This paper deals with gas-liquid mass transfer in an airlift via CFD simulations in the context of photobioreactor (PBR) scale-up. Two aspects are emphasized. Firstly, since carbon uptake by microalgae is of crucial importance as part of PBRs, CO2 transfer is in focus, and numerical simulations are developed to take into account CO2 gas-liquid transfer and dissociation in the aqueous phase. Secondly, since estimating kLa is of crucial importance when scaling up PBRs, different ways to evaluate kLa are discussed using numerical experiments. Firstly, kLa may be estimated as the volume average value of the local mass transfer coefficients calculated from steady-state hydrodynamics and Higbie penetration model. Secondly, kLa can be deduced from classical dynamic gassing-out/gassing-in experiments. This second method is simulated for O-2, as commonly performed experimentally, and also with CO2 since it is the transferred species in PBRs. Results show that kLa field is strongly heterogeneous, as expected in airlifts where gas is mainly present in the riser. Performed with O-2, the gassing-in method leads to quite accurate estimation of the spatial average value of local kLa. But, gassing-in methods performed with O-2 and CO2 lead to discordant results. In fact, CFD shows that the CO2 depletion in the gas phase has to be accounted for to predict kLa from CO2 gassing-in method, especially at large scale. This study also puts into evidence the potentialities of CFD which allows to get detailed image of local gas-liquid mass transfer, depending on two-phase hydrodynamics, gas phase distribution and transferred species solubility. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.