Control of phosphate uptake by zooxanthellae and host cells in the scleractinian coral Stylophora pistillata

Depletion experiments were used to assess the uptake rates of phosphate by the tropical coral Stylophora pistillata and its symbiotic zooxanthellae under different conditions. Our results showed the presence of active phosphate carriers both in the animal and the algal fractions, since uptake kinetics followed uniphasic saturation curves in both compartments (Michaelis‐Menten shape). Transporters in the animal and the algae presented different affinities for phosphate, with higher half‐saturation constants for the animal compartment than for the isolated zooxanthellae (K = 1.08 ± 0.42 vs. 0.57 ± 0.18 µmol L−1). The velocity of phosphate absorption increased in the light, suggesting a relationship with the photosynthetic activity of the zooxanthellae. A correlation was found between phosphate uptake rates and the organic or inorganic feeding history of the corals; rates were indeed 4.6 times higher in 8‐week starved than in fed corals and depended on the repletion status of phosphorus stocks within the symbionts. We report evidence showing that zooxanthellae act as a sink of phosphate within the symbiosis.