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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Structural and Dynamical Design Principles for Iron-Sulfur Clusters in Sequence Defined Polymers

This integrated synthetic and computational effort uses sequence defined polymers such as peptoids, and triazines, capable of mimicking a flexible protein environment to control the positioning of side chains around redox cofactors. This strategy will allow modulation of redox properties through both direct and indirect mechanisms. To this end the aim is defined to test the underlying hypothesis: that principles and critical functionalities governing the binding and tuning of naturally occurring electron carrier co-factors can be identified and used to predictably design and synthesize maquettes based on non-natural polymers resulting in new modular biomimetic scaffolds for Electron Transfer Materials. This will be achieved through design and synthesis of ligands that can stabilize iron-sulfur clusters and tune the redox potential based on sequence.

36 MATERIALS SCIENCE↗
DOE OSTI2022

MISPR : an open-source package for high-throughput multiscale molecular simulations

Computational tools provide a unique opportunity to study and design optimal materials by enhancing our ability to comprehend the connections between their atomistic structure and functional properties. However, designing materials with tailored functionalities is complicated due to the necessity to integrate various computational-chemistry software (not necessarily compatible with one another), the heterogeneous nature of the generated data, and the need to explore vast chemical and parameter spaces. The latter is especially important to avoid bias in scattered data points-based models and derive statistical trends only accessible by systematic datasets. Here, we introduce a robust high-throughput multi-scale computational infrastructure coined MISPR (Materials Informatics for Structure–Property Relationships) that seamlessly integrates classical molecular dynamics (MD) simulations with density functional theory (DFT). By enabling high-performance data analytics and coupling between different methods and scales, MISPR addresses critical challenges arising from the needs of automated workflow management and data provenance recording. The major features of MISPR include automated DFT and MD simulations, error handling, derivation of molecular and ensemble properties, and creation of output databases that organize results from individual calculations to enable reproducibility and transparency. In this work, we describe fully automated DFT workflows implemented in MISPR to compute various properties such as nuclear magnetic resonance chemical shift, binding energy, bond dissociation energy, and redox potential with support for multiple methods such as electron transfer and proton-coupled electron transfer reactions. The infrastructure also enables the characterization of large-scale ensemble properties by providing MD workflows that calculate a wide range of structural and dynamical properties in liquid solutions. MISPR employs the methodologies of materials informatics to facilitate understanding and prediction of phenomenological structure–property relationships, which are crucial to designing novel optimal materials for numerous scientific applications and engineering technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2021

High Ammonia Adsorption in MFM-300 Materials: Dynamics and Charge Transfer in Host–Guest Binding

Ammonia (NH 3 ) is a promising energy resource owing to its high hydrogen density. However, its widespread application is restricted by the lack of efficient and corrosion-resistant storage materials. In this work, we report high NH 3 adsorption in a series of robust metal-organic framework (MOF) materials, MFM-300(M) (M = Fe, V, Cr, In). MFM-300(M) (M = Fe, V III , Cr) show fully reversible capacity for >20 cycles, reaching capacities of 16.1, 15.6, and 14.0 mmol g -1 , respectively, at 273 K and 1 bar. Under the same conditions, MFM-300(V IV ) exhibits the highest uptake among this series of MOFs of 17.3 mmol g -1 . In situ neutron powder diffraction, single-crystal X-ray diffraction, and electron paramagnetic resonance spectroscopy confirm that the redox-active V center enables host-guest charge transfer, with V IV being reduced to V III and NH 3 being oxidized to hydrazine (N 2 H 4 ). A combination of in situ inelastic neutron scattering and DFT modeling has revealed the binding dynamics of adsorbed NH 3 within these MOFs to afford a comprehensive insight into the application of MOF materials to the adsorption and conversion of NH 3 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Defect-Driven Redox Interplay on Anatase TiO 2 : Surface-Structure Dependent Activation for CO 2 Hydrogenation Catalysis

