for amino acid reactivity, this study provides a detailed description of the organic reaction kinetics and mechanisms for deamination of amine functional groups at acidic hydrothermal conditions, which favors their protonated aminium forms. Time series experiments yield hydrothermal deamination rates for model aminum compounds benzylaminium (BAH+) and α-methylbenzylaminium (α-CH3-BAH+), buffered at pH 3.3 at 250 °C and 40 bar (Psat). Both compounds are primary amines, but the amine group in the former is bonded to a primary carbon while in the latter to a secondary carbon; this difference has implications for reactivity that are relevant for naturally occurring amines. Deamination of the aminiums under these conditions forms alcohols as the major primary products. The deamination mechanisms were investigated by determining the reaction kinetics of ring-substituted BAH+ and α-CH3-BAH+ derivatives, which provided information on the nature of charge buildup in the transition state. The results for the deamination of BAH+ to form benzyl alcohol support nearly equal contribution from two reaction mechanisms, specifically unimolecular nucleophilic substitution (SN1, kSN1 ≈ 2.4 × 10−6 s−1) and bimolecular nucleophilic substitution (SN2, kSN2 ≈ 2.7 × 10−6 s−1), while α-CH3-BAH+ deaminates almost exclusively via a much faster SN1 mechanism (kα ≈ 7.6 × 10−4 s−1). These observed rates reinforce the notion that previously reported amino acid deamination rates may have resulted from catalysis mechanisms associated with certain reaction vessel materials (e.g., Pyrex and stainless steel). The observation of two competing mechanisms for BAH+ deamination/hydration implies that extrapolation of deamination rates to other temperatures is currently unreliable, since this is likely to be accompanied by a change in the predominant mechanism. However, this work establishes that mechanistic contributions can be quantified, which will enhance the accuracy of extrapolating deamination rates across the temperature and pH ranges common to aqueous geochemical environments.

Bulk gold was found to catalyze the Cannizzaro reaction of benzaldehyde and related disproportionation reactions in superheated water. At 200 °C and 1.5 MPa for ∼48 hours, benzaldehyde conversions increased linearly with added gold at low loadings, but this trend reversed at higher gold loadings. Ratios of the primary products benzoic acid and benzyl alcohol exceeded unity, increasing with increasing amounts of gold. These observations are attributed to the reactivity of benzyl alcohol in the presence of gold, which is proposed to react via cross-disproportionation with benzaldehyde to yield toluene and benzoic acid, transitioning to benzyl alcohol disproportionation that forms toluene and reforms benzaldehyde at higher gold loadings. Turnover frequencies for both benzaldehyde (0.00013 s−1) and benzyl alcohol (0.0011 s−1) were quite low and experiments of varied duration demonstrated that very few turnovers occurred under the reaction conditions. Proposed mechanisms for the catalyzed reactions include hydride transfer from gold-bonded alkoxides to surface gold atoms. These observations expand the known catalytic capabilities of bulk gold to include hydride transfer, a fundamental process in organic chemistry and biochemistry. The simple, environmentally benign reaction conditions are reminiscent of organic geochemical processes, with implications for both geomimicry and green chemistry approaches to chemical challenges.

Reduction of carbon–carbon π-bonds has been demonstrated using iron powder as the reductant and simple powdered nickel as the catalyst in water as the solvent at 250 °C and the saturated water vapor pressure, 40 bars. Stereochemical, kinetic and electronic probes of the mechanism suggest reaction via a conventional Horiuti–Polyani process for hydrogenation at the nickel metal surface. Selective reduction of carbon–carbon π-bonds is observed in the presence of other functional groups. The reactions use benign and Earth-abundant reagents that are at low depletion risk and take place in water as the only solvent under conditions that are characteristic of many geochemical processes.

The search for life on exoplanets is one of the grand scientific challenges of our time. The strategy to date has been to find (e.g., through transit surveys like Kepler) earthlike exoplanets in their stars’ habitable zone, then use transmission spectroscopy to measure biosignature gases, especially oxygen, in the planets’ atmospheres (e.g., using James Webb Space Telescope (JWST)). Already there are more such planets than can be observed by JWST, and missions like the Transiting Exoplanet Survey Satellite and others will find more. A better understanding of the geochemical cycles relevant to biosignature gases is needed, to prioritize targets for costly follow-up observations and to help design future missions. We define a Detectability Index to quantify the likelihood that a biosignature gas could be assigned a biological versus nonbiological origin. We apply this index to the case of oxygen gas, O2, on earthlike planets with varying water contents. We demonstrate that on earthlike exoplanets with 0.2 weight percent (wt%) water (i.e., no exposed continents) a reduced flux of bioessential phosphorus limits the export of photosynthetically produced atmospheric O2 to levels indistinguishable from geophysical production by photolysis of water plus hydrogen escape. Higher water contents >1 wt% that lead to high-pressure ice mantles further slow phosphorus cycling. Paradoxically, the maximum water content allowing use of O2 as a biosignature, 0.2 wt%, is consistent with no water based on mass and radius. Thus, the utility of an O2 biosignature likely requires the direct detection of both water and land on a planet.

