Uptake kinetics and storage capacity of dissolved inorganic phosphorus and corresponding N:P dynamics in <i>Ulva lactuca</i> (Chlorophyta)

Dissolved inorganic phosphorus (DIP) is an essential macronutrient for maintaining metabolism and growth in autotrophs. Little is known about DIP uptake kinetics and internal P‐storage capacity in seaweeds, such as <i>Ulva lactuca</i> (Chlorophyta). <i>Ulva lactuca</i> is a promising candidate for biofiltration purposes and mass commercial cultivation. We exposed <i>U. lactuca</i> to a wide range of DIP concentrations (1–50 μmol · L<super>−1</super>) and a nonlimiting concentration of dissolved inorganic nitrogen (DIN; 5,000 μmol · L<super>−1</super>) under fully controlled laboratory conditions in a “pulse‐and‐chase” assay over 10 d. Uptake kinetics were standardized per surface area of <i>U. lactuca</i> fronds. Two phases of responses to DIP‐pulses were measured: (i) a surge uptake (V_S) of 0.67 ± 0.10 μmol · cm<super>−2</super> · d<super>−1</super> and (ii) a steady state uptake (V_M) of 0.07 ± 0.03 μmol · cm<super>−2</super> · d<super>−1</super>. Mean internal storage capacity (ISC_P) of 0.73 ± 0.13 μmol · cm−2 was calculated for DIP. DIP uptake did not affect DIN uptake. Parameters of DIN uptake were also calculated: V_S = 12.54 ± 1.90 μmol · cm<super>−2</super> · d<super>−1</super>, V_M = 2.26 ± 0.86 μmol · cm<super>−2</super> · d<super>−1</super>, and ISCN = 22.90 ± 6.99 μmol · cm<super>−2</super>. Combining ISC and V_M values of P and N, nutrient storage capacity of <i>U. lactuca</i> was estimated to be sufficient for ~10 d. Both P and N storage capacities were filled within 2 d when exposed to saturating nutrient concentrations, and uptake rates declined thereafter at 90% for DIP and at 80% for DIN. Our results contribute to understanding the ecological aspects of nutrient uptake kinetics in <i>U. lactuca</i> and quantitatively evaluating its potential for bioremediation and/or biomass production for food, feed, and energy