Titanium dioxide (TiO 2 ) is one of the most extensively studied oxides as an active catalyst or catalyst support, particularly in energy and environmental applications, but the atomistic mechanisms governing its dynamic response to reactive environments and their correlation to reactivity remain largely elusive. Using in situ environmental transmission electron microscopy (ETEM), synchrotron X-ray diffraction (XRD), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), temperature-programmed reduction (TPR), reactivity measurements, and theoretical modeling, we reveal the dynamic interplay between oxygen loss and replenishment of anatase TiO 2 under varying reactive conditions. Under H 2 exposure, anatase TiO 2 undergoes surface reduction via lattice oxygen loss, forming Ti 3 O 5 . In contrast, CO 2 exposure induces oxygen replenishment, reversing stoichiometry. In mixed H2/CO 2 environments, the reverse water–gas shift (RWGS) reaction proceeds selectively on stepped and high-indexed TiO 2 surfaces, whereas the thermodynamically stable TiO 2 (101) surface remains inactive and intact. Critically, H 2 pretreatment generates oxygen vacancies on TiO 2 (101), transforming it into an active Ti 3 O 5 or defect-rich surface that catalyzes RWGS. By correlating surface structure, defect dynamics, and gas-phase interactions, this work deciphers the competition between H 2 -driven reduction and CO 2 -driven oxidation pathways at the atomic scale. Furthermore, these insights establish defect engineering as a strategic lever to activate inert TiO 2 facets, advancing the design of adaptive catalysts for sustainable fuel synthesis technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Direct Evidence for Excited Ligand Field State-based Oxidative Photoredox Chemistry of a Cobalt(III) Polypyridyl Photosensitizer

Increasing interest in sustainable chemistry coupled with the quest to explore new reactivity has spurred research on first-row transition metal complexes for potential applications in a variety of settings. One of the more active areas of research is photoredox catalysis, where the synthetically tunable nature of their electronic structures provides a rich palette of options for tailoring their reactivity to a desired chemical transformation. Understanding the mechanism of excited-state reactivity is critical for the informed development of next-generation catalysts, which in turn requires information concerning the propensity of their electronic excited states to engage in the desired electron or energy transfer processes. Herein we provide direct evidence of the highly oxidizing nature of the lowest-energy ligand-field (LF) excited state of a first-row d 6 -low-spin Co(III) photosensitizer [Co(4,4'-Br 2 bpy) 3 ] 3+ (where 4,4'-Br 2 bpy is 4,4'-dibromo-2,2'-bipyridine). The redox potential associated with the LF excited state of the Co(III) complex was bracketed by performing bimolecular quenching studies by using a series of simple organic electron donors. Time-resolved absorption spectroscopy confirmed a dynamic quenching process attributed to reductive quenching of the lowest-energy ligand-field excited state of the Co(III) chromophore. Analysis of the Stern–Volmer plots for each chromophore-quencher pair revealed a limiting value of E red * ~ 1.25 V vs Fc/Fc + for the metal-centered excited state, which is significantly stronger than that of more commonly employed transition metal-based photoredox agents such as [Ru(bpy) 3 ] 2+ (E red * = 0.32 V vs Fc/Fc + ) and [Ir(ppy) 2 (bpy)] + (E red * = 0.27 V vs Fc/Fc + ). Finally, these results suggest that this class of chromophores could find utility in applications requiring the activation of oxidatively resistant organic substrates for photoredox catalysis.

14 SOLAR ENERGY↗
DOE OSTI2021

Soil gas probes for monitoring trace gas messengers of microbial activity

Abstract Soil microbes vigorously produce and consume gases that reflect active soil biogeochemical processes. Soil gas measurements are therefore a powerful tool to monitor microbial activity. Yet, the majority of soil gases lack non-disruptive subsurface measurement methods at spatiotemporal scales relevant to microbial processes and soil structure. To address this need, we developed a soil gas sampling system that uses novel diffusive soil probes and sample transfer approaches for high-resolution sampling from discrete subsurface regions. Probe sampling requires transferring soil gas samples to above-ground gas analyzers where concentrations and isotopologues are measured. Obtaining representative soil gas samples has historically required balancing disruption to soil gas composition with measurement frequency and analyzer volume demand. These considerations have limited attempts to quantify trace gas spatial concentration gradients and heterogeneity at scales relevant to the soil microbiome. Here, we describe our new flexible diffusive probe sampling system integrated with a modified, reduced volume trace gas analyzer and demonstrate its application for subsurface monitoring of biogeochemical cycling of nitrous oxide (N 2 O) and its site-specific isotopologues, methane, carbon dioxide, and nitric oxide in controlled soil columns. The sampling system observed reproducible responses of soil gas concentrations to manipulations of soil nutrients and redox state, providing a new window into the microbial response to these key environmental forcings. Using site-specific N 2 O isotopologues as indicators of microbial processes, we constrain the dynamics of in situ microbial activity. Unlocking trace gas messengers of microbial activity will complement -omics approaches, challenge subsurface models, and improve understanding of soil heterogeneity to disentangle interactive processes in the subsurface biome.

