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.