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UP: Fachverband Umweltphysik

UP 4: Measurement Techniques & Miscellaneous

UP 4.5: Talk

Friday, September 3, 2021, 12:00–12:15, H7

Using the limits of photosynthesis to understand planetary habitability — •Axel Kleidon — MPI für Biogeochemie, Jena, Germany

Photosynthesis is the dominant process which supports life on Earth with the chemical energy it needs to sustain its metabolic activities. Here, I evaluate the factors that limit photosynthesis, focusing on terrestrial ecosystems, as these include the most productive ecosystems on Earth. I first use satellite-based datasets of gross carbon uptake by terrestrial ecosystems and solar radiation to show that the median efficiency of photosynthesis of terrestrial ecosystems of converting energy is less than 1%, consistent with long-standing ecological observations, but far below the maximum efficiency derived from thermodynamics. I show that this low efficiency can be explained by the thermodynamic limit on gas exchange, as ecosystems need to take up carbon dioxide from the atmosphere to produce chemical energy in form of glucose, and inevitably lose water when doing so. Gas exchange is limited by turbulent transport within the lower atmosphere, which in turn is limited by the heating of the surface. I show that the geographic variations of this gas exchange with respect to water can be explained by the thermodynamic limit of maximum power very well. What this implies is that the photosynthetic activity of the most productive ecosystems on Earth appear to be strongly constrained by thermodynamics through gas exchange. For habitability, this interpretation emphasizes the importance of mass exchange to sustain high levels of chemical activity that are needed to sustain life in planetary environments.