04 OIL SHALES AND TAR SANDS↗
DOE OSTI2021

Cryo-EM structures of Escherichia coli cytochrome bo 3 reveal bound phospholipids and ubiquinone-8 in a dynamic substrate binding site

Two independent structures of the proton-pumping, respiratory cytochrome bo3 ubiquinol oxidase (cyt bo 3 ) have been determined by cryogenic electron microscopy (cryo-EM) in styrene–maleic acid (SMA) copolymer nanodiscs and in membrane scaffold protein (MSP) nanodiscs to 2.55- and 2.19-Å resolution, respectively. The structures include the metal redox centers (heme b, heme o 3 , and Cu B ), the redox-active cross-linked histidine–tyrosine cofactor, and the internal water molecules in the proton-conducting D channel. Each structure also contains one equivalent of ubiquinone-8 (UQ8) in the substrate binding site as well as several phospholipid molecules. The isoprene side chain of UQ8 is clamped within a hydrophobic groove in subunit I by transmembrane helix TM0, which is only present in quinol oxidases and not in the closely related cytochrome c oxidases. Both structures show carbonyl O1 of the UQ8 headgroup hydrogen bonded to D75I and R71 I . In both structures, residue H98 I occupies two conformations. In conformation 1, H98 I forms a hydrogen bond with carbonyl O4 of the UQ8 headgroup, but in conformation 2, the imidazole side chain of H98 I has flipped to form a hydrogen bond with E14 I at the N-terminal end of TM0. We propose that H98 I dynamics facilitate proton transfer from ubiquinol to the periplasmic aqueous phase during oxidation of the substrate. Computational studies show that TM0 creates a channel, allowing access of water to the ubiquinol headgroup and to H98 I .

bioenergetics↗
DOE OSTI2022

Theoretical Modeling of Electrochemical Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) plays an essential role in a wide range of electrocatalytic processes. Here, a vast array of theoretical and computational methods have been developed to study electrochemical PCET. These methods can be used to calculate redox potentials and pKa values for molecular electrocatalysts, proton-coupled redox potentials and bond dissociation free energies for PCET at metal and semiconductor interfaces, and reorganization energies associated with electrochemical PCET. Periodic density functional theory can also be used to compute PCET activation energies and perform molecular dynamics simulations of electrochemical interfaces. Various approaches for maintaining a constant electrode potential in electronic structure calculations and modeling complex interactions in the electric double layer (EDL) have been developed. Theoretical formulations for both homogeneous and heterogeneous electrochemical PCET spanning the adiabatic, nonadiabatic, and solvent-controlled regimes have been developed and provide analytical expressions for the rate constants and current densities as functions of applied potential. The quantum mechanical treatment of the proton and inclusion of excited vibronic states have been shown to be critical for describing experimental data, such as Tafel slopes and potential dependent kinetic isotope effects. The calculated rate constants can be used as input to microkinetic models and voltammogram simulations to elucidate complex electrocatalytic processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Adaptive_Evolution_2026

Thermoanaerobacterium saccharolyticum, an anaerobic and thermophilic bacterium capable of metabolizing sugar monomers and soluble oligomers into ethanol, has been proposed for use in consolidated bioprocessing in coculture with compatible cellulolytic bacteria such asClostridium thermocellum. Although the mixed acid fermentation of both of these strains has been engineered to produce ethanol as the major fermentation product, the maximum titer produced thus far byC. thermocellum, about 3% (w/v) ethanol, is half that produced byT. saccharolyticum. There is thus motivation to understand the mechanistic basis of the robust ethanol pathway inT. saccharolyticumso that key features can be recapitulated inC. thermocellumand other organisms.Previously, we characterized theindividualrole of the main genes responsible for electron transfer in the ethanol production ofT. saccharolyticum. However, the consequences of thecombinedloss of function of all these genes have not been investigated, nor has the way in which fermentative metabolism adapts to such constraints. In this work, we combined knockouts of ferredoxin nicotinamide oxidoreductase (fnor) genes (nfnAandnfnB) and hydrogenase genes (hydAandhfsD) and studied their effects on fermentation and grow. We showed that these genetic modifications together impair growth and decrease electron transfer from reduced ferredoxin, thereby redirecting flux from the pyruvate ferredoxin oxidoreductase enzyme to the pyruvate formate lyase enzyme. We also performed adaptive evolution of these mutants to rescue their growth, and most notably, we observed a single nucleotide variation in the alcohol dehydrogenaseadhAgene. Through molecular dynamics simulations and enzymatic assays, we determined that this point mutation causes a structural change that impairs the AdhA specificity for the NADPH cofactor and increases NADH-linked activity to restore redox balance. These findings consolidate our understanding of the functioning of electron transfer pathways in this organism.

