Plant Physiol, December 1999, Vol. 121, pp. 1247-1255

Evidence for an Inorganic Carbon-Concentrating Mechanism in the Symbiotic Dinoflagellate Symbiodinium sp.¹

Biochemistry and Molecular Biology, James Cook University, Townsville, Queensland 4811, Australia (W.L., D.Y.); and Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University P.O. Box 475, Canberra, Australian Capital Territory 0200, Australia (M.R.B.)

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.