Fractionation of clumped isotopes of CO2 during photosynthesis

Stable isotope (δ¹³C and δ¹⁸O) and mole fraction measurements of CO₂ are used to constrain the carbon cycle. However, the gross fluxes of the carbon cycle, especially photosynthesis and respiration, remain uncertain due to the challenging task of distinguishing individual flux terms from each other. The clumped isotope composition (Δ₄₇) of CO₂ has been suggested as an additional tracer for gross CO₂ fluxes since it depends mainly on temperature but not on the bulk isotopic composition of leaf, soil and surface water, unlike δ¹⁸O of CO₂.

In this study, we quantify the effect of photosynthetic gas exchange on Δ₄₇ of CO₂ using leaf cuvette experiments with two C₃ and one C₄ plants and discuss challenges and possible applications of clumped isotope measurements. The experimental results are supported by calculations with a leaf cuvette model. Our results demonstrate how the effect of gas exchange on Δ₄₇ is controlled by CO₂-H₂O isotope exchange (using plants with different carbonic anhydrase activity), and kinetic fractionation as CO₂ diffuses into and out of the leaf (using plants with different stomatal and mesophyll conductance). We experimentally confirm the previously suggested dependence of Δ₄₇­­ on the stomatal conductance and back-diffusion flux.