alcohol dehydrogenase↗
DOE OSTI2023

Stabilizing lattice oxygen redox in layered sodium transition metal oxide through spin singlet state

Abstract Reversible lattice oxygen redox reactions offer the potential to enhance energy density and lower battery cathode costs. However, their widespread adoption faces obstacles like substantial voltage hysteresis and poor stability. The current research addresses these challenges by achieving a non-hysteresis, long-term stable oxygen redox reaction in the P3-type Na 2/3 Cu 1/3 Mn 2/3 O 2 . Here we show this is accomplished by forming spin singlet states during charge and discharge. Detailed analysis, including in-situ X-ray diffraction, shows highly reversible structural changes during cycling. In addition, local CuO 6 Jahn-Teller distortions persist throughout, with dynamic Cu-O bond length variations. In-situ hard X-ray absorption and ex-situ soft X-ray absorption study, along with density function theory calculations, reveal two distinct charge compensation mechanisms at approximately 3.66 V and 3.99 V plateaus. Notably, we observe a Zhang-Rice-like singlet state during 3.99 V charging, offering an alternative charge compensation mechanism to stabilize the active oxygen redox reaction.

25 ENERGY STORAGE↗
DOE OSTI2023

Correlations between Molecular Structure, Solvation Topology, and Transport Properties of Aqueous Organic Flow Battery Electrolyte Solutions

Aqueous organic redox flow batteries (AORFBs) are considered promising technologies for storing energy generated from renewable resources. However, designing organic electrolyte molecules is limited by gaps between fundamental understanding of coupling between solvation structure and dynamics, and macroscopic transport properties like viscosity. Herein, we used molecular dynamics simulations to understand correlations between ionic molecular structures, ion clustering, and transport properties in 2,3-dihydrophenazine (2,3-DHP), a promising AORFB anolyte. We show that experimentally measured viscosity can be reproduced from simulations at relevant concentrations and that the asymmetric structure of 2,3-DHP leads to unique inhomogeneity in the solvation topology. However, order parameters and metrics need to be developed for better correlations over spatiotemporal scales, with careful consideration of the inhomogeneity of organic anolyte molecules. In conclusion, we show that the increased size and asymmetry of the anolyte leads to breakdown of assumptions within methods for determining ion transport mechanisms previously developed for Li-ion batteries.

25 ENERGY STORAGE↗
DOE OSTI2025

Time-resolved chemically-selective spectroscopic investigation of the redox reaction between hematite and aluminium

Thermite reactions –highly energetic redox processes between a metal and an oxide—are used in welding, propulsion, and the fabrication of advanced materials. When reduced to the nanoscale, these reactions exhibit enhanced energetic performance, but their ultrafast dynamics remain poorly understood. Gaining insight into charge transfer during these processes is essential for advancing applications in energy conversion and materials design. Here we show that the reaction between aluminium and hematite, a common iron oxide, can be tracked with femtosecond resolution using extreme ultraviolet (EUV) time-resolved absorption spectroscopy at the Fe M 2,3 and Al L 2,3 edges. By exciting the system with an ultrashort optical pulse and probing element-specific absorption changes, we observe an early spectral shift that reveals the formation of localized charge carriers (polarons). Comparing samples with different supporting substrates highlights ultrafast electron transfer from aluminium to hematite. These results demonstrate an approach to investigating charge flow in energetic materials and provide a basis for studying fast chemical reactions with chemical specificity.

