Quantifying terrestrial carbon storage and predicting the sensitivity of ecosystems to climate change relies on our ability to obtain observational constraints on photosynthesis and respiration at large scales (ecosystem, regional and global). Photosynthesis (GPP), the largest CO2 flux from the land surface, is currently estimated with considerable uncertainty (1-3). Robust estimates of global GPP can be obtained from an atmospheric budget of the oxygen isotopic composition (δ18O) of atmospheric CO2, provided that we have a good knowledge of the δ18O signatures of the terrestrial CO2 fluxes (1, 4). The latter reflect the δ18O of leaf and soil water pools because CO2 exchanges “isotopically” with water [CO2+H2 18OÄH2O+CO18O]. This exchange can be accelerated by the enzyme carbonic anhydrase (CA). In leaves, where CA is present and abundant, this isotopic equilibrium is reached almost instantaneously. As a consequence, and because soil and leaf water pools havedifferent δ18O signatures, CO2 fluxes from leaves and soils carry very distinct δ18O signals and can thus be tracked from the fluctuations in the δ18O of atmospheric CO2 (δa). The accelerated isotopic exchange between CO2 and water due toCA activity has recently been shown to be a widespread phenomenon in soils as well (4). Across a range of ecosystems, we found that CO2 hydration was 10 to 1000 times faster than the un-catalysed rate, with highest values in the hottest ecosystems. At the global scale, accounting for soil CA activity dramatically shifts the influence of soil and leaf fluxes on δa, thus changing the estimates of terrestrial gross CO2 fluxes. In this talk we will present the current state of understanding of the environmental and ecological causes behind the variability in CA activity observed in soils and illustrate how this variability can influence our estimates of global GPP inferred from δa budgets.