Aldehydes represent an intermediate redox state of organic carbon and can be precursors to carboxylic acids via disproportionation. A model aldehyde, benzaldehyde, was subjected to hydrothermal experiments (250–350 °C, saturation pressure) to assess the kinetics and mechanisms of the reactions leading to carboxylic acids. The concentration dependence demonstrates the kinetics are second-order in benzaldehyde, consistent with a disproportionation reaction, which is reminiscent of the base-promoted Cannizzaro reaction known at lower temperatures. Arrhenius parameters for these rate constants trend well with data from most, but not all, previous studies for the reaction under supercritical conditions. The rate constants yielded an entropy of activation (ΔS‡) of −161 J mol–1 K–1, consistent with a bimolecular transition state at the rate-limiting step. Experimental yields of benzoic acid and benzyl alcohol were not equal, unlike what is expected for the disproportionation reaction. A kinetic model that includ...

Hydrothermal organic transformations occurring in geochemical processes are influenced by the surrounding environments including rocks and minerals. This work is focused on the effects of five common minerals on reactions of a model ketone substrate, dibenzylketone (DBK), in an experimental hydrothermal system. Ketones play a central role in many hydrothermal organic functional group transformations, such as those converting hydrocarbons to oxygenated compounds; however, how these minerals control the hydrothermal chemistry of ketones is poorly understood. Under the hydrothermal conditions of 300 °C and 70 MPa for up to 168 h, we observed that, while quartz (SiO2) and corundum (Al2O3) had no detectable effect on the hydrothermal reactions of DBK, iron-containing minerals, such as hematite (Fe2O3), magnetite (Fe3O4), and troilite (synthetic FeS), accelerated the reaction of DBK by up to an order of magnitude. We observed that fragmentation products, such as toluene and bibenzyl, dominated in the presence of hematite or magnetite, while use of troilite gave primarily the reduction products, e.g., 1, 3-diphenyl-propane and 1, 3-diphenyl-2-propanol. The roles of the three iron minerals in these transformations were further explored by (1) control experiments with various mineral surface areas, (2) measuring H2 in hydrothermal solutions, and (3) determining hydrogen balance among the organic products. These results suggest the reactions catalyzed by iron oxides (hematite and magnetite) are promoted mainly by the mineral surfaces, whereas the sulfide mineral (troilite) facilitated the reduction of ketone in the reaction solution. Therefore, this work not only provides a useful chemical approach to study and uncover complicated hydrothermal organic-mineral interactions, but also fosters a mechanistic understanding of ketone reactions in the deep carbon cycle.

Volatile Organic Compounds (VOCs) are ubiquitously present at low but detectable concentrations in hydrothermal fluids from volcanic and geothermal systems. Although their behavior is strictly controlled by physical and chemical parameters, the mechanisms responsible for the production of most VOCs in natural environments are poorly understood. Among them, benzene, whose abundances were found to be relatively high in hydrothermal gases, can theoretically be originated from reversible catalytic reforming processes, i.e. multi-step dehydrogenation reactions, involving saturated hydrocarbons. However, this hypothesis and other hypotheses are difficult to definitively prove on the basis of compositional data obtained by natural gas discharges only. In this study, therefore, laboratory experiments were carried out to investigate the production of benzene from cyclic hydrocarbons at hydrothermal conditions, specifically 300 °C and 85 bar. The results of experiments carried out in the presence of water and selected powdered minerals, suggest that cyclohexane undergoes dehydrogenation to form benzene, with cyclohexene and cyclohexadiene as by-products, and also as likely reaction intermediates. This reaction is slow when carried out in water alone and competes with isomerization and hydration pathways. However, benzene formation was increased compared to these competing reactions in the presence of sulfide (sphalerite and pyrite) and iron oxide (magnetite and hematite) minerals, whereas no enhancement of any reaction products was observed in the presence of quartz. The production of thiols was observed in experiments involving sphalerite and pyrite, suggesting that sulfide minerals may act both to enhance reactivity and also as reactants after dissolution. These experiments demonstrate that benzene can be effectively produced at hydrothermal conditions through dehydrogenation of saturated cyclic organic structures and highlight the crucial role played by minerals in this process.

