Uptake kinetics and storage capacity of dissolved inorganic phosphorus and corresponding N:P dynamics in Ulva lactuca (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 Ulva lactuca (Chlorophyta). Ulva lactuca is a promising candidate for biofiltration purposes and mass commercial cultivation. We exposed U. lactuca to a wide range of DIP concentrations (1–50 μmol · L−1) and a nonlimiting concentration of dissolved inorganic nitrogen (DIN; 5,000 μmol · L−1) under fully controlled laboratory conditions in a “pulse‐and‐chase” assay over 10 d. Uptake kinetics were standardized per surface area of U. lactuca fronds. Two phases of responses to DIP‐pulses were measured: (i) a surge uptake (VS) of 0.67 ± 0.10 μmol · cm−2 · d−1 and (ii) a steady state uptake (VM) of 0.07 ± 0.03 μmol · cm−2 · d−1. Mean internal storage capacity (ISCP) 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: VS = 12.54 ± 1.90 μmol · cm−2 · d−1, VM = 2.26 ± 0.86 μmol · cm−2 · d−1, and ISCN = 22.90 ± 6.99 μmol · cm−2. Combining ISC and VM values of P and N, nutrient storage capacity of U. lactuca 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 U. lactuca and quantitatively evaluating its potential for bioremediation and/or biomass production for food, feed, and energy.