Sources and mechanisms of inorganic carbon transport for coral calcification and photosynthesis

The sources and mechanisms of inorganic carbon transport for scleractinian coral calcificaticion and photosynthesis were studied using a double labell ing technique with (HCO3)-C-14 and Ca-45. Clones of Stylophora pistillata t hat had developed into microcolonies were examined. Compartmental and pharm acological analyses of the distribution of Ca-45 and (HCO3)-C-14 in the coe lenteron, tissues and skeleton were performed in dark or Eight conditions o r in the presence of various seawater HCO3-concentrations. For calcification, irrespective of the lighting conditions, the major sourc e of dissolved inorganic carbon (DIC) is metabolic CO2 (70-75 % of total Ca CO3 deposition), while only 25-30% originates from the external medium (sea water carbon pool). These results are in agreement with the observation tha t metabolic CO2 production in the light is at least six times greater than is required for calcification, This source is dependent on carbonic anhydra se activity because it is sensitive to ethoxyzolamide. Seawater DIC is tran sferred from the external medium to the coral skeleton by two different pat hways: from sea water to the coelenteron, the passive paracellular pathway is largely sufficient, while a DIDS-sensitive transcellular pathway appears to mediate the flux across calicoblastic cells, Irrespective of the source , an anion exchanger performs the secretion of DIC at the site of calcifica tion. Furthermore, a fourfold light-enhanced calcification of Stylophora pi stillata microcolonies was measured. This stimulation was only effective af ter a lag of 10 min. These results are discussed in the context of light-en hanced calcification. Characterisation of the DIC supply for symbiotic dinoflagellate photosynthe sis demonstrated the presence of a DIC pool within the tissues. The size of this pool was dependent on the lighting conditions, since it increased 39- fold after 3h of illumination. Passive DIC equilibration through oral tissu es between sea water and the coelenteric cavity is insufficient to supply t his DIC pool, suggesting that there is an active transepithelial absorption of inorganic carbon sensitive to DIDS, ethoxyzolamide and iodide, These re sults confirm the presence of CO2-concentrating mechanisms in coral cells. The tissue pool is not, however, used as a source for calcification since n o significant lag phase Tn the incorporation of external seawater DIC was m easured.