Electron transfer↗
DOE OSTI2021

Effective kinetics driven by dynamic concentration gradients under coupled transport and reaction

Biogeochemical reaction kinetics are generally established from batch reactors where concentrations are uniform. In natural systems, many biogeochemical processes are characterized by spatially and temporally variable concentration gradients that often occur at scales which are not resolved by field measurements or biogeochemical and reactive transport models. Yet, it is not clear how these sub-scale chemical gradients affect reaction kinetics compared to batch kinetics. Here we investigate this question by studying the paradigmatic case of localized pulses of solute reacting with a solid or a dissolved species in excess. Additionally, we consider non-linear biogeochemical reactions, representative of mineral dissolution, adsorption and redox reactions, which we quantify using simplified power-law kinetics. The combined effect of diffusion and reaction leads to effective kinetics that differ quantitatively and qualitatively from the batch kinetics. Depending on the nonlinearity (reaction order) of the local kinetics, these effects lead to either enhancement or decrease of the overall reaction rate, and result in a rich variety of reaction dynamics. We derive analytical results for the effective kinetics, which are validated by comparison to direct numerical simulations for a broad range of Damköhler numbers and reaction order. Our findings provide new insights into the interpretation of imperfectly mixed lab experiments, the effective kinetics of field systems characterized by intermittent reactant release and the integration of sub-scale concentration gradients in reactive transport models.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Polarons are probes of the dynamic nanoscale environments found in electrochemically doped π-conjugated polymers

Charge carriers (i.e., polarons) in electrochemically doped organic (semi)conductors are proposed to be regulated by several physicochemical features, including the chemical compositions and structures of the π-conjugated frameworks of the semiconductor building blocks, the chemistry of the electrolyte (considering both the salt and solvent), multiscale and time-dependent morphology variations across the material as a function of electrochemical processes, and assorted permutations of these and other factors. To address these hypotheses, we investigate the energetic, optoelectronic, chemical, and local structural properties of negative polarons (radical anions) as a function of electrochemical doping in the donor–acceptor, π-conjugated redox copolymer P(NDI2OD-T2), also referred to as N2200. A critical finding is that there is not just “one type” of polaron in electrochemically doped P(NDI2OD-T2). Rather, an ensemble of polarons exists, with the polarons having optoelectronic signatures that are defined by their nanoscale environments. Importantly, the polaron optical signatures serve as local probes for how the operando electrochemical environments are dynamically working in concert to facilitate charge transport. Collectively, the distinctive and extensive integration of theory and measurement science presented here establishes a baseline for the roles that semiconductor and electrolyte chemistries and dynamic structural features have on polarons in electrochemically doped organic semiconductors and how these factors influence the energetics and rates of polaron transport.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Atomic Origin of the Autocatalytic Reduction of Monoclinic CuO in a Hydrogen Atmosphere

Reducibility is key for the use of bulk metal oxides in chemical transformation involving redox reactions but probing microscopic processes of oxide reduction is challenging. This is because the insulating nature of bulk oxides restricts ion and electron spectroscopic measurements of oxide surfaces. Herein, using a combination of environmental transmission electron microscopy and atomistic modeling, we report direct in-situ atomic-scale observations of the surface and subsurface dynamics and show that the hydrogen induced CuO reduction occurs via the surface decay of Cu-O/Cu bilayer atomic steps, formation of partially reduced CuO superstructure by the self-ordering of O vacancies in the subsurface, and collapse of Cu-O layers in the bulk. In this work, all these substeps can be traced back to the progressively increased concentration and activity of O vacancies in the surface and subsurface of the oxide, thereby leading to the self-accelerated oxide reduction. These results demonstrate the microscopic details that may have broader applicability in modulating various redox processes.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Dynamic tunability of phase-change material transition temperatures using ions for thermal energy storage

Thermal energy storage (TES) based on phase-change materials (PCMs) has many current and potential applications, such as climate control in buildings, thermal management for batteries and electronics, thermal textiles, and transportation of pharmaceuticals. Despite its promise, the adoption of TES has been limited, in part due to limited tunability of the transition temperature, which hinders TES performance for varying use temperatures. Transition temperature tuning of a material using an external stimulus, such as pressure or an electric field, typically requires very large stimuli. To circumvent this problem, here, we report on the dynamic transition temperature tunability of a PCM using ions. We achieve a transition temperature tunability up to 6°C in polyethylene glycol (PEG) by using the salt lithium oxalatodifluoroborate at a low voltage of 2.5 V, which may enable simpler and safer devices/system designs. We also explain the thermal properties of the salt/PCM solution using the Flory-Huggins theory.

