Exogenous abscisic (ABA) acid differentially altered HCO3− uptake by C. reinhardtii in a light-intensity-dependent manner. Mid log phase (A750 = 0.3) algal cultures were immobilised in alginate gel beads containing the concentrations of ABA indicated and in (A) were incubated for 1 h under different light intensities (7.6 to 223.2 µmoles photons m−2 s−1) with bicarbonate indicator buffer in TAP media. Depletion of HCO3− in the media was monitored as the change in absorbance in the indicator buffer at 550 nm after the incubation period with conversion to the change in the concentration of HCO3− in the TAP media by reference to a standard calibration. Shown are the mean δHCO3− (mM) h−1 of n = 5 replicates with errors as the 95% confidence intervals around the means in each case. In (B) mid log phase (A750 = 0.3) algal cultures were sampled 1 h into the photoperiod and subsequently exposed for 1 h to the different light intensities indicated. Algal cells were pelleted from n = 3 replicate 25 mL aliquots of culture. Pellets were extracted and assessed for ABA content by competitive ELISA (MyBioSource Inc.). Data are shown as the mean ABA content cell−1 with error bars shown as +/− the 95% confidence interval around the mean in each case.