Photosynthetic properties of the common red alga Gracilaria conferta, collected from the eastern Mediterranean Sea were investigated in 1989, in order to begin evaluating its adaptative strategies with regard to the inorganic carbon composition of seawater, and to test whether the alleged C4 photosynthesis of another Gracilaria species is common within the genus. Net photosynthetic rates of G. conferta were, under ambient conditions of inorganic carbon (ca. 10 μM, CO2 and 2.2 m M HCO3-), not sensitive to O2 over the range 10 to 300 μM, and the CO2 compensation point was low (ca. 0.005 μM). Ribulose-1,5-bisphosphate carboxylase/oxygenase was the major carboxylating enzyme, with a crude extract activity of 175 μmol CO2 g-1 fresh wt h-1 while phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase were present at 70 and 20%, respectively, of that activity. No activities of the decarboxylases NAD-and NADP-malic enzyme could be detected. The14C pulse-chase incorporation pattern showed that G. conferta fixes inorganic carbon via the photosynthetic carbon reduction cycle only, with no evidence for photosynthetic C4 acid metabolism. Photosynthesis at the natural seawater pH of 8.2 was, at 25°C and saturating light, saturated at the ambient inorganic carbon concentration of 2.5 m M. It is proposed that, under ambient inorganic carbon conditions, a CO2 concentrating system other than C4 metabolism provides an internal CO2 concentration sufficient to suppress the O2 effect on ribulose-1,5-bisphosphate carboxylase/oxygenase and, thus, on photorespiration, in a medium where the external free CO2 concentration is lower than the Km(CO2) of the carboxylating enzyme. Since inorganic carbon, under natural saturating light conditions, seems not to be a limiting factor for photosynthesis of G. conferta, it likely follows that other nutrients limit the growth of this alga in nature.