In the next decades, the astrobiological community will debate whether the first observations of oxygen in an exoplanet$'$s atmosphere signifies life, so it is critical to establish procedures now for collection and interpretation of such data. We present a step-by-step observational strategy for using oxygen as a robust biosignature, to prioritize exoplanet targets and design future observations. It is premised on avoiding planets lacking subaerial weathering of continents, which would imply geochemical cycles drastically different from Earth$'$s, precluding use of oxygen as a biosignature. The strategy starts with the most readily obtained data: semi-major axis and stellar luminosity to ensure residence in the habitable zone; stellar XUV flux, to ensure an exoplanet can retain a secondary (outgassed) atmosphere. Next, high-precision mass and radius information should be combined with high-precision stellar abundance data, to constrain the exoplanet$'$s water content; those incompatible with less than 0.1 wt % H$_{2}$O can be deprioritized. Then, reflectance photometry or low-resolution transmission spectroscopy should confirm an optically thin atmosphere. Subsequent long-duration, high-resolution transmission spectroscopy should search for oxygen and ensure that water vapor and CO$_{2}$ are present only at low (10$^{2}$-10$^{4}$ ppm levels). Assuming oxygen is found, attribution to life requires the difficult acquisition of a detailed, multispectral light curve of the exoplanet to ensure both surface land and water. Exoplanets failing some of these steps might be habitable, even have observable biogenic oxygen, but should be deprioritized because oxygen could not be attributed unambiguously to life. We show how this is the case for the Solar System, the 55 Cnc System, and the TRAPPIST-1 System, in which only the Earth and TRAPPIST-1e successfully pass through our procedure., Comment: 27 Pages, 1 Figure, 0 Tables (accepted 09 June 2020, in press)

When searching for exoplanets and ultimately considering their habitability, it is necessary to consider the planet's composition, geophysical processes, and geochemical cycles in order to constrain the bioessential elements available to life. Determining the elemental ratios for exoplanetary ecosystems is not yet possible, but we generally assume that planets have compositions similar to those of their host stars. Therefore, using the Hypatia Catalog of high-resolution stellar abundances for nearby stars, we compare the C, N, Si, and P abundance ratios of main sequence stars with those in average marine plankton, Earth's crust, as well as bulk silicate Earth and Mars. We find that, in general, plankton, Earth, and Mars are N-poor and P-rich compared with nearby stars. However, the dearth of P abundance data, which exists for only ~1% of all stars and 1% of exoplanet hosts, makes it difficult to deduce clear trends in the stellar data, let alone the role of P in the evolution of an exoplanet. Our Sun has relatively high P and Earth biology requires a small, but finite, amount of P. On rocky planets that form around host stars with substantially less P, the strong partitioning of P into the core could rule out the potential for surface P and, consequently, for life on that planet's surface. Therefore, we urge the stellar abundance community to make P observations a priority in future studies and telescope designs., Comment: 8 pages, 4 figures, 1 table, accepted to ApJL

It is likely that half of the urban areas that will exist in 2050 have not yet been designed and built. This provides tremendous opportunities for enhancing urban sustainability, and using "nature in cities" is critical to more resilient solutions to urban challenges. Terms for "urban nature" include Green Infrastructure (GI), Green-Blue Infrastructure (GBI), Urban Green Space (UGS), and Nature-Based Solutions (NBS). These terms, and the concepts they represent, are incomplete because they tend to reduce the importance of non-terrestrial ecological features in cities. We argue that the concept of Urban Ecological Infrastructure (UEI), which came from a 2013 forum held in Beijing and from several subsequent 2017 publications, is a more inclusive alternative. In this paper we refine the 2013 definition of UEI and link the concept more directly to urban ecosystem services. In our refined definition, UEI comprises all parts of a city that support ecological structures and functions, as well as the ecosystem services provided by UEI that directly affect human outcomes and wellbeing. UEI often includes aspects of the built environment, and we discuss examples of this "hybrid infrastructure". We distinguish terrestrial, aquatic, and wetland UEI because each type provides different ecosystem services. We present several examples of both "accidental" UEI and UEI that was explicitly designed and managed, with an emphasis on wetland UEI because these ecotonal ecosystems are uniquely both terrestrial and aquatic. We show how both accidental and planned UEI produces unexpected ecosystem services, which justifies recognizing and maintaining both purposeful and serendipitous types of UEI in cities. Finally, we posit that by incorporating both "ecological" and "infrastructure", UEI also helps to bridge urban scientists and urban practitioners in a more transdisciplinary partnership to build more resilient and sustainable cities. Funding Agencies|U.S. National Science FoundationNational Science Foundation (NSF) [DEB-1026865, DEB-1637590, DEB-1832016]; FORMAS, the Swedish research council for sustainable development [942-2016-69 992023]; RhinMeuse Water Agency; French Biodiversity Agency; ZAEU (French LTSER network); Urban Resilience to Extreme Weather-Related Events Sustainability Research Network (UREx SRN; NSF) [SES-1444755]; SETS Resilience project (NSF) [GCR-1934933]; NATURA network (NSF) [Accel-Net 1927167]

Dust storms known as ‘haboobs’ occur in Tempe, AZ during the North American monsoon season. This work presents a catalog of haboob occurrence over the time period 2005–2014. A classification method based on meteorological and air quality measurements is described. The major factors that distinguish haboobs events from other dust events and from background conditions are event minimum visibility, maximum wind or gust speed, and maximum PM10 (particulate matter with aerodynamic diameters of 10 μm or less) concentration. We identified from 3 to 20 haboob events per year over the period from 2005 to 2014. The calculated annual TSP (total suspended particulate) dry deposition ranged from a low of 259 kg ha−1 in 2010 to a high of 2950 kg ha−1 in 2011 with a mean of 950 kg ha−1 yr−1. The deposition of large particles (PM>10) is greater than the deposition of PM10. The TSP dry deposition during haboobs is estimated to contribute 74% of the total particulate mass deposited in Tempe.

