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The Origins of Molecules in Diverse Space and Planetary Environments and Their Intramolecular Isotope Signatures

The NAI Penn State team seeks to discover and document isotope patterns in organic molecules found in meteorites, dissolved in deep Earth fluids, from individual living organisms, within microbial ecosystems, and in organics associated with minerals and ice. Employing advanced computational tools and a rich observation portfolio, they will build a predictive understanding of how abiotic and biotic processes and environments are encoded in the isotopes of simple to complex organic compounds. Their work will lead to new understanding of organics and the isotopes they carry from space and planetary environments, in metabolic systems and modern biotic communities, and over Earth’s history.

Task 1. Meteorites and Cosmochemistry. This research will develop knowledge needed to recognize the ‘fingerprints’ of abiotic chemical and physical processes that culminate in the formation of extraterrestrial organics in space and planetary environments. The study of extra-terrestrial organic compounds is central to our efforts to understand the rise of life on Earth and the search for life elsewhere in the universe. This large and complex research area is poised to expand dramatically with the planned return to Earth of organic-bearing samples from Mars, the asteroid Bennu, and perhaps other solar system bodies. The organic constituents of such samples will reflect pre-biotic organic synthesis, which begins with precursors inherited from the interstellar medium, themselves products of high temperature reactions in stellar envelopes and low temperature reactions in molecular clouds. Primordial organics are processed and complemented by new components in circumstellar disks, and the mixture of these components contributes to the volatile-rich constituents of accreting planets and smaller bodies. Planetary processes, principally hydrothermal reactions and atmospheric photochemistry, transform and add to the inventory of planetary organics. It is critical to the success of coming decades of solar system exploration that the community works in advance to develop the methods and contextual understanding that will maximize the information retrieved from returned organics. The Penn State team will focus on organics carried by meteorites, and building from these analyses, they will develop experimental and theoretical models to address their origins.

Task 2. Methane. The Penn State team will evaluate equilibrium and kinetic drivers behind the unique clumped isotopic character of methane as it is cycled via generative and oxidative microbial processes. They seek to understand inter- and intra-molecular patterns and in single and clustered cells, in light of methane assimilation, metabolic and energetic biochemistry, and spatial variability in microbial communities. When methane is produced slowly by microbes or by thermal processes, its clumped isotope abundances reflect formation temperatures, information that is proving highly valuable to energy research and exploration. However, biotic methane is also a major component of biogeochemical cycles, and when generated rapidly or oxidized aerobically, it deviates from clumped-isotope equilibrium. Further, our understanding of substrates, metabolism and energetics during the anaerobic oxidation of methane (AOM) is still emerging, and little is known how it impacts clumped isotopes of residual methane. AOM cell clusters reveal a wide range of d13C values (>50‰); and their biochemicals carry isotopic differences >60 ‰ (i.e., between coenzyme F430, a tetrapyrrole and the lipid archaeol). Given the exceptionally wide range of isotope values observed within and between cells, the Penn State team anticipate isotope differences at specific positions within molecules are even further amplified by metabolism and assimilation processes.

Task 3. Organics in Earth Fluids. New isotope techniques will be used by this team to reveal compositions of complex molecules and to overcome analytical sensitivity limits. Using the kinetic and equilibration properties of isotopes will deepen our understanding of the chemical and biotic processes that create and transform compounds in reducing fluids. Fluids rich in dissolved H2 emanating from deep-sea hydrothermal systems and ancient terrestrial fracture waters have a favorable thermodynamic drive for the abiotic reduction of inorganic carbon into organic compounds. In such fluids, thermogenic biomass degradation, and abiotic chemical processes can modify dissolved organic carbon budgets in the ocean, and in low temperature mixing zones, can foster a deep biosphere. Reactions between water and rock links geologic processes with extreme ecosystems, constitute some of our best natural modern analogues for conditions on early Earth, and inform life detection strategies. Members of this team will lead sampling of hydrothermal fluids at the Pescadero Basin, where microbial and thermogenic influences, sediments, and abiotic serpentinization processes may contribute to fluids rich in hydrocarbons, carboxylic acids, lipids, and other organic compounds at the deepest hydrothermal field in the Pacific Ocean (3800 m). Other target marine sites include caldera plume samples from the active Vailulu’u seamount near the Samoan hotspot, the active Socorro volcano, and black smoker vents at the Western Galapagos Spreading Center. Continental serpentinite-hosted hydrothermal systems, such as Hakuba-Hppoa in Japan, will also be investigated. Fluids and gases emitted from a 2.7 Ga banded iron formation in the Soudan iron mine in Minnesota present an opportunity to examine carbon and energy flows in an extreme deep crustal microbial ecosystem. This work will build on relatively well characterized whole-molecule isotope signatures in fluid systems, and isotopomer techniques that have yielded temperatures of CH4 formation in vent and mine fluids, and mechanistic insights for abiotic organics production.

Task 4. Organics, Ice, and Minerals. The Penn State team will use spectroscopic analyses and position-specific isotopic data to constrain modeled mechanisms and isotopic consequences of organics-surface interactions. Organic matter can interact with mineral surfaces by diverse mechanisms, including ligand exchange, polyvalent cations bridges, and intermolecular forces (i.e., hydrogen bonding and London forces). These interactions vary in strength for different compound classes, with more polar molecules typically forming stronger bonds than the less reactive, non-polar hydrocarbons. Given this, it is unclear if mineral interactions with non-polar hydrocarbons are strong enough to drive preservation of compounds relevant to both biomarker studies and energy exploration. Team members conjecture that non-polar hydrocarbon preservation is likely favored in areas with higher noncrystalline mineral abundances, or where structures can partition into hydrophobic zones created by amphiphilic molecules (which possess both hydrophilic and hydrophobic properties), bound to mineral surfaces. Polar and ionic interactions may facilitate exchange of isotopes of elements in functional groups (i.e., especially C, H, O, N, S). Vapor pressure isotope effects measured on low-temperature liquids, and observations of isotope effects during liquid phase partitioning suggest organic-mineral surface mechanisms may impart subtle isotope fractionation at the scale of whole compounds. It is expected that such influences are amplified at polar molecular positions directly involved in sorption to mineral surface and in organic-ice interactions.

