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Isotopic evidence for elevated photorespiration during the last glacial period

Nature Geoscience volume 18, pages 1232–1238 (2025)Cite this article

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Abstract

Low atmospheric carbon dioxide levels are thought to suppress land plant productivity in part by promoting photorespiration, wherein illuminated C3 plants uptake molecular oxygen and release carbon dioxide. This could act as a negative feedback that limits atmospheric carbon dioxide decline during glacial periods. However, colder glacial temperatures would suppress photorespiration, potentially counteracting this feedback. Here we tested the hypothesis that land plants photorespired more during glacial periods by applying a proxy for photorespiration rate based on clumped isotope compositions of wood methoxyl groups, validated in modern and recent trees, to North American subfossil tree specimens from the last glacial period. We find that, across most of ice-free North America, trees from the last glacial period photorespired more than more recent trees from similar locations and more than contemporary trees from higher latitudes. We reconcile these differences using a single model relationship between temperature, atmospheric carbon dioxide levels and photorespiration, which suggests that, during glacial periods, photorespiration increased primarily in warmer growing environments that cooled by about 6 °C or less. This supports the hypothesis of a negative feedback that regulates atmospheric carbon dioxide by increasing photorespiration and restricting land plant productivity during glacial periods.

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Fig. 1: Spatial variability in the glacial photorespiration hypothesis and the samples we used to test it.
Fig. 2: Premise of a photorespiration proxy in clumped isotopes of wood.
Fig. 3: Comparison of clumped isotopic compositions and model-predicted relative photorespiration rates for glacial, Holocene and modern samples from two locations: NW Alaska (Arctic willows) and S. California (Glacial (La Brea Tar Pits) and Modern S. CA).
Fig. 4: Clumped isotope compositions of all samples studied, organized by latitude and time period.
Fig. 5: Clumped isotope compositions compared with model predictions of relative photorespiration rate.

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Data availability

All data are available in the Supplementary Information and via ScholarSphere at https://doi.org/10.26207/dwkr-pw46 (ref. 83).

Code availability

The Python code needed to reproduce the figures and analysis is available via ScholarSphere at https://doi.org/10.26207/dwkr-pw46 (ref. 83).

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Acknowledgements

X. Feng and J. Landis (Dartmouth University), along with G. Wang (Clemson University), provided subfossil wood samples. L. Santiago (UC Riverside) enabled sampling of modern Juniperus specimens. M.K.L. and R.S.S. each acknowledge support from Aguoron Institute Geobiology Postdoctoral Fellowships. B.E.W. acknowledges support from a NOAA Climate and Global Change Postdoctoral Fellowship. Additional funding was provided by NSF grant EAR-2047003 to D.A.S. The Thermo 253 Ultra at UC Berkeley was funded in part by the Heising-Simons Foundation.

Author information

Author notes

  1. Rebekah S. Sprengel

    Present address: Travertine Technologies, Boulder, CO, USA

Authors and Affiliations

  1. Department of Earth and Planetary Science, University of California, Berkeley, CA, USA

    Max K. Lloyd, Rebekah S. Sprengel, Barbara E. Wortham, Daniel E. Ibarra & Daniel A. Stolper

  2. Department of Geosciences, The Pennsylvania State University, University Park, PA, USA

    Max K. Lloyd

  3. La Brea Tar Pits and Museum, Natural History Museums of Los Angeles County, Los Angeles, CA, USA

    Regan E. Dunn

  4. Department of Earth, Environmental, and Planetary Sciences, and Institute at Brown for Environment and Society, Brown University, Providence, RI, USA

    Daniel E. Ibarra

  5. Department of Integrative Biology, University of California, Berkeley, CA, USA

    Todd E. Dawson

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Contributions

M.K.L., T.E.D. and D.A.S. conceived the study. M.K.L., R.S.S. and B.E.W. collected the data. D.E.I. assisted M.K.L in processing and interpreting climate model outputs. R.E.D. supervised B.E.W. in sampling and interpreting La Brea Tar Pits samples. M.K.L. generated the figures and wrote the original draft of the paper. All authors contributed to the review and editing of the paper.

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Correspondence to Max K. Lloyd.

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Extended data

Extended Data Fig. 1 Model predictions for VO/VC difference between Glacial and Holocene [CO2]atm levels.

The difference between Glacial and Holocene VO/VC is predicted to be positive or negligible depending on assumptions regarding growing season photosynthesis temperature, the extent of glacial cooling, and the response of ci/ca to a change in pCO2. Orange, blue, and purple lines are model predictions assuming photosynthesis temperatures in the Last Glacial period were 3, 6, or 9 °C colder than in the Holocene. 3–9 °C spans the range of mean annual surface temperature differences observed in recent modeling efforts and proxy reconstructions21,22. Dotted lines use the FvCB model9 assuming a single set of Rubisco kinetics11, a temperature-dependent mesophyll conductance term48,49, and a fixed ci/ca of 0.713,30,84,85. Upper and lower bounds of envelopes use the same kinetics but assume that ci/ca was lower (0.6, upper bound)32,86 or higher (0.8, lower bound)33,87,88 during the Last Glacial period compared to the Holocene (0.7). Pentagons use the average output of P-model v.1.053 for all sample sites (see Methods, Extended Data Fig. 2). Grey line denotes no net change in VO/VC between Glacial and Holocene climate states.

