The intricate dance between land plants and Earth’s atmosphere has long fascinated scientists, particularly the mechanisms that regulate atmospheric carbon dioxide levels over geological timescales. A recent groundbreaking study sheds new light on one such process—photorespiration—and its pivotal role during the last glacial period. This research uncovers compelling isotopic evidence indicating that land plants experienced significantly elevated rates of photorespiration during ice ages, a finding that challenges preconceived notions about plant productivity under low carbon dioxide conditions and cold climates.
Photorespiration, a biochemical pathway in which plants consume oxygen and release carbon dioxide while assimilating light energy, has traditionally been viewed as a costly and somewhat wasteful process in C3 plants—the dominant photosynthetic pathway among most terrestrial vegetation. Under contemporary conditions, photorespiration is considered a limiting factor for photosynthesis efficiency, especially when atmospheric CO2 concentrations are low and temperatures are high. This inefficiency has inspired decades of research aimed at genetically modifying crops to suppress photorespiration and improve productivity.
However, during glacial periods, atmospheric CO2 levels dropped dramatically, a shift with profound implications for plant physiology. Lower CO2 concentrations generally promote increased photorespiration because the enzyme Rubisco, responsible for fixing carbon dioxide, tends to react with oxygen instead when CO2 is scarce. This could have suppressed land plant productivity, acting as a negative feedback mechanism that slowed further CO2 drawdown. Yet, colder glacial temperatures would theoretically reduce photorespiration rates because enzymatic activities slow with temperature. This paradox raised important questions: did photorespiration actually increase or decrease during the last glacial period, and how did temperature and CO2 concentration interact to influence plant metabolic behavior?
To address this scientific conundrum, researchers developed and applied an innovative proxy that quantifies historical photorespiration rates by analyzing clumped isotope compositions within wood methoxyl groups. Clumped isotopes are unique molecular signatures that form under certain environmental and physiological conditions, and they can serve as reliable markers of past biochemistry. Validated meticulously on modern and more recent wood samples from diverse geographic locations, this proxy was then applied to well-preserved subfossil wood specimens from North America, dating back to the last glacial maximum around 20,000 years ago.
The results of this isotopic detective work were striking. Across a wide swath of ice-free North America, subfossil trees from the glacial period exhibited isotope signals consistent with substantially elevated photorespiration rates compared to modern and recent trees from similar latitudes, as well as contemporary trees from higher latitudes that typically experience cooler climates. These findings suggest that photorespiration was not uniformly suppressed by colder glacial temperatures. Instead, it increased in many regions, implying a more nuanced interplay between atmospheric and climatic factors than previously appreciated.
Interpreting these findings required the development and application of a sophisticated model integrating temperature, atmospheric CO2 partial pressures, and photorespiration kinetics. This model revealed that the decisive factor governing photorespiration during glacial times was the balance between temperature decline and CO2 scarcity. Specifically, photorespiration rates increased most dramatically in warmer growing environments that experienced cooling of roughly six degrees Celsius or less. In regions where temperatures dropped more severely, photorespiration did not increase as much, indicating that cold temperatures indeed suppressed this process.
This nuanced picture reconciles earlier conflicting ideas about the regulation of photorespiration and plant productivity during ice ages. It supports the theory that photorespiration provided a critical negative feedback mechanism, whereby low atmospheric CO2 bolstered the enzymatic oxygenation reaction, increasing photorespiration and thus limiting photosynthetic carbon assimilation by land plants. This limitation, in turn, would slow the drawdown of atmospheric CO2 during glacial periods, acting as a stabilizing force in Earth’s carbon cycle.
The significance of this study extends beyond paleoclimatology and plant physiology. Understanding the dynamics of photorespiration under low CO2 and cooler climates informs predictions about how modern ecosystems and agricultural systems might respond to future changes in atmospheric composition and temperature. As atmospheric CO2 rises due to anthropogenic emissions, photorespiration is expected to decline, potentially enhancing photosynthetic efficiency and crop yields. However, this historical insight into photorespiration’s regulation could offer critical clues about feedbacks in global carbon cycling that are not currently accounted for in climate models.
Moreover, the innovative use of clumped isotopes as proxies for plant metabolic processes opens exciting avenues for paleobotanical research. By decoding biochemical signals locked within subfossil wood, scientists can reconstruct physiological responses of ancient vegetation to changing environments with unprecedented precision. This methodological advance promises to illuminate the interplay of climate, atmospheric chemistry, and terrestrial ecosystems across vast temporal scales.
One fascinating implication of elevated glacial photorespiration is its role in constraining land plant productivity during periods of global cooling. Photorespiration’s increase likely contributed to reduced net carbon uptake by soils and biomass, influencing not only atmospheric CO2 concentrations but also terrestrial ecosystem dynamics. Lower plant productivity during glacials would have cascading effects on herbivores, nutrient cycling, and ultimately ecosystem resilience—factors that shape the trajectory of Earth’s biosphere evolution.
This study’s geographically broad dataset, encompassing subfossil trees from various North American sites, demonstrates that elevated photorespiration was a widespread phenomenon rather than a localized anomaly. This spatial consistency reinforces the robustness of the observations and highlights the global relevance of the underlying physiological processes. Future investigations could expand this approach to other continental regions to establish the global footprint of photorespiration changes throughout glacial–interglacial cycles.
In sum, this research elegantly illustrates a feedback loop embedded within the Earth system, in which organic chemistry at the scale of leaf cells influences the planet’s atmosphere and climate over millennia. By unveiling how photorespiration intensified during the last glacial period, driven by a delicate balance between cooling and CO2 scarcity, it underscores the complexity of interactions between life and environment. Such insights are crucial as humanity grapples with rapid anthropogenic climate change and seeks to understand the past to better anticipate the future.
As the scientific community continues to refine models of Earth’s carbon cycle, incorporating these refined photorespiration dynamics will improve the fidelity of paleoenvironmental reconstructions and future climate projections. This study, therefore, represents a paradigm shift in our understanding of plant–atmosphere feedbacks and a testament to the power of integrating isotopic geochemistry, plant physiology, and climate science.
With climate challenges mounting globally, the revelations about photorespiration’s role during past ice ages emphasize that terrestrial ecosystems harbor intricate self-regulatory mechanisms. Exploring these biological pathways not only enriches our knowledge of Earth system science but also inspires innovative strategies for sustainable ecosystem management and crop improvement in a changing world.
The future trajectory of this research will likely include refinement of isotopic proxies for other metabolic pathways, multi-proxy integration for atmospheric reconstructions, and application to key intervals beyond the last glacial period. Such efforts will deepen our grasp of the interdependence between climate and life, helping humanity navigate a precarious environmental future with enhanced scientific understanding.
Subject of Research: Elevated photorespiration during the last glacial period and its impact on land plant productivity and atmospheric carbon dioxide regulation.
Article Title: Isotopic evidence for elevated photorespiration during the last glacial period
Article References:
Lloyd, M.K., Sprengel, R.S., Wortham, B.E. et al. Isotopic evidence for elevated photorespiration during the last glacial period. Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01841-x
Image Credits: AI Generated
