Acidification and CO₂ production in the boundary layer during photosynthesis in Ulva rigida (Chlorophyta) C. Agardh

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.