As the Earth's climate has changed, Arctic sea ice extent has decreased drastically. It is likely that the late-summer Arctic will be ice-free as soon as the 2030s. This loss of sea ice represents one of the most severe positive feedbacks in the climate system, as sunlight that would otherwise be reflected by sea ice is absorbed by open ocean. It is unlikely that CO2 levels and mean temperatures can be decreased in time to prevent this loss, so restoring sea ice artificially is an imperative. Here we investigate a means for enhancing Arctic sea ice production by using wind power during the Arctic winter to pump water to the surface, where it will freeze more rapidly. We show that where appropriate devices are employed, it is possible to increase ice thickness above natural levels, by about 1 m over the course of the winter. We examine the effects this has in the Arctic climate, concluding that deployment over 10% of the Arctic, especially where ice survival is marginal, could more than reverse current trends of ice loss in the Arctic, using existing industrial capacity. We propose that winter ice thickening by wind-powered pumps be considered and assessed as part of a multipronged strategy for restoring sea ice and arresting the strongest feedbacks in the climate system.

Dissolved organic carbon in hot springs reflects a range of sources and biogeochemical processes. We evaluated ~200 continental hot spring samples, with a range in pH and temperature, collected from the Tengchong hydrothermal region, Yunnan Province, China and Yellowstone National Park, Wyoming, USA. Dissolved organic carbon concentrations ranged from 16.7 µM to 2.97 mM. Acidic springs displayed the highest values and widest range in carbon concentration. Alkaline springs had a narrower range and lower average concentrations. Carbon composition was evaluated using ultraviolet absorption and 3D-fluorescence spectroscopy. Total fluorescence was correlated (p 97% of the total fluorescence. Our model includes three humic-like components, one protein-like component, and one novel component exclusively observed in highly acidic springs. The closest spectral match to the novel component is an acid-soluble lignin produced during high-temperature, acid digestion of wood pulp. Humic-like components were dominant in mid-pH springs (4

Collaborations between biologists and geologists are key to understanding and projecting how landscapes function and change over time. Such collaborations are stimulated by on-going scientific developments, advances in instrumentation and technology, and the growing recognition that environmental problems necessitate interdisciplinary investigation. Here, we show how the biogeosciences are well placed to answer more completely the core questions that motivate the world's invaluable environmental research networks: specifically, the venerable Long Term Ecological Research networks (LTERs), the newer surveillance facilities of the Earth Observatory Networks (EONs including the USA's NEON), and the geosciences' interdisciplinary network of Critical Zone Observatories (CZOs). Because LTER and EON programs have been supported largely by ecological and biological communities and CZOs largely by the geological community, we assert that a concerted biogeoscience approach across these invaluable networks can benefit both their scientific productivity and usefulness to the wider public.

