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1. Reimport of carbon from cytosolic and vacuolar sugar pools into the Calvin–Benson cycle explains photosynthesis labeling anomalies

   https://doi.org/10.1073/pnas.2121531119  

   Xu, Yuan; Wieloch, Thomas; Kaste, Joshua A.; Shachar-Hill, Yair; Sharkey, Thomas D. (March 2022, Proceedings of the National Academy of Sciences)

   When isotopes of carbon are fed to photosynthesizing leaves, metabolites of the Calvin–Benson cycle (CBC) are rapidly labeled initially, but then the rate of labeling slows considerably, raising questions about the integration of the CBC within leaf metabolism. We have used 2-h time courses of labeling of Camelina sativa leaf metabolites to test models of 12 C washout when the CO 2 source is rapidly switched to 13 CO 2 . Fitting exponential functions to the time course of CBC metabolites, we found evidence for three temporally distinct processes contributing to the labeling but none for metabolically inactive pools. We next modeled the data of all metabolites by 13 C isotopically nonstationary metabolic flux analysis, testing a variety of flux networks. In the model that best explains measured data, three processes determine CBC metabolite labeling. First is fixation of incoming 13 CO 2 ; second is dilution by weakly labeled carbon in cytosolic glucose reentering the CBC following oxidative pentose phosphate pathway reactions, which forms a shunt bypassing much of the CBC. Third, very weakly labeled carbon from the vacuole further dilutes the labeling. This model predicts the shunt proceeds at about 5% of the rate of net CO 2 fixation and explains the three phases of labeling. In showing the interconnection of three compartments, we have drawn a more complete picture of how carbon moves through photosynthetic metabolism in a way that integrates the CBC, cytosolic sugar pools, glucose-6-phosphate shunt, and vacuolar sugars into a single system. 

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2. Revisiting Elevated δ ¹³ C Values of Sediment on Modern Carbonate Platforms

   https://doi.org/10.1029/2023GL107703  

   Trower, Elizabeth_J; Hibner, Brianna_M; Lincoln, Tyler_A; Dodd, Jacqueline_E; Hagen, Cedric_J; Cantine, Marjorie_D; Gomes, Maya_L (July 2024, Geophysical Research Letters)

   Abstract The measured carbon isotopic compositions of carbonate sediments (δ¹³C_carb) on modern platforms are commonly¹³C‐enriched compared to predicted values for minerals forming in isotopic equilibrium with the dissolved inorganic carbon (DIC) of modern seawater. This offset undermines the assumption that δ¹³C_carbvalues of analogous facies in the rock record are an accurate archive of information about Earth's global carbon cycle. We present a new data set of the diurnal variation in carbonate chemistry and seawater δ¹³C_DICvalues on a modern carbonate platform. These data demonstrate that δ¹³C_carbvalues on modern platforms are broadly representative of seawater, but only after accounting for the recent decrease in the δ¹³C value of atmospheric CO₂and shallow seawater DIC due to anthropogenic carbon release, a phenomenon commonly referred to as the¹³C Suess effect. These findings highlight an important, yet overlooked, aspect of some modern carbonate systems, which must inform their use as ancient analogs. 

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3. Imaging the Water Snowline around Protostars with Water and HCO ⁺ Isotopologues

   https://doi.org/10.3847/1538-4357/ac3080  

   van ’t Hoff, Merel L.; Harsono, Daniel; van Gelder, Martijn L.; Hsieh, Tien-Hao; Tobin, John J.; Jensen, Sigurd S.; Hirano, Naomi; Jørgensen, Jes K.; Bergin, Edwin A.; van Dishoeck, Ewine F. (January 2022, The Astrophysical Journal)

