Ammonium, but not nitrate, stimulates an increase in glutamine concentration in the haemolymph of Tridacna gigas |
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Authors: | D Shepherd W Leggat T A V Rees D Yellowlees |
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Institution: | (1) Department of Biochemistry and Molecular Biology, James Cook University, Townsville, Queensland 4811, Australia Fax: +61 7 4725 1394 e-mail: david.yellowlees@jcu.edu.au, AU;(2) Leigh Marine Laboratory, University of Auckland, P.O. Box 349, Warkworth, New Zealand, NZ |
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Abstract: | The concentration of glutamine in Tridacna gigas haemolymph increased >35-fold following exposure to sea water supplemented with ammonium (20 μM), but no increase was observed with nitrate (20 μM). Lack of a diel cycle, no decrease in haemolymph glucose levels, the expression patterns of glutamine synthetase in zooxanthellae
and host, and the lack of glutamine release in response to nitrate supplementation all support the proposition that the increase
in haemolymph glutamine is a product of the host and not the zooxanthellae. 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, suggesting the release of these essential amino acids from zooxanthellae. Glutamine
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 μM). In contrast, GS activity in zooxan- thellae doubled. The response of zooxanthellae in situ was confirmed by incubating
freshly isolated zooxanthellae for 4 d in ammonium, which resulted in a ten-fold increase in GS activity. Comparison of the
in situ response of zooxanthellae with that obtained in vitro indicates that the symbionts are likely to be exposed to ammonium
concentrations lower than that found in the haemolymph.
Received: 14 November 1997 / Accepted: 28 April 1998 |
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