Calcified macroalgae are essential components of marine ecosystem, yet much of their physiology remains to be understood. Here, the nutrient (NO₃^– and PO₄^–3) uptake physiologies of two branched macroalgae, Actinotrichia fragilis (Nemaliophycidae) and Amphiroa fragilissima (Corallinophycidae), and the non-geniculate rhodolith Sporolithon sp. (Corallinophycidae) were examined. Sporolithon sp. had the lowest uptake rate through time and the three calcified macroalgae had a surge in NO₃^– and PO₄^–3 uptake that occurred between 3 and 20 min, with a maximum uptake at 3 min, after which the nutrient uptake rates declined. The NO₃^– uptake of the three calcified macroalgae followed Michaelis-Menten kinetics. For NO₃^– uptake, Sporolithon sp. had the lowest K_m (2.72 ± 0.97 µM), V_max (0.08 ± 0.01 µmol gDW^–1 h^–1), V_max/K_m (0.05 ± 0.03 µmol gDW^–1 h^–1 µM⁻¹) and α (0.01 ± 0.00 µmol gDW^–1 h^–1 µM⁻¹), while A. fragilis had the highest K_m (12.35 ± 0.71 µM) and V_max (6.41 ± 0.23 µmol gDW^–1 h^–1), and A. fragilissima had the highest V_max/K_m (1.52 ± 0.26 µmol gDW^–1 h–1 µM⁻¹) and α (0.37 ± 0.01 µmol gDW^–1 h^–1 µM⁻¹). Moreover, the PO₄^–3 uptake rate of the three species was faster at higher PO₄^–3 levels. These differences in species-specific nutrient uptake traits are likely caused by differences in morphology. These traits are important for survival and proliferation of this group of marine organisms, particularly in a nutrient-variable environment.

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Nitrate and phosphate uptake of morphologically distinct calcified macroalgae	7

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