Abstract
The substrate analogue [14C]-methylammonium was used to study ammonium/methylammonium uptake by Symbiodinium microadriaticum (zooxanthellae). The value of the Michaelis constant (K m) for the uptake system was approximately 35 μM with methylammonium as substrate; ammonium was a competitive inhibitor of methylammonium uptake, and the K m for ammonium uptake (determined as the inhibition constant, K i, for methylammonium) was 6.6 μM. Methylammonium uptake by zooxanthellae was light-dependent. Methylammonium uptake rates of zooxanthellae which had been freshly isolated from the hermatypic coral Acropora formosa (0.85±0.05x10-10 μmol min-1 cell-1) were lower than those of axenic cultures of the zooxanthellae from Montipora verrucosa (Acroporidae) grown under various nitrogen regimes (1.6 to 12x10-10 μmol min-1 cell-1). Maximum uptake rates were found for ammonium-starved cultured M. verrucosa zooxanthellae (10.2 to 12x10-10 μmol min-1 cell-1); M. verrucosa zooxanthellae growing with ammonium as nitrogen source and zooxanthellae which had been freshly isolated from A. formosa gave similar and considerably lower uptake rates (0.85 to 1.6x10-1 μmol min-1 cell-1). These results suggest that either coral tissue contains sufficient ammonium to repress synthesis of the uptake system of the algal symbionts or, alternatively, there are additional barriers to ammonium transport for zooxanthellae in vivo.
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Communicated by G. F. Humphrey, Sydney
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Gunnersen, J., Yellowlees, D. & Miller, D.J. The ammonium/methylammonium uptake system of Symbiodinium microadriaticum . Marine Biology 97, 593–596 (1988). https://doi.org/10.1007/BF00391057
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DOI: https://doi.org/10.1007/BF00391057