Evidence for an inorganic carbon-concentrating mechanism in the symbiotic dinoflagellate Symbiodinium sp.

The presence of a carbon-concentrating mechanism in the symbiotic dinoflagellate Symbiodinium sp. was investigated. Its existence was postulated to explain how these algae fix inorganic carbon (C-i) efficiently despite the presence of a form II Rubisco. When the dinoflagellates were isolated from their host, the giant clam (Tridacna gigas), CO2 uptake was found to support the majority of net photosynthesis (45%-80%) at pH 8.0; however, 2 d after isolation this decreased to 5% to 65%, with HCO3- uptake supporting 35% to 95% of net photosynthesis. Measurements of intracellular C-i concentrations showed that levels inside the cell were between two and seven times what would be expected from passive diffusion of C-i into the cell. Symbiodinium also exhibits a distinct light-activated intracellular carbonic anhydrase activity. This, coupled with elevated intracellular C-i and the ability to utilize both CO2 and HCO3- from the medium, suggests that Symbiodinium sp. does possess a carbon-concentrating mechanism. However, intracellular C-i levels are not as large as might be expected of an alga utilizing a form II Rubisco with a poor affinity for CO2.