INORGANIC CARBON UPTAKE FOR PHOTOSYNTHESIS BY THE SYMBIOTIC CORAL DINOFLAGELLATE ASSOCIATION .1. PHOTOSYNTHETIC PERFORMANCES OF SYMBIONTS AND DEPENDENCE ON SEA-WATER BICARBONATE/

The aim of this and the accompanying paper is to investigate the mecha nisms of dissolved inorganic carbon (DIC) uptake by the scleractinian coral Galaxea fascicularis, and its delivery to the endosymbiotic phot osynthetic dinoflagellates (zooxanthellae). For this purpose, a compar ison was made between the photosynthetic performance of zooxanthellae in intact symbiosis within microcolonies of Galaxea fascicularis, fres hly 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 v ariations, pharmacology or modifications of ion concentration in seawa ter. In this paper, the photosynthesis/irradiance curves of G. fascicu laris microcolonies, FIZ and CZ are presented. It is shown that zooxan thellae inside their host have lower photosynthetic performance than i solated zooxanthellae. Light saturation (I-k) occurred at higher irrad iance in the intact association than in isolated symbionts. Light util ization efficiency (alpha) was minimum in the intact association and i ncreased in FIZ and CZ. G. fascicularis microcolonies, FIZ and CZ were tested for their ability to utilize HCO3- as a source of DIC for phot osynthesis. Two main approaches were used, the first consisting of cha nging the bicarbonate concentration by adding HCO3- to bicarbonate-fre e artificial seawater at constant pH, and the second of modifying the pH of the seawater in a closed or open system. At saturating light int ensity, the DIC concentration saturating for photosynthesis is no more than that of normal sea water. At pH 8.2, a half-maximal rate of phot osynthetic O-2 evolution is achieved at 408, 71 and 178 mu M HCO3- for coral, FIZ and CZ respectively. The photosynthetic O-2 production wit h 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 substrate s for transport. The rate of non-enzymatic dehydration of HCO3- exceed s the rate of photosynthesis in coral and FIZ. but not in CZ. The resu lts presented in this paper suggest that C. fascicularis microcolonies are able to take up bicarbonate to supply symbiont photosynthesis, al though zooxanthellae in hospite seems DIC-limited. FIZ seem to absorb CO2 and HCO3- indiscriminately while CZ use HCO3-.