Ammonium, but not nitrate, stimulates an increase in glutamine concentration in the haemolymph of Tridacna gigas

The concentration of glutamine in Tridacna gigas haemolymph increased > 35- fold following exposure to sea water supplemented with ammonium (20 mu M), but no increase was observed with nitrate (20 mu M). Lack of a diel cycle, no decrease in haemolymph glucose levels, the expression patterns of glutam ine synthetase in zooxanthellae and host, and the lack of glutamine release in response to nitrate supplementation all support the proposition that th e increase in haemolymph glutamine is a product of the host and not the zoo xanthellae. Unlike ammonium, nitrate accumulates rapidly in the haemolymph. It has no effect on the concentration of glutamine in the haemolymph, but there is an increase in arginine, histidine and lysine in the haemolymph, s uggesting the release of these essential amino acids from zooxanthellae. Gl utamine synthetase (GS) activity decreased markedly in the gill and less so in the mantle over a period of 6 d exposure to elevated ammonium (20 mu M) . In contrast, CS activity in zooxanthellae doubled. The response of zooxan thellae in situ was confirmed by incubating freshly isolated zooxanthellae for 4 d in ammonium, which resulted in a tenfold increase in GS activity. C omparison of the in situ response of zooxanthellae with that obtained in vi tro indicates that the symbionts are likely to be exposed to ammonium conce ntrations lower than that found in the haemolymph.