  Photosynthesis converts sunlight into the chemical free energy that feeds the
Earth's biosphere, yet at levels much lower than what thermodynamics would
allow for. I propose here that photosynthesis is nevertheless thermodynamically
limited, but this limit acts indirectly on the material exchange of water and
carbon dioxide. I substantiate this interpretation using global
observation-based datasets of radiation, photosynthesis, precipitation and
evaporation. I first calculate the conversion efficiency of photosynthesis in
terrestrial ecosystems and its climatological variation, with a median
efficiency of 0.78% (n = 13445). The rates tightly correlate with evaporation
(r2 = 0.89), which demonstrates the importance of the coupling of
photosynthesis to material exchange. I then infer evaporation from the maximum
material exchange between the surface and the atmosphere that is
thermodynamically possible using datasets of solar radiation and precipitation.
This inferred rate closely correlates with the observation-based evaporation
dataset (r2 = 0.85). When this rate is converted back into photosynthetic
activity, the resulting patterns correlate highly with the observation-based
dataset (r2 = 0.56). This supports the interpretation that it is not energy
directly that limits terrestrial photosynthesis, but rather the material
exchange that is driven by sunlight. This interpretation can explain the very
low, observed conversion efficiency of photosynthesis in terrestrial ecosystems
as well as its spatial variations. More generally, this implies that one needs
to take the necessary material flows and exchanges associated with life into
account to understand the thermodynamics of life. This, ultimately, requires a
perspective that links the activity of the biosphere to the thermodynamic
constraints of transport processes in the Earth system.
