Inorganic carbon uptake for photosynthesis by the symbiotic coral/dinoflagellate association: I. Photosynthetic performances of symbionts and dependence on sea water bicarbonate

The aim of this and the accompanying paper is to investigate the mechanisms of dissolved inorganic carbon (DIC) uptake by the scleractinian coral Galaxea fascicularis, and its delivery to the endosymbiotic photosynthetic dinoflagellates (zooxanthellae). For this purpose, a comparison was made between the photosynthetic performance of zooxanthellae in intact symbiosis within microcolonies of Galaxea fascicularis, freshly isolated zooxanthellae (FIZ) and cultured zooxanthellae (CZ) under different conditions. Discrimination between CO2 or HCO3/- uptake was achieved by several means including changes in DIC concentration, pH variations, pharmacology or modifications of ion concentration in seawater. In this paper, the photosynthesis/irradiance curves of G. fascicularis microcolonies, FIZ and CZ are presented. It is shown that zooxanthellae inside their host have lower photosynthetic performance than isolated zooxanthellae. Light saturation (I(k)) occurred at higher irradiance in the intact association than in isolated symbionts. Light utilization efficiency (α) was minimum in the intact association and increased in FIZ and CZ. G. fascicularis microcolonies, FIZ and CZ were tested for their ability to utilize HCO3/- as a source of DIC for photosynthesis. Two main approaches were used, the first consisting of changing the bicarbonate concentration by adding HCO3/- to bicarbonate-free artificial seawater at constant pH, and the second of modifying the pH of the seawater in a closed or open system. At saturating light intensity, the DIC concentration saturating for photosynthesis is no more than that of normal sea water. At pH 8.2, a half- maximal rate of photosynthetic O2 evolution is achieved at 408, 71 and 178 μM HCO3/- for coral, FIZ and CZ respectively. The photosynthetic O2 production with constant inorganic carbon but varying pH reached an optimum at pH 8 to 9 suggesting that HCO3/- is the main species taken up initially. FIZ and CZ possess the ability to utilize both CO2 and HCO3/- as substrates for transport. The rate of non-enzymatic dehydration of HCO3/- exceeds the rate of photosynthesis in coral and FIZ, but not in CZ. The results presented in this paper suggest that G. fascicularis microcolonies are able to take up bicarbonate to supply symbiont photosynthesis, although zooxanthellae in hospite seems DIC- limited. FIZ seem to absorb CO2 and HCO3/- indiscriminately while CZ use HCO3-.