Atkin Owen K.

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Owen K.

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  • Preprint
    Macromolecular rate theory (MMRT) provides a thermodynamics rationale to underpin the convergent temperature response in plant leaf respiration
    ( 2017-10) Liang, Liyin L. ; Arcus, Vickery ; Heskel, Mary ; O'Sullivan, Odhran S. ; Weerasinghe, Lasantha K. ; Creek, Danielle ; Egerton, John J. G. ; Tjoelker, Mark ; Atkin, Owen K. ; Schipper, Louis A.
    Temperature is a crucial factor in determining the rates of ecosystem processes, e.g. leaf respiration (R) − the flux of plant respired CO2 from leaves to the atmosphere. Generally, R increases exponentially with temperature and formulations such as the Arrhenius equation are widely used in earth system models. However, experimental observations have shown a consequential and consistent departure from an exponential increase in R. What are the principles that underlie these observed patterns? Here, we demonstrate that macromolecular rate theory (MMRT), based on transition state theory for enzyme-catalyzed kinetics, provides a thermodynamic explanation for the observed departure and the convergent temperature response of R using a global database. Three meaningful parameters emerge from MMRT analysis: the temperature at which the rate of respiration would theoretically reach a maximum (the optimum temperature, Topt), the temperature at which the respiration rate is most sensitive to changes in temperature (the inflection temperature, Tinf) and the overall curvature of the log(rate) versus temperature plot (the change in heat capacity for the system, ∆Cp‡). On average the highest potential enzyme-catalyzed rates of respiratory enzymes for R is predicted to occur at 67.0±1.2 °C and the maximum temperature sensitivity at 41.4±0.7 °C from MMRT. The average curvature (average negative ∆Cp‡) was -1.2±0.1 kJ.mol-1K-1. Interestingly, Topt, Tinf and ∆Cp‡ appear insignificantly different across biomes and plant functional types (PFTs), suggesting that thermal response of respiratory enzymes in leaves could be conserved. The derived parameters from MMRT can serve as thermal traits for plant leaves that represents the collective temperature response of metabolic respiratory enzymes and could be useful to understand regulations of R under a warmer climate. MMRT extends the classic transition state theory to enzyme-catalyzed reactions and provides an accurate and mechanistic model for the short-term temperature response of R around the globe.
  • Article
    Differential physiological responses to environmental change promote woody shrub expansion
    (John Wiley & Sons, 2013-03-13) Heskel, Mary ; Greaves, Heather ; Kornfeld, Ari ; Gough, Laura ; Atkin, Owen K. ; Turnbull, Matthew H. ; Shaver, Gaius R. ; Griffin, Kevin L.
    Direct and indirect effects of warming are increasingly modifying the carbon-rich vegetation and soils of the Arctic tundra, with important implications for the terrestrial carbon cycle. Understanding the biological and environmental influences on the processes that regulate foliar carbon cycling in tundra species is essential for predicting the future terrestrial carbon balance in this region. To determine the effect of climate change impacts on gas exchange in tundra, we quantified foliar photosynthesis (Anet), respiration in the dark and light (RD and RL, determined using the Kok method), photorespiration (PR), carbon gain efficiency (CGE, the ratio of photosynthetic CO2 uptake to total CO2 exchange of photosynthesis, PR, and respiration), and leaf traits of three dominant species – Betula nana, a woody shrub; Eriophorum vaginatum, a graminoid; and Rubus chamaemorus, a forb – grown under long-term warming and fertilization treatments since 1989 at Toolik Lake, Alaska. Under warming, B. nana exhibited the highest rates of Anet and strongest light inhibition of respiration, increasing CGE nearly 50% compared with leaves grown in ambient conditions, which corresponded to a 52% increase in relative abundance. Gas exchange did not shift under fertilization in B. nana despite increases in leaf N and P and near-complete dominance at the community scale, suggesting a morphological rather than physiological response. Rubus chamaemorus, exhibited minimal shifts in foliar gas exchange, and responded similarly to B. nana under treatment conditions. By contrast, E. vaginatum, did not significantly alter its gas exchange physiology under treatments and exhibited dramatic decreases in relative cover (warming: −19.7%; fertilization: −79.7%; warming with fertilization: −91.1%). Our findings suggest a foliar physiological advantage in the woody shrub B. nana that is further mediated by warming and increased soil nutrient availability, which may facilitate shrub expansion and in turn alter the terrestrial carbon cycle in future tundra environments.