Photosynthesis and calcification in the calcifying algae Halimeda discoidea studied with microsensors

With microsensors, we measured the steady-state microprofiles of O-2, pH an d Ca2+ on the topside of young segments of Halimeda discoidea, as well as t he surface dynamics upon light-dark shifts. The effect of several inhibitor s was studied. The steady-state measurements showed that under high light i ntensity, calcium and protons were taken up, while O-2 was produced. In the dark, O-2 was consumed, the pH decreased to below seawater level and Ca2uptake was reduced to 50%. At low light intensity (12 mu mol photons m(-2) s(-t)), Ca2+ efflux was observed. Upon light-dark shifts, a complicated pat tern of both the pH and calcium surface dynamics was observed. Illumination caused an initial pH decrease, followed by a gradual pH increase: this ind icated that the surface pH of H. discoidea is determined by more than one l ight-induced process. When photosynthesis was inhibited by dichlorophenyl d imethyl urea (DCMU), a strong acidification was observed upon illumination. The nature and physiological function of this putative pump is not known. The calcium dynamics followed all pH dynamics closely, both in the presence and absence of DCMU. The Ca-channel blockers verapamil and nifedipine had no effect on the Ca2+ dynamics and steady-state profiles. Thus, in H. disco idea, calcification is not regulated by the alga, but is a consequence of p H increase during photosynthesis. Acetazolamide had no effect on photosynth esis, whereas ethoxyzolamide inhibited photosynthesis at higher light inten sities. Therefore, all carbonic anhydrase activity is intracellular. Carbon ic anhydrase is required to alleviate the CO2 limitation. Calcification can not supply sufficient protons and CO2 to sustain photosynthesis.