Long-term environmental research networks are one approach to advancing local, regional, and global environmental science and education. A remarkable number and wide variety of environmental research networks operate around the world today. These are diverse in funding, infrastructure, motivating questions, scientific strengths, and the sciences that birthed and maintain the networks. Some networks have individual sites that were selected because they had produced invaluable long-term data, while other networks have new sites selected to span ecological gradients. However, all long-term environmental networks share two challenges. Networks must keep pace with scientific advances and interact with both the scientific community and society at large. If networks fall short of successfully addressing these challenges, they risk becoming irrelevant. The objective of this paper is to assert that the biogeosciences offer environmental research networks a number of opportunities to expand scientific impact and public engagement. We explore some of these opportunities with four networks: the International Long-Term Ecological Research Network programs (ILTERs), critical zone observatories (CZOs), Earth and ecological observatory networks (EONs), and the FLUXNET program of eddy flux sites. While these networks were founded and expanded by interdisciplinary scientists, the preponderance of expertise and funding has gravitated activities of ILTERs and EONs toward ecology and biology, CZOs toward the Earth sciences and geology, and FLUXNET toward ecophysiology and micrometeorology. Our point is not to homogenize networks, nor to diminish disciplinary science. Rather, we argue that by more fully incorporating the integration of biology and geology in long-term environmental research networks, scientists can better leverage network assets, keep pace with the ever-changing science of the environment, and engage with larger scientific and public audiences. Fil: Richter, Daniel D.. University of Duke; Estados Unidos Fil: Billings, Sharon A.. University of Kansas; Estados Unidos Fil: Groffman, Peter M.. Brooklyn College; Estados Unidos Fil: Kelly, Eugene F.. State University of Colorado - Fort Collins; Estados Unidos Fil: Lohse, Kathleen A.. Idaho State University; Estados Unidos Fil: McDowell, William H.. University Of New Hampshire; Estados Unidos Fil: White, Timothy S.. State University of Pennsylvania; Estados Unidos Fil: Anderson, Suzanne. State University of Colorado at Boulder; Estados Unidos Fil: Baldocchi, Dennis D.. University of California at Berkeley; Estados Unidos Fil: Banwart, Steve. University Of Leeds; Reino Unido Fil: Brantley, Susan. State University of Pennsylvania; Estados Unidos Fil: Braun, Jean J.. University of Yaounide; Camerún. Universite de Toulouse; Francia Fil: Brecheisen, Zachary S.. University of Duke; Estados Unidos Fil: Cook, Charles S.. University of Duke; Estados Unidos Fil: Hartnett, Hilairy E.. Arizona State University; Estados Unidos Fil: Hobbie, Sarah E.. University of Minnesota; Estados Unidos Fil: Gaillardet, Jerome. Institut Universitaire de France. Institut de Physique du Globe de Paris; Francia Fil: Jobbagy Gampel, Esteban Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Matemática Aplicada de San Luis "Prof. Ezio Marchi". Universidad Nacional de San Luis. Facultad de Ciencias Físico, Matemáticas y Naturales. Instituto de Matemática Aplicada de San Luis "Prof. Ezio Marchi"; Argentina Fil: Jungkunst, Hermann F.. Universitat Koblenz; Alemania Fil: Kazanski, Clare E.. University of Minnesota; Estados Unidos Fil: Krishnaswamy, Jagdish. Ashoka Trust For Research In Ecology And The Environment; India Fil: Markewitz, Daniel. University of Georgia; Estados Unidos Fil: O'Neill, Katherine. Roanoke College; Estados Unidos Fil: Riebe, Clifford S.. University Of Wyoming; Fil: Schroeder, Paul. University of Georgia; Estados Unidos Fil: Siebe, Christina. Universidad Nacional Autónoma de México; México Fil: Silver, Whendee L.. University of California at Berkeley; Estados Unidos Fil: Thompson, Aaron. University of Georgia; Estados Unidos Fil: Verhoef, Anne. University of Reading; Reino Unido Fil: Zhang, Ganlin. Chinese Academy of Sciences; República de China

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets—Biosignatures—Habitability markers—Photosynthesis—Planetary surfaces—Atmospheres—Spectroscopy—Cryptic biospheres—False positives. Astrobiology 18, 663–708., Table of Contents 1. Introduction 1.1. Requirements for life 1.2. Exoplanet biosignature definitions 1.3. Biosignature categories 2. Evaluating Planetary Habitability 3. Overview of Terrestrial Exoplanet Modeling Studies 3.1. Observations of earth 3.2. Spectral models 3.3. Photochemical studies of terrestrial atmospheres 3.4. Earth through time 4. Gaseous Biosignatures 4.1. Gaseous biosignature overview 4.2. Earth-like atmospheres 4.2.1. Oxygen (O2) 4.2.2. Ozone (O3) 4.2.3. Methane (CH4) 4.2.4. Nitrous oxide (N2O) 4.2.5. Sulfur gases (DMS, DMDS, CH3SH) and relation to detectable C2H6 4.2.6. Methyl chloride (CH3Cl) 4.2.7. Haze as a biosignature 4.2.8. Other gases 4.3. “False positives” for biotic O2/O3 and possible spectral discriminators 4.4. Biosignatures in other types of atmospheres 4.5. Effects of the host star spectrum on photochemistry 4.6. Impacts of flares and particle events on biosignature gases 5. Surface Biosignatures 5.1. Photosynthesis 5.1.1. Principles of photosynthesis 5.1.1.1. Relationship between band gap wavelength and reductant in the generation of biogenic gases and pigment color 5.1.1.2. Uniqueness of OP 5.1.2. Photosynthetic pigments and the color of phototrophs 5.1.2.1. Structure 5.1.2.2. Light absorption 5.1.3. The vegetation “red edge” 5.1.4. Speculation about photosynthesis and pigment signatures on exoplanets 5.2. Retinal pigments 5.3. Alternative surface biosignatures: nonphotosynthetic pigments and reflectance features 5.4. False positive surface biosignatures 5.5. Chiral and polarization biosignatures 5.6. Fluorescence and bioluminescence 6. Temporal Biosignatures 6.1. Oscillations of atmospheric gases 6.2. Oscillations in surface signatures 7. Assessing Biosignature Plausibility 7.1. Chemical disequilibrium 7.2. Biomass estimation 7.3. Applications of network theory to biosignatures 8. Cryptic Biospheres: “False Negatives” for Life? 9. Prospects for Detecting Exoplanet Biosignatures 10. Summary

