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

Abstract

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 PM) 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.

Meeting the photosynthetic demand for inorganic carbon in an alga–invertebrate association: preferential use of CO 2 by symbionts in the giant clam Tridacna gigas

“…While zooxanthellae in hospite are not bathed directly in haemolymph, previous results indicate that the haemolymph composition re£ects the environment surrounding these dino£agellates (Rees et al 1993a;Fitt et al 1995). The haemolymph C i concentration is therefore a measure of the size of the C i pool available to the zooxanthellae.…”

“…We investigated the e¡ect of pH on the photosynthetic rate of zooxanthellae because of the diel £uctuation of haemolymph pH, between pH 7.2 and 8.1 (Fitt et al 1995), and the pH-dependent variation in C i species.…”

“…The suspension medium was bubbled with N 2 gas in the electrode prior to addition of zooxanthellae (¢nal volumeˆ1.4 ml). A ¢nal zooxanthellae density of 5 £10 6 cells ml ¡1 was used to simulate the relative ratio of cell density to haemolymph volume in the clam (Fitt et al 1995). Following addition of zooxanthellae the cells were allowed to photosynthesize at a photon £ux density of 4450 mE m ¡2 s ¡1 provided by a Schott KL1500 (Massachusetts, USA) optic ¢bre light source until the CO 2 -compensation point was reached.…”

“…However, that study did not take into account the pH of the haemolymph and therefore the relative concentrations of CO 2 and HCO ¡ 3 . We have since demonstrated that the pH of the haemolymph £uctuates and is inversely proportional to the photon £ux density (Fitt et al 1995). The pH increases during the day (maximum pH 8.1) and decreases at night (minimum pH 7.2).…”

“…The pH increases during the day (maximum pH 8.1) and decreases at night (minimum pH 7.2). This is presumably caused by the depletion of C i in the haemolymph due to zooxanthellae photosynthesis (Fitt et al 1995). This change will a¡ect the equilibrium between CO 2 and HCO ¡ 3 in the haemolymph and Z-tubes, and hence the supply of C i to the zooxanthellae.…”