Uptake rates of dissolved inorganic phosphorus and dissolved inorganic nitrogen under unsaturated and saturated conditions were studied in young sporophytes of the seaweeds Saccharina latissima and Laminaria digitata (Phaeophyceae) using a “pulse‐and‐chase” assay under fully controlled laboratory conditions. In a subsequent second “pulse‐and‐chase” assay, internal storage capacity (ISC) was calculated based on VM and the parameter for photosynthetic efficiency Fv/Fm. Sporophytes of S. latissima showed a VS of 0.80 ± 0.03 μmol · cm−2 · d−1 and a VM of 0.30 ± 0.09 μmol · cm−2 · d−1 for dissolved inorganic phosphate (DIP), whereas VS for DIN was 11.26 ± 0.56 μmol · cm−2 · d−1 and VM was 3.94 ± 0.67 μmol · cm−2 · d−1. In L. digitata, uptake kinetics for DIP and DIN were substantially lower: VS for DIP did not exceed 0.38 ± 0.03 μmol · cm−2 · d−1 while VM for DIP was 0.22 ± 0.01 μmol · cm−2 · d−1. VS for DIN was 3.92 ± 0.08 μmol · cm−2 · d−1 and the VM for DIN was 1.81 ± 0.38 μmol · cm−2 · d−1. Accordingly, S. latissima exhibited a larger ISC for DIP (27 μmol · cm−2) than L. digitata (10 μmol · cm−2), and was able to maintain high growth rates for a longer period under limiting DIP conditions. Our standardized data add to the physiological understanding of S. latissima and L. digitata, thus helping to identify potential locations for their cultivation. This could further contribute to the development and modification of applications in a bio‐based economy, for example, in evaluating the potential for bioremediation in integrated multitrophic aquacultures that produce biomass simultaneously for use in the food, feed, and energy industries.

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 ) and a nonlimiting concentration of dissolved inorganic nitrogen (DIN; 5,000 μmol · L ) 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 (V ) of 0.67 ± 0.10 μmol · cm · d and (ii) a steady state uptake (V ) of 0.07 ± 0.03 μmol · cm · d . Mean internal storage capacity (ISC ) of 0.73 ± 0.13 μmol · cm was calculated for DIP. DIP uptake did not affect DIN uptake. Parameters of DIN uptake were also calculated: V = 12.54 ± 1.90 μmol · cm · d , V = 2.26 ± 0.86 μmol · cm · d , and ISC = 22.90 ± 6.99 μmol · cm . Combining ISC and V 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.