Summary Archaea can respond to changes in the environment by altering the composition of their membrane lipids, for example, by modification of the abundance and composition of glycerol dialkyl glycerol tetraethers (GDGTs). Here, we investigated the abundance and proportions of polar GDGTs (P-GDGTs) and core GDGTs (C-GDGTs) sampled in different seasons from Tengchong hot springs (Yunnan, China), which encompassed a pH range of 2.5-10.1 and a tempera- ture range of 43.7-93.6°C. The phylogenetic com- position of the archaeal community (reanalysed from published work) divided the Archaea in spring sediment samples into three major groups that corre- sponded with spring pH: acidic, circumneutral and alkaline. Cluster analysis showed correlation between spring pH and the composition of P- and C-GDGTs and archaeal 16S rRNA genes, indicating an intimate link between resident Archaea and the distri- bution of P- and C-GDGTs in Tengchong hot springs. The distribution of GDGTs in Tengchong springs was also significantly affected by temperature; however, the relationship was weaker than with pH. Analysis of published datasets including samples from Tibet, Yellowstone and the US Great Basin hot springs revealed a similar relationship between pH and GDGT content. Specifically, low pH springs had higher concentrations of GDGTs with high numbers of cyclopentyl rings than neutral and alkaline springs, which is consistent with the predominance of high cyclopentyl ring-characterized Sulfolobales and Thermoplasmatales present in some of the low pH springs. Our study suggests that the resident Archaea in these hot springs are acclimated if not adapted to low pH by their genetic capacity to effect the packing density of their membranes by increasing cyclopentyl rings in GDGTs at the rank of community.

We report the design, fabrication, and application of temperature and electrical conductivity sensor arrays for studying the harsh environments of hot springs at Yellowstone National Park. Centimeter-scale measurements are necessary to measure sharp gradients in geochemical parameters across the photosynthetic/nonphotosynthetic transition zone known as the photosynthetic fringe. Platinum, parylene-C, and fused quartz were the critical materials used in fabrication of both sensor arrays. More than 700 temperature and 90 conductivity measurements were taken in the mixing zone, where two geochemically distinct hot spring channels converged. We present a new technique, whereby microscale temperature measurements can be used to predict ion concentrations that represent the bulk ion chemistry via electrical conductivity (or other conservative chemicals) in this mixing zone. High-spatial-resolution data confirm that the strong temperature gradients across these regions largely determine where photosynthetic organisms appear.appear.

The LUSI mud volcano has been erupting since May 2006 in the densely populated Sidoarjo regency (East Java, Indonesia), forcing the evacuation of 40,000 people and destroying industry, farmland, and over 10,000 homes. Mud extrusion rates of 180,000 m(3) d(-1) were measured in the first few months of the eruption, decreasing to a loosely documented 4000 m for methane and approximately 600 m for carbon dioxide; however, the mass fractions of these gases are insufficient to explain the observed dynamics. Rather, the primary driver of the cyclic bubble-bursting activity is decompressional boiling of water, which initiates a few tens of meters below the surface, setting up slug flow in the upper conduit. Our measured gas flux and conceptual model lead to a corresponding upper-bound estimate for the mud-water mass flux of 10(5) m(3) d(-1).

Lakes are important for storage of the essential micronutrient molybdenum (Mo) during its transfer from the continents to the oceans, but little is known about the major sources and sinks for Mo in lacustrine ecosystems. We studied Mo cycling in Castle Lake, a small subalpine lake in the Klamath-Siskiyou Mountains of Northern California underlain primarily by mafic and ultramafic rocks where primary productivity is limited by Mo bioavailability. The deeper water of the lake becomes dysoxic (9–90 μM dissolved oxygen) during the summer. This study was undertaken to identify the sources of Mo to Castle Lake and establish a Mo budget. We measured Mo concentrations in a suite of bulk solids (lake sediments, soils and bedrock) and aqueous samples (sediment porewaters, soil runoff, spring waters, snow and ice) from Castle Lake and its watershed. Lake sediments have elevated Mo (7–36 ppm) compared to soils and bedrock (0.2–2 ppm) and Mo/Al values were nearly two orders of magnitude higher than the crustal abundance. Sediment porewaters had higher Mo (4–15 nM) than lake water (2–4 nM), soil runoff (0.1–6.2 nM), snowmelt (⩽0.1 nM), lake ice (0.3–2.2 nM) and local spring waters (0.03–2.72 nM). Bulk lake sediments had negative δ^(98/95)Mo values, ranging from −0.5 to −1.0‰ (±0.1). We used the numerical model PROFILE to estimate the net reaction rate of Mo in the porewater. Model calculations ruled out diagenesis as a source of Mo to lake sediments; diagenetic Mo always represented ⩽5% of the total Mo content in sediment. We also ruled out dissolved Mo inputs from groundwater and watershed inflow as important sources of Mo. Two whole-lake experimental Mo additions in the 1960’s could have contributed a sizeable amount of Mo to the lake sediments, but only over a short period. Atmospheric deposition of anthropogenic Mo from extensive copper smelting that occurred south of Castle Lake from 1896 to 1919 and from major Californian urban centers today were negligible Mo sources. Mo flux from the sediments (0.4–0.5 nmol cm^(−2) yr^(−1)) was roughly equal to Mo fluxes from surface inflow and outflow, whereas Mo burial fluxes were significantly higher (11.5 nmol cm^(−2) yr^(−1)). Because dissolved Mo fluxes were minimal, and atmospheric Mo deposition was estimated to be a minor source of Mo (

