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 (Cᵢ) in order to support their symbiotic population of photosynthetic dinoflagellates. Their capacity to meet this demand will be reflected in the Cᵢ concentration of their haemolymph during periods of high photosynthesis. The Cᵢ concentration in haemolymph was found to be inversely proportional to irradiance with a minimum Cᵢ concentration of 0.75 mM at peak light levels increasing to 1.2 mM in the dark. The photosynthetic rate of isolated zooxanthellae under conditions that prevail in the haemolymph at peak light levels was significantly less than the potential Pmax (maximum photosynthetic rate) indicating that zooxanthellae are carbon limited in hospite. This is consistent with previous studies on the hermatypic coral symbiosis. The Pmax was not affected by pH but there was a dramatic increase in the half-saturation constant for Cᵢ (K₀.₅ (Cᵢ)) with increasing pH (6.5-9.0) and only a small decrease in K₀.₅ (CO₂) over the same range. These results indicate that zooxanthellae in giant clams use CO₂ as the primary source of their Cᵢ in contrast to symbionts in corals, which use bicarbonate. The physiological implications are discussed and comparison is made with the coral symbiosis