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

The uptake kinetics of NO3-, NH4+, and PO3- (4) 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 NH4+ and PO3- (4) in specimens from all localities increased with elevated substrate concentration, which were analyzed assuming Michaelis-Menten kinetics. In all samples from the four localities, NO3- uptake rates increased until about 20 mu M and then gradually decreased as the substrate concentration increased, which followed single-enzyme Michaelis-Menten kinetics with substrate inhibition. The Vmax/Ks ratio (a measure of adaptation to low nutrient concentrations) was different among all samples. Uptake rates of NO(3)(-)and PO3- (4) varied with temperature in samples at three localities, however the 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 NH4+ exceeded NO3- . The relatively higher uptake rates for NH4+ may indicate an adaptive property for efficient absorption of transient NH4+ supply from groundwater discharge in the subtropical Ryukyu Islands under low nitrate conditions.