Black carbon (BC) is a poorly understood type of organic carbon but it is present in almost all environmental systems (i.e., atmosphere, soil and water). This work focuses on soot BC in desert soils and, in particular, urban soils from the Phoenix, Arizona metropolitan area. Soot BC is that fraction of black carbon formed from the condensation of gas phase molecules produced during burning. Soot BC in Phoenix area soils exhibits a range in both concentration and isotopic composition. Soot BC concentration in 52 soils (desert, agricultural and urban) ranges from 0.02–0.54 wt% and comprises from

The environmental effects and bioavailability of nanoparticulate iron (Fe) to plants are currently unknown. Here, plant bioavailability of synthesized hematite Fe nanoparticles was evaluated using Arabidopsis thaliana ( A. thaliana ) as a model. Over 56-days of growing wild-type A. thaliana , the nanoparticle-Fe and no-Fe treatments had lower plant biomass, lower chlorophyll concentrations, and lower internal Fe concentrations than the Fe-treatment. Results for the no-Fe and nanoparticle-Fe treatments were consistently similar throughout the experiment. These results suggest that nanoparticles (mean diameter 40.9 nm, range 22.3–67.0 nm) were not taken up and therefore not bioavailable to A. thaliana . Over 14-days growing wild-type and transgenic (Type I/II proton pump overexpression) A. thaliana , the Type I plant grew more than the wild-type in the nanoparticle-Fe treatment, suggesting Type I plants cope better with Fe limitation; however, the nanoparticle-Fe and no-Fe treatments had similar growth for all plant types.

Many transformation reactions involving hydrocarbons occur in the presence of H 2 O in hydrothermal systems and deep sedimentary systems. We investigate these reactions using laboratory-based organic chemistry experiments at high temperature and pressure (300 °C and 100 MPa). Organic functional group transformation reactions using model organic compounds based on cyclohexane with one or two methyl groups provided regio- and stereochemical markers that yield information about reversibility and reaction mechanisms. We found rapidly reversible interconversion between alkanes, alkenes, dienes, alcohols, ketones, and enones. The alkane-to-ketone reactions were not only completely reversible, but also exhibited such extensive reversibility that any of the functional groups along the reaction path (alcohol, ketone, and even the diene) could be used as the reactant and form all the other groups as products. There was also a propensity for these ring-based structures to dehydrogenate; presumably from the alkene, through a diene, to an aromatic ring. The product suites provide strong evidence that water behaved as a reactant and the various functional groups showed differing degrees of reactivity. Mechanistically-revealing products indicated reaction mechanisms that involve carbon-centered cation intermediates. This work therefore demonstrates that a wide range of organic compound types can be generated by abiotic reactions at hydrothermal conditions.

The problems of antibiotic overuse compel us to seek alternative antibacterial agents. Some clays have been shown to kill antibiotic-resistant human pathogens and may provide an alternative to known antibiotics. Here we show that Al toxicity plays a central role in the antibacterial action of a kaolin-rich clay from the Colombian Amazon (AMZ). Antibacterial susceptibility testing shows minimum inhibitory concentrations of 80 mg/mL against a model Escherichia coli (ATCC 25922). The clay buffered the media pH to ∼4.6 and Eh values to +360 mV. Chemical analysis of AMZ and bacteria showed that Al, P, and transition metals (Fe, Cu, Mn, and Zn) were exchanged during incubation at 37 °C. Only Al derived from the clay exceeded the minimum inhibitory concentrations for E. coli under acidic conditions. Ion imaging showed elevated Al levels in the bacterial membrane, and high intracellular Fe levels, relative to those of untreated controls. Phosphorus depletion in E. coli after reaction with AMZ, together with evidence of membrane permeabilization, suggests that Al reacts with membrane phospholipids, enhancing intracellular transport of metals. These results highlight the importance of dissolved Al for amplifying the toxicity of transition metals to human pathogens.

Studies of hydrothermal reactions involving organic compounds suggest complex, possibly reversible, reaction pathways that link functional groups from reduced alkanes all the way to oxidized carboxylic acids. Ketones represent a critical functional group because they occupy a central position in the reaction pathway, at the point where CAC bond cleavage is required for the formation of the more oxidized carboxylic acids. The mechanisms for the critical bond cleavage reactions in ketones, and how they compete with other reactions are the focus of this experimental study. We studied a model ketone, dibenzylketone (DBK), in H2O at 300 C and 70 MPa for up to 528 h. Product analysis was performed as a function of time at low DBK conversions to reveal the primary reaction pathways. Reversible interconversion between ketone, alcohol, alkene and alkane functional groups is observed in addition to formation of radical coupling products derived from irreversible CAC and CAH homolytic bond cleavage. The product distributions are time-dependent but the bond cleavage products dominate. The major products that accumulate at longer reaction times are toluene and larger, dehydrogenated structures that are initially formed by radical coupling. The hydrogen atoms generated by dehydrogenation of the coupling products are predominantly consumed in the formation of toluene. Even though bond cleavage products dominate, no carboxylic acids were observed on the timescale of the reactions under the chosen experimental conditions.

