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- 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 CO2, apparently generated from an acidification process in its boundary layer. Release of CO2 during light periods was measured using it membrane inlet mass spectrometer, while acidification at the boundary layer was detected in pH-drift experiments. Under repetitive light (4, 70, 300 mu mol photons m(-2) s(-1))and dark cycles, O-2 production rates exceeded by 10-fold that of CO2 production under illumination. Photosynthetic release of O-2 was largely dependent oil the experimental range of light intensities, while CO2 evolution under illumination was significantly reduced at 24 degrees C as compare to 15 degrees C. Rapid acidification occuring within the unstirred layer and the consequent CO2 release may account for all additional mechanism of inorganic carbon utilization strategies in Ulva rigida, which previously had been reported to posses in efficient carbonic anhydrase modulated carbon concentrating mechanism.