Uptake and assimilation of ¹⁵NH ⁺₄ by a variety of corals

Abstract

Corals from the Swain Reefs of the Great Barrier Reef were exposed to ¹⁵NH ⁺₄ in sea water, and the disappearance of NH ⁺₄ from the medium and the appearance of ¹⁵N in the corals was followed. Disappearance of NH ⁺₄ usually followed a reasonably smooth curve with rate of disappearance decreasing as the concentration of NH ⁺₄ decreased; the decrease in rate was not the result of damage to the corals. The rate of uptake of ¹⁵NH ⁺₄ vs concentration did not describe a normal substrate saturation curve, as uptake may have been diffusion limited. ¹⁵N was assimilated into the corals in the form of stable organic nitrogenous compounds. Its assimilation usually exhibited an initial lag. An ahermatypic coral did not remove NH ⁺₄ from the medium. Differences in NH ⁺₄ uptake with a 1-h cycle of light and dark were minor. Isolated zooxanthellae assimilated ¹⁵NH ⁺₄ as indicated by accumulation of ¹⁵N with no lag.