Task 5. Biochemicals. This large effort will evaluate isotope abundances in biochemicals in the context of assimilation, metabolism, and conditions within the growth environment. International collaborators, Robins, Remaud, Gilbert and Yoshida, have pioneered studies of the metabolic patterns expressed in biochemicals that build on early studies, such as acetogenic lipids, amino acids, and intermolecular isotope differences in organisms. Collectively, the Penn State team will construct a dataset of biochemical isotope patterns from simple to complex structures. Their goal will be to characterize the metabolic characteristics and constraints underlying position-specific isotope patterns in biochemical building blocks, and extend studies of sugars and organic acids and add studies of amino acids, and nitrogen bases. Knowledge derived from such biochemical intermediate compounds can then be extended to larger structures such as lipids and pigments, as well as assembled biopolymers such as polysaccharides, proteins, and nucleic acids. Biochemical studies will include biotic (e.g., prokaryote, algae, yeast, plant) and abiotic processes of environmental relevance (e.g., diffusion, ab/ad-sorption, and state changes, such as evaporation, condensation, or sublimation). These will be carried out in experiments where the biology/physical chemistry is rigorously controlled, and using cultured microorganisms and isolated enzymes.

Task 6. Predictive Models. The NAI Penn State team will use experimental data to anchor models of abiotic physical and chemical processes and metabolic systems to build a predictive understanding of how abiotic and biotic processes are encoded in molecular isotope patterns. Theory based on equilibrium isotope effects backs up observed multiple isotope substitution in methane, and has been used to predict patterns in abiotic amino acids. Yet, there is little concrete data that confidently constrains isotope exchange equilibria involving hydrocarbons larger than methane. This team will approach this issue with quantum mechanical calculations of vibrational energies in position-specific and multiple substitutions for molecules of interest, and using experiments targeting compounds and molecular sites that can be driven to equilibrium isotope distributions on laboratory timescales. Approximations of density-functional theory (DFT) cause errors in calculated isotope effects when applied to structurally complex compounds due to second-order isotope effects or dynamics of surfaces and solvated compounds; however, the relative equilibrium fractionation and kinetic isotope effects should be reasonably accurate and help guide interpretation of experimental results. Previous experience in modeling adsorption and solvation effects on vibrational frequencies has demonstrated that these factors can be reproduced accurately even in relatively complex molecules. Theoretical studies will consider approximations and errors inherent to the methods and will encompass alternate visions of how such problems could be approached. Biotically, the inherent complexity of metabolism means many variables can drive isotope patterns, and any given isotope distribution we measure is unlikely to be unique. Isotope distributions in biotic structures require metabolic insights before they can be used to drive expectations for related or alternative biochemistries. The quantitation and modeling of metabolic fluxes with isotope label methods is a rapidly expanding field in biochemistry and biomedical research. The Penn State team will take advantage of metabolic flux modeling approaches, and their own molecular isotope fractionation data, starting with amino acids, to initiate development of isotope-enabled metabolic flux models.

Task 7. Ancient Life and Environments. This collaboration will enable intramolecular studies of lipids in modern organisms to be applied to ancient hydrocarbon biomarkers. Their first application will focus on steranes that represent early animal life in the Neoproterozoic, and their analogues in modern sponges. Partnering with the NAI team at UC-Riverside, the Penn State team will investigate the integrity of intramolecular isotope abundances for ancient biomarkers in both thermally mature and immature matrices, and incorporated into kerogen. This collaboration leverages comprehensive biomarker stratigraphic records data and sample sets for Proterozoic and Paleozoic intervals from free and kerogen-bound pools via hydropyrolysis (HyPy) and MRM-GC-MS analysis. Thermal maturities of rock samples have been independently constrained by Rock-Eval pyrolysis and fidelity of biomarker records have been verified by analytical self-consistency checks. Supporting studies with model compounds and biomass indicate carbon atoms in the hydrocarbon skeleton of lipids remain intact under HyPy conditions and may be preserved over long timescales, in contrast with hydrogen and functional groups that are highly susceptible to isotopic alteration. The improved sensitivity of the latest Pico-CSIA opens the possibility of obtaining compound-specific d13C records for a broader suite of ancient biomarker compounds than has been possible up till now.

Task 8. Analytical Capacity. Analytical developments will be developed to foster improved precision, sensitivity, and molecular isolation capability needed to advance molecular isotope analyses, and to address anticipated demands of future solar system exploration. This team brings together researchers who have, independently, developed a suite of powerful molecular isotopic analytical tools. They will develop cross-comparison of molecular isotope data, by analyzing common standards and using methods to chemically isolate target functional groups. Team members will also seek to improve sensitivity of methods with the aim of decreasing sample sizes, recognizing that sample return missions will be constrained by extremely small amounts of material. Additionally, method development in mass spectrometry with high spatial resolution (e.g., MALDI-MS, TOF-SIMS, and/or laser ablation-MS) will explore ways that preserve molecular fragments as part of a path toward rover capability for in situ measurements of isotopes, isotopologues, and molecules. Finally, facilities will be developed at Penn State for molecule isolation and characterization to enable py-PSIA, irm-NMR, and high-resolution MS analyses and for molecular research needs of the NAI community.