Photosynthesis in balance with respiration

As an ordinary biologist, I assumed that living organisms' impacts on atmospheric CO2 and O2 levels were more or less “in balance,” with plant photosynthesis being equalled by the summed respiration of plants, animals, and soil and aquatic microbes. Thus, I find puzzling the attempt by A. W. King et al. (“Plant respiration in a warmer world,” Perspectives, 28 Apr., p. [536][1]) to use an adaptation of plant respiration to higher temperatures as compensation for increased CO2 production owing to temperature-stimulated increases in photosynthesis. Surely, temperature also affects rates of respiration in almost all organisms that utilize photosynthates for their energy source? Thus, only the small handful of animals capable of thermal control of body temperature could effectively offset rises in body temperature to lower respiration rates—and even those capacities can add to respiration-derived energy demands. Why is plant adaptation by lowering temperature-induced increases in respiration a necessary hypothesis to offset higher photosynthetic rates? Surely, if we are to estimate the production of CO2 as a function of ambient temperatures, we must also consider the impacts of such temperatures on photosynthesis, as well as on the rates of respiration not only of plants, but also of all other lifeforms—from microbes to humans. How well do they adapt their metabolic needs to persistent temperature increases? On balance, over eons of time, the photosynthate has more or less been “in balance” (once the great quantities of reduced carbon were sequestered in fossil fuels, creating an oxygen-rich atmosphere)—through periods of warming and cooling—to provide relatively stable CO2 to O2 ratios in the atmosphere. Shouldn't the temperature-dependent responses of all these metabolic regimes be part of any meaningful analysis? If all reactions are more or less equally affected by temperature, how can there be a net “problem” from increased plant respiration? # Response {#article-title-2} Clark's assumption that global photosynthesis is more or less “in balance” with total plant and animal respiration holds as an approximation only when those processes are not being forced from their quasi-equilibrium by disturbance. The ongoing anthropogenic perturbation of the atmosphere by fossil-fuel burning is a major disturbance of Earth's carbon cycle ([1][2]). Rising atmospheric CO2 increases photosynthesis. The concurrent increases in temperature alter photosynthesis and respiration, but with different sensitivities. These perturbations, combined with deforestation accompanying large-scale agriculture, are large enough that the world's terrestrial ecosystems are not in equilibrium with respect to CO2 and O2 fluxes. We did not investigate “adaptation” of plant respiration, as suggested by Clark, but rather acclimation to higher temperatures. Acclimation commonly refers to physiological and metabolic adjustments to environmental change, distinguishing these responses from genetic adaptation. Nor did we examine “photosynthesis,” but rather temperature-stimulated respiration. Clark asks, “Surely, temperature also affects rates of respiration in almost all organisms…?” Yes, it does, and all rates of metabolic respiration in our model are functions of temperature ([2][3], [3][4]). Furthermore, organisms can indeed lower respiration rates in the facing of rising temperatures. The concept of thermal acclimation applies to respiratory rates (and rates of other enzymatic-based processes) in any poikilothermic organism ([4][5], [5][6]), which includes plants. Acclimation of plant respiration to warmer temperatures is not included in global models of carbon cycle response and feedback to climate change. It is important to understand how including or not including it influences the simulation and interpretation of positive feedback between Earth's carbon cycle and future climate change. The argument can be made that if one's purpose is to estimate the production of CO2 as a function of changes in temperature, then one must consider the impacts of temperature (and temperature acclimation) not only on plant respiration, but also photosynthesis and respiration of all lifeforms. The respiration of all lifeforms in the simulations was modeled as a function of temperature, but we purposefully designed the simulation experiments to isolate the contribution of acclimation of plant (leaf) respiration to temperature. Nonetheless, the differential effects of increases in atmospheric CO2 and changes in climate on cellular reactions could result in plants making an additional net contribution to the imbalance in atmospheric CO2. 1. 1.[↵][7] 1. J. T. Houghton , Ed. Climate Change 2001: The Scientific Basis (Cambridge Univ. Press, Cambridge, 2001) et al., Eds. 2. 2.[↵][8] 1. A. W. King, 2. W. M. Post, 3. S. D. Wullschleger , Clim. Change 33, 199 (1997). [OpenUrl][9] 3. 3.[↵][10] 1. J. S. Amthor 2. et al. , J. Geophys. Res. 106, 33623 (2001). [OpenUrl][11] 4. 4.[↵][12] 1. H. Precht, 2. J. Christophersen, 3. H. Hensel, 4. W. Larcher 1. A. Pisek , in Temperature and Life, H. Precht, J. Christophersen, H. Hensel, W. Larcher, Eds. (Springer-Verlag, New York, 1973), pp. 102-194. 5. 5.[↵][13] 1. K. Y. H. Lagerspetz , J. Thermal Biol. 31, 332 (2006). 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