Extended Data Fig. 2 Modeled relative photorespiration rate (VO/VC) vs. monthly daytime growing temperature, grouped by [CO2]atm bin.

Daytime temperatures were predicted from a downscaling of a time-evolving earth system model (see Methods) for every month in a typical year for every sample at the time of growth. Relative photorespiration rates predicted from [CO2]atm and daytime temperature along with other required climate variables (see Supplementary Information Fig. 2) using P-model v.1.051. Lines are 2nd-order polynomial fits to the data and associated ± 95% prediction intervals where data are binned by [CO2]. The strong correlations (r2 = 0.98 for all bins) justify the use of [CO2] bins to explain our data and suggest that other P-model inputs that vary naturally between sample sites (for example, vapor pressure deficit, light intensity, soil moisture) do not meaningfully impact relative photorespiration rates.

Extended Data Fig. 3 Plant acclimation has a limited ability to suppress high relative photorespiration rates.

a ∆13CH2D vs. estimated daytime showing season temperature, showing a systematic offset between the temperatures at which low ∆13CH2D values occur in different time periods. Data are presented as mean values ± 1 s.e.m. of each measurement. b-d Comparison of four ways to estimate relative photorespiration rate (VO/VC) using different sets of model assumptions. Abscissa in b-d assumes a constant ci/ca of 0.7 and a universal temperature dependence of rubisco selectivity regardless of time period11,48,49. Ordinate in b) assumes ci/ca was 0.8 in glacial and deglacial samples and 0.7 in more recent samples33. Ordinate in c) assumes that rubisco selectivity (S) was optimized for lower temperature and [CO2]atm in glacial and deglacial samples by using an equation for the temperature dependence of S from in vivo measurements of A. thaliana vs. N. tabacum50. Ordinate in d) assumes that ci/ca was optimized to balance carboxylation capacity and water loss at a given light level in every sample separately35,52,53.

Extended Data Fig. 4 The finding that glacial ∆13CH2D data are more variable than Holocene data from the same latitude band is insensitive to how latitudes are grouped.

a Kernel density estimates (KDEs) of Holocene (pink) and glacial (blue) ∆13CH2D data grouped latitude in 10° ranges. This is the same grouping as in Fig. 4b in the Main text, except here 10–20 and 20–30°N groups are separated. Asterisk denotes group where glacial ∆13CH2D data are significantly more variable than Holocene data (P < 0.05 in two-sided Levene test for equal variances). b same as a) except using 15° ranges. c same as a) except using 20° ranges, beginning at 10°N because we have no data below 10°N.

Extended Data Fig. 5 Comparisons of methoxyl ∆13CH2D, δ13C, and concentration, colored by sample set.

a Methoxyl ∆13CH2D versus methoxyl concentration. b Methoxyl ∆13CH2D versus methoxyl δ13C. c Methoxyl δ13C versus methoxyl concentration. Methoxyl δ13C and concentration (in wt.%) are both potential tracers of O-demethylation reactions37. We interpret the lack of correlation for ∆13CH2D vs. δ13C and ∆13CH2D vs. concentration to indicate that any partial O-demethylation (if it occurred) did not influence the ∆13CH2D values of these samples. Error bars on ∆13CH2D omitted for clarity (typically ± 0.25‰, 1 s.e.m. of measurement). Error bars on δ13C and wt.% methoxyl are smaller than symbol size.

Extended Data Fig. 6 Interpretation of clumped isotope data from different time periods is insensitive to how relative photorespiration rate is calculated.

We observe a similar relationship between ∆13CH2D and VO/VC across time periods regardless of how VO/VC is estimated. a Methoxyl ∆13CH2D versus VO/VC assuming a constant ci/ca of 0.7 and a universal temperature dependence of rubisco selectivity regardless of time period. b Same as a) except abscissa assumes that ci/ca was optimized to balance carboxylation capacity and water loss at a given light level in every sample separately. c Same as a) except abscissa assumes that ci/ca was 0.8 in glacial and deglacial samples and 0.7 in more recent samples. d Same as a) except rubisco selectivity (S) was optimized for lower temperature and [CO2]atm in glacial and deglacial samples by using an equation for the temperature dependence of S from in vivo measurements of A. thaliana vs. N. tabacum. See Extended Data Fig. 3, Methods for details. On all panels, data are presented as mean values ± 1 s.e.m. of each measurement.

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Supplementary Information

Supplementary Figs. 1–3 and references.

Supplementary Tables 1–3

Supplementary Table 1. Age and location data for every newly presented sample. Supplementary Table 2. Methoxyl isotopic data for every newly presented sample. Supplementary Table 3. Climate and photosynthesis model predictions for every sample presented, along with a reference list for these tables and notes.

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Lloyd, M.K., Sprengel, R.S., Wortham, B.E. et al. Isotopic evidence for elevated photorespiration during the last glacial period. Nat. Geosci. 18, 1232–1238 (2025). https://doi.org/10.1038/s41561-025-01841-x

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