Mechanisms of inorganic carbon utilization (i.e., carbon-concentrating mechanisms) in seaweeds involve uptake of HCO3- and carbonic anhydrase-mediated production of CO2. These mechanisms are needed to cope with the low concentrations of CO2 dissolved in seawater and the low affinities of Rubisco for CO2 in the macroalgal chloroplasts. The current study shows, for the first time, direct evidence that while Ulva rigida photosynthesizes there is continuous production of CO 2, apparently generated from an acidification process in its boundary layer. Release of CO2 during light periods was measured using a membrane inlet mass spectrometer, while acidification at the boundary layer was detected in pH-drift experiments. Under repetitive light (4, 70, 300 μmol photons m-2 s-1) and dark cycles, O2 production rates exceeded by 10-fold that of CO2 production under illumination. Photosynthetic release of O2 was largely dependent on the experimental range of light intensities, while CO2 evolution under illumination was significantly reduced at 24°C as compare to 15°C. Rapid acidification occuring within the unstirred layer and the consequent CO 2 release may account for an additional mechanism of inorganic carbon utilization strategies in Ulva rigida, which previously had been reported to posses an efficient carbonic anhydrase modulated carbon concentrating mechanism.