Nutrient uptake characteristics of Cladosiphon okamuranus (Phaeophyceae) from the Ryukyu Islands of Japan

Abstract

The uptake kinetics of ${\mathrm{NO}}_{3}^{-}$, ${\mathrm{NH}}_{4}^{+}$, and ${\mathrm{PO}}_{4}^{3-}$ were investigated for the brown alga Cladosiphon okamuranus, which is a major cultivated species in the Ryukyu Islands, Japan. Macroscopic sporophytes from individuals obtained at four different localities were used for the experiment. Uptake rates of ${\mathrm{NH}}_{4}^{+}$ and ${\mathrm{PO}}_{4}^{3-}$ in specimens from all localities increased with elevated substrate concentration, which were analyzed assuming Michaelis–Menten kinetics. In all samples from the four localities, ${\mathrm{NO}}_{3}^{-}$ uptake rates increased until about 20 μM and then gradually decreased as the substrate concentration increased, which followed single-enzyme Michaelis–Menten kinetics with substrate inhibition. The V_max/K_s ratio (a measure of adaptation to low nutrient concentrations) was different among all samples. Uptake rates of ${\mathrm{NO}}_{3}^{-}$ and ${\mathrm{PO}}_{4}^{3-}$ varied with temperature in samples at three localities, however the ${\mathrm{NH}}_{4}^{+}$ uptake rate was invariant with temperature. There were differences in the nutrient uptake kinetics of macroscopic sporophytes of C. okamuranus among nutrients. Uptake rates for ${\mathrm{NH}}_{4}^{+}$ exceeded ${\mathrm{NO}}_{3}^{-}$. The relatively higher uptake rates for ${\mathrm{NH}}_{4}^{+}$ may indicate an adaptive property for efficient absorption of transient ${\mathrm{NH}}_{4}^{+}$ supply from groundwater discharge in the subtropical Ryukyu Islands under low nitrate conditions.