25 ENERGY STORAGE↗
DOE OSTI2025

Redox Couples Control Band Bending, Photovoltage, and Quasi-Fermi Levels in Tungsten Oxide (WO 3 ) Photoanodes

Tungsten oxide (WO 3 ) is a well-known photoanode and photocatalyst for photoelectrochemical (PEC) water oxidation. Because the compound has a deep valence band, it can facilitate the oxygen evolution reaction without added cocatalysts, and it can drive the oxidation of species with much higher electrochemical potentials, including the conversion of water to hydrogen peroxide, sulfate to persulfate, and iodate to meta-periodate. Here, we use the liquid vibrating Kelvin probe surface photovoltage (liquid VK-SPV) technique in combination with open circuit potential (OCP) and photoelectrochemical (PEC) scans to assess the possibility of reaching such oxidizing potentials in aqueous electrolytes and at open circuit. Here, this is done by mapping the quasi-Fermi levels of electrons and holes at the interfaces as a function of the light intensity. Nanostructured WO 3 photoelectrodes for this purpose were fabricated by thermal annealing of a tungstic acid solution on fluorine-doped tin oxide. Electrochemical measurements are conducted at open circuit and 400 nm LED light illumination in electrolytes containing fast (O 2 /H 2 O 2 ), slow (O 2 /H 2 O), and very oxidizing (NaIO 4 /NaIO 3 ) redox couples. Photovoltage values scale with the light intensity and with the built-in potential for each redox couple and reach values up to 0.61 V under 20 mW cm –2 illumination for the NaIO 4 electrolyte. This shows that the photoelectrodes behave like Schottky-type diodes whose maximum possible energy output is determined mainly by the built-in voltage of each junction. For slow redox couples, the quasi-Fermi level of the holes increases with light intensity due to hole accumulation at the WO 3 –liquid interface. For example, for the O 2 /H 2 O electrolyte, interfacial hole accumulation and removal occur on the 90–300 s time scale. For the fast hole acceptor H 2 O 2 , on the other hand, the quasi-Fermi level of the photoholes is pinned to the electrochemical potential of the O 2 /H 2 O 2 couple. This limits the energy conversion efficiency of the electrode. Overall, these results reveal the influence of charge transfer thermodynamics and kinetics on the photovoltage of WO 3 . Furthermore, the work further establishes VK-SPV as a contactless method to observe the photovoltage, carrier dynamics, and quasi-Fermi levels of semiconductor-liquid junctions.

Electrodes↗
DOE OSTI2020

Dynamic changes in dissolved organic matter composition in a Mountain Lake under ice cover and relationships to changes in nutrient cycling and phytoplankton community composition

We studied Bear Lake, a sub-alpine oligotrophic lake in Rocky Mountain National Park, Colorado U.S.A, during the winter ice cover period of 2018. Our goal was to understand relationships between biogeochemical processes and the chemical quality of the dissolved organic matter, a major pool of organic carbon in all lakes. The lake exhibited a highly stratified environment with limited light availability, a consistent oxycline at 7 meters, and dilute hydrochemistry. Nutrient concentrations were low and depletion of nitrogen, particularly for NH4 to a lesser extent NO3, were related to increasing phytoplankton abundance. The deep chlorophyll maxima that developed above the oxycline was dominated by a potentially mixotrophic dinoflagellate. DOM chemical quality as measured through high-resolution mass spectrometry was related to these shifts in aquatic ecosystem structure. However, CDOM spectroscopy of the whole water was a poor indicator of biogeochemical changes in the DOM pool. The spectrometry data highlighted an evolving and contrasting reservoir of phosphorus, nitrogen, and sulfur-containing organic compounds sensitive to nutrient cycling and redox conditions. We also observed reduced chemodiversity between the surface and bottom water DOM molecular formulas over time. Homogeneity increased in the surface and more unique compounds were produced in the bottom waters, where an overall increase occurred in the favorability of carbon to support microbial respiration. Our results show dynamic chemical changes in the DOM pool reflect dynamic nutrient cycling under ice cover, potentially driven by mixotrophic growth of the dinoflagellate Gymnodinium sp.

Rue, Garrett↗