   Abstract The water snowline location in protostellar envelopes provides crucial information about the thermal structure and the mass accretion process as it can inform about the occurrence of recent (≲1000 yr) accretion bursts. In addition, the ability to image water emission makes these sources excellent laboratories to test indirect snowline tracers such as H 13 CO + . We study the water snowline in five protostellar envelopes in Perseus using a suite of molecular-line observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) at ∼0.″2−0.″7 (60–210 au) resolution. B1-c provides a textbook example of compact H 2 18 O (3 1,3 −2 2,0 ) and HDO (3 1,2 −2 2,1 ) emission surrounded by a ring of H 13 CO + ( J = 2−1) and HC 18 O + ( J = 3−2). Compact HDO surrounded by H 13 CO + is also detected toward B1-bS. The optically thick main isotopologue HCO + is not suited to trace the snowline, and HC 18 O + is a better tracer than H 13 CO + due to a lower contribution from the outer envelope. However, because a detailed analysis is needed to derive a snowline location from H 13 CO + or HC 18 O + emission, their true value as a snowline tracer will lie in the application in sources where water cannot be readily detected. For protostellar envelopes, the most straightforward way to locate the water snowline is through observations of H 2 18 O or HDO. Including all subarcsecond-resolution water observations from the literature, we derive an average burst interval of ∼10,000 yr, but high-resolution water observations of a larger number of protostars are required to better constrain the burst frequency. 

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4. NMR Crystallography: Evaluation of Hydrogen Positions in Hydromagnesite by ¹³ C{ ¹ H} REDOR Solid‐State NMR and Density Functional Theory Calculation of Chemical Shielding Tensors

   https://doi.org/10.1002/anie.201813306  

   Cui, Jinlei; Olmsted, David L.; Mehta, Anil K.; Asta, Mark; Hayes, Sophia E. (February 2019, Angewandte Chemie International Edition)

   Abstract Solid‐state NMR measurements coupled with density functional theory (DFT) calculations demonstrate how hydrogen positions can be refined in a crystalline system. The precision afforded by rotational‐echo double‐resonance (REDOR) NMR to interrogate¹³C–¹H distances is exploited along with DFT determinations of the¹³C tensor of carbonates (CO₃²⁻). Nearby¹H nuclei perturb the axial symmetry of the carbonate sites in the hydrated carbonate mineral, hydromagnesite [4 MgCO₃⋅Mg(OH)₂⋅4 H₂O]. A match between the calculated structure and solid‐state NMR was found by testing multiple semi‐local and dispersion‐corrected DFT functionals and applying them to optimize atom positions, starting from X‐ray diffraction (XRD)‐determined atomic coordinates. This was validated by comparing calculated to experimental¹³C{¹H} REDOR and¹³C chemical shift anisotropy (CSA) tensor values. The results show that the combination of solid‐state NMR, XRD, and DFT can improve structure refinement for hydrated materials. 

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5. Calibration Strategies for Detecting Macroscale Patterns in NEON Atmospheric Carbon Isotope Observations

   https://doi.org/10.1029/2020JG005862  

   Fiorella, Richard P.; Good, Stephen P.; Allen, Scott T.; Guo, Jessica S.; Still, Christopher J.; Noone, David C.; Anderegg, William R. L.; Florian, Christopher R.; Luo, Hongyan; Pingintha‐Durden, Natchaya; et al (March 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract Carbon fluxes in terrestrial ecosystems and their response to environmental change are a major source of uncertainty in the modern carbon cycle. The National Ecological Observatory Network (NEON) presents the opportunity to merge eddy covariance (EC)‐derived fluxes with CO₂isotope ratio measurements to gain insights into carbon cycle processes. Collected continuously and consistently across >40 sites, NEON EC and isotope data facilitate novel integrative analyses. However, currently provisioned atmospheric isotope data are uncalibrated, greatly limiting ability to perform cross‐site analyses. Here, we present two approaches to calibrating NEON CO₂isotope ratios, along with an R package to calibrate NEON data. We find that calibrating CO₂isotopologues independently yields a lowerδ¹³C bias (<0.05‰) and higher precision (<0.40‰) than directly correctingδ¹³C with linear regression (bias: <0.11‰, precision: 0.42‰), but with slightly higher error and lower precision in calibrated CO₂mole fraction. The magnitude of the corrections toδ¹³C and CO₂mole fractions vary substantially by site, underscoring the need for users to apply a consistent calibration framework to data in the NEON archive. Post‐calibration data sets show that site mean annualδ¹³C correlates negatively with precipitation, temperature, and aridity, but positively with elevation. Forested and agricultural ecosystems exhibit larger gradients in CO₂andδ¹³C than other sites, particularly during the summer and at night. The overview and analysis tools developed here will facilitate cross‐site analysis using NEON data, provide a model for other continental‐scale observational networks, and enable new advances leveraging the isotope ratios of specific carbon fluxes. 

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