Nitrogen uptake by phytoplankton and zooxanthellae in a coral atoll

Abstract

This paper describes the nitrogen uptake, measured by assimilation of @u15@@N - labelled nitrate, ammonium, urea and @u14@@C - labelled urea, by phytoplankton, intact corals and freshly isolated zooxanthellae from the Kavaratti Atoll of the Lakshadweep Archipelago (Indian Ocean). This paper also presents the ambient concentrations of these nitrogenous nutrients in the lagoon and surrounding waters. In addition, the excretion of these nitrogen compounds by major coral species of the atoll was also measured. Based on these data, and in conjunction with microbiological studies carried out, nitrogen dynamics in the coral atoll ecosystem located in the nitrogen-impoverished oceanic waters is discussed. NH@d4@ is the principal form in which N is excreted by corals, and the rates varied from 223 to 775 ng-at (mg coral tissue) @u-1@@h@u-1@@ in 4 species. Urea excretion forms about 5% of total N excreted. N balance (NH@d4@@, urea, NO@d3@@) calculated from 4 species of corals shows that zooxanthellae can derive all their N requirements from coral excretion alone, and that about 80% of the N taken up by zooxanthellae is translocated to corals in 4 hours