Meeting the photosynthetic demand for inorganic carbon in an alga-invertebrate association: preferential use of CO2 by symbionts in the giant clam Tridacna gigas

Unlike most marine invertebrates which excrete respiratory CO2, giant clams (Tridacna gigas) must acquire inorganic carbon (C-i) in order to support t heir symbiotic population of photosynthetic dinoflagellates. Their capacity to meet this demand will be reflected in the C-i concentration of their ha emolymph during periods of high photosynthesis. The C-i concentration in ha emolymph was found to be inversely proportional to irradiance with a minimu m C-i concentration of 0.75 mM at peak light levels increasing to 1.2 mM in the dark. The photosynthetic rate of isolated zooxanthellae under conditio ns that prevail in the haemolymph at peak light levels was significantly le ss than the potential P-max (maximum photosynthetic rate) indicating that z ooxanthellae are carbon limited in hospite. This is consistent with previou s studies on the hermatypic coral symbiosis. The P-max was not affected by pH but there was a dramatic increase in the half-saturation constant for C- i (K-0.5(C-i)) with increasing pH (6.5-9.0) and only a small decrease in K- 0.5(CO2) over the same range. These results indicate that zooxanthellae in giant clams use CO2 as the primary source of their C-i in contrast to symbi onts in corals, which use bicarbonate. The physiological implications are d iscussed and comparison is made with the coral symbiosis.