Ammonium Metabolism in the Zooxanthellate Coral, Stylophora pistillata

O. Rahav, Z. Dubinsky, Y. Achituv, P. G. Falkowski

Abstract

The cycling of ammonium between the enidarian animal host and intracellular symbiotic algae was investigated in the zooxanthellate coral Stylophora pistillata, obtained from the Gulf of Eilat. Excretion of ammonium into the surrounding water by freshly collected corals was not detectable because of efficient recycling of inorganic nitrogen between the symbionts. Excretion into surrounding water was induced by incubation in the dark, by treatment with the photosynthetic electron transport inhibitor DCMU, and by treatment with azaserine, an inhibitor of glutamine 2-oxoglutarate amido transferase (GOGAT). Methionine sulphoxamine, an inhibitor of glutamine synthetase (GS), killed the animals. The three non-lethal treatments gave similar excretion rates of 0.0112 $\pm$ 0.0011 $\mu$ mol NH$^+_4$ cm$^{-2}$ h$^{-1}$. Whole coral respiration, and animal glutamate dehydrogenase (GDH) activity, which reversibly catalyses the reductive deamination of glutamate, were also measured. The ratios of GDH activity excretion and respiration excretion were remarkably constant, averaging 6.2 $\pm$ 1.2 mol NH$^+_4$ mol$^{-1}$ NH$^+_4$ and 57 $\pm$ 7 mol O$_2$ mol$^{-1}$ NH$^+_4$ respectively. Our results suggest that GDH activity and dark respiration rates may be used to estimate quantitatively the rate of ammonium excretion in S pistillata. From knowledge of the nitrogen content per unit area of zooxanthellae and animal tissue, the mitotic index of the algae, and the calculated excretion rate of the animal, we constructed a nitrogen budget for the coral. Our calculations suggest that recycled nitrogen accounts for 90% of the zooxanthellae nitrogen demand, and that nitrogen turns over at a rate of 0.13 per day in the algae and 0.013 per day within the animal tissue.