Relationship between pH and the availability of dissolved inorganic nitrogen in the zooxanthella-giant clam symbiosis

Giant clams of the family Tridacnidae live in symbiosis with intercellular dinoflagellates of the genus Symbiodinium, often referred to by the trivial name “zooxanthellae.” Intact clams take up both ammonium and nitrate when they are offered separately. When zooxanthellae were removed from the mantle and incubated in hemolymph, they took up ammonium, and they took up nitrate when ammonium concentrations were low enough (< 1 µM) not to inhibit uptake. The pH of hemolymph collected from the venous sinuses (immediately below the mantle tissue where most of the zooxanthellae reside) varied on a diel cycle, with highest values (∼ 8.1) measured during the middle of the day and lowest (∼ 7.2) detected at night. Zooxanthellae are probably responsible for the alkalization of the hemolymph since the increase in pH is coincident with maximum light levels and photosynthesis. Natural concentrations of ammonium in the hemolymph of freshly collected clams were inversely correlated with pH, with highest values detected at night when pH was lowest. Rates of diffusion of ammonia from seawater (pH 8.1) into animal compartments of lower pH were proportional to the pH differential. Diffusion of ammonia over a pH gradient, with diel variation in pH, may act to regulate availability of nitrogen to the algal symbionts in all zooxanthellae symbioses, including reef corals, thereby controlling the timing and rate of algal division.