[NiFe]-hydrogenases are enzymes that catalyze the reversible interconversion of protons and hydrogen at a heterobimetallic site containing Ni and Fe. This organometallic site has served as an inspiration for the synthesis of a number of biomimetic complexes, but, unfortunately, most close structural mimics have shown little to no reactivity with either of the substrates for hydrogenases. This suggests that interactions between the metallo-active site and the protein scaffold are crucial in tuning reactivity. As a first step toward development of peptide-based models, in this paper we demonstrate a synthetic strategy for construction of peptide coordinated, cysteinyl thiolate bridged Ni-M complexes in which M is a hetero-organometallic fragment. We utilize the seven amino acid peptide ACDLPCG as a scaffold for construction of these peptide-coordinated metallocenters. This peptide binds Ni in an N(2)S(2) environment consisting of the amino terminus, an amide nitrogen, and the two cysteinyl thiolates. We show that these thiolates serve as reactive sites for formation of heterometallic complexes in which they serve as bridging ligands. The method is general, and a number of heterometallic fragments including Ru(η(6)-arene)(2+), M(CO)(4)(piperidine) for M = Mo and W, and Fe(2)(CO)(6) were successfully incorporated, and the resulting metallopeptides characterized via a range of spectroscopic techniques. This methodology serves as the first step to construction of hydrogenase peptidomimetics that incorporate defined outer coordination sphere interactions intended to tune reactivity.

Summary Genes encoding nitrogenase (nifH) were amplified from sediment and photosynthetic mat samples collected in the outflow channel of Mound Spring, an alkaline thermal feature in Yellowstone National Park. Results indicate the genetic capacity for nitrogen fixation over the entire range of temperatures sampled (57.2°C to 80.2°C). Amplification of environmental nifH transcripts revealed in situ expression of nifH genes at temperatures up to 72.7°C. However, we were unable to amplify transcripts of nifH at the higher-temperature locations (> 72.7°C). These results indicate that microbes at the highest temperature sites contain the genetic capacity to fix nitrogen, yet either do not express nifH or do so only transiently. Field measurements of nitrate and ammonium show fixed nitrogen limitation as temperature decreases along the outflow channel, suggesting nifH expression in response to the downstream decrease in bioavailable nitrogen. Nitrogen stable isotope values of Mound Spring sediment communities further support geochemical and genetic data. DNA and cDNA nifH amplicons form several unique phylogenetic clades, some of which appear to represent novel nifH sequences in both photosynthetic and chemosynthetic microbial communities. This is the first report of in situ nifH expression in strictly chemosynthetic zones of terrestrial (non-marine) hydrothermal systems, and sets a new upper temperature limit (72.7°C) for nitrogen fixation in alkaline, terrestrial hydrothermal environments.

A general method for immobilization of synthetic analogues of the [FeFe]‐hydrogenase in designed peptides via on resin modification of an amino acid side chain with a dithiol functional group is described. Utilizing a unique amine side chain as anchor, the dithiol unit is coupled to the peptide via formation of an amide. This dithiol unit precisely positions the two required sulfur atoms for the formation of a [(μ‐SRS){Fe(CO)3}2] cluster on reaction with [Fe3(CO)12]. UV/Vis and FTIR spectroscopy demonstrate formation of the desired complex.

Biological soil crusts (BSCs) are topsoil biosedimentary structures built by photosynthetic microbes commonly found today on arid soils. They play a role in soil stabilization and the fertility of arid lands, and are considered modern analogues of ancient terrestrial microbial communities. We determined the concentrations of four biogenic and 21 other elements, mostly metals, in surface soils that hosted BSCs, in the soils underneath those crusts, and in proximate but non-crusted surface soils. The samples were from six sites in the Colorado Plateau highlands and the Sonoran Desert lowlands. In spite of the variability in climate and geologic setting, we found statistically significant overall trends of enrichment in biogenic elements and depletion in non-biogenic elements when BSCs were compared with non-crusted soils. The differences between crusted and non-crusted soils were statistically significant at approximately 95% confidence for C, N (enrichments) and for Ca, Cr, Mn, Cu, Zn, As, and Zr (depletions). These trends are best explained by the activity of microbes. As expected, no differences in the concentrations of C, N, P, and S were detected between the soils underneath the crusts and the non-crusted soils, but the former showed depletion of non-biogenic elements, indicating that the leaching effect of crust microbes extends downward in the soil. These patterns speak to the need for a sustained input of allochthonous material, possibly dust, to maintain BSC fertility. These elemental patterns can be considered a biosignature that may be preserved in the rock record and might help identify ancient microbial communities on land.