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

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.