Uptake and assimilation of dissolved inorganic nitrogen by a symbiotic sea anemone

Abstract

The tropical sea anemone,Aiptasia pulchella, harbours symbiotic dinoflagellates (zooxanthellae). Animals in their natural habitat in Hawaii and those maintained in the laboratory in Los Angeles took up ammonium from nutrient enriched seawater. That the uptake experiment was done in the dark did not influence uptake although prolonged (19 h) dark treatment caused the animals to release ammonium. Aposymbiotic anemones (lacking zooxanthellae) typically excreted ammonium in the light. Freshly isolated zooxanthellae were also able to deplete enriched seawater of ammonium, indicating that the capacity for uptake by the symbiosis is due to the zooxanthellae. Uptake rates of freshly isolated zooxanthellae exceeded those of zooxanthellae in symbiosis. There was evidence that uptake of ammonium follows diffusion kinetics rather than the Michaelis-Menten model, since there was no saturation component. Neither symbioticA.pulchellanor their isolated algae were able to remove nitrate from enriched seawater, even when pretreated for 24 h or one month with 5 or 10 µm nitrate. Symbiotic anemones that were not fed zooplankton for a month, to reduce the internal pools of excretory ammonium, showed a slight ability to deplete the medium of nitrate. Enzymes involved in assimilation of dissolved inorganic nitrogen were assayed in cell-free zooxanthellae extracts. Neither nitrate reductase nor glutamate synthase activity was found. There was glutamate dehydrogenase activity, with aKmfor ammonium of 4—5 mm;Kmfor α-ketoglutarate of 5-7 mm. The zooxanthellae also showed glutamine synthetase activity with aKmfor glutamate of 12.5 µM but exhibiting negative cooperativity with ammonium.