Acidification and CO<sub>2</sub> production in the boundary layer during photosynthesis in <i>Ulva rigida</i> (Chlorophyta) C. Agardh

Abstract

Mechanisms of inorganic carbon utilization (i.e., carbon-concentrating mechanisms) in seaweeds involve uptake of HCO 3 - and carbonic anhydrase-mediated production of CO 2 . These mechanisms are needed to cope with the low concentrations of CO 2 dissolved in seawater and the low affinities of Rubisco for CO 2 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 CO 2 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, O 2 production rates exceeded by 10-fold that of CO 2 production under illumination. Photosynthetic release of O 2 was largely dependent on the experimental range of light intensities, while CO 2 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.