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

Abstract

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 (V_S ) of 0.67 ± 0.10 μmol · cm^-2 · d^-1 and (ii) a steady state uptake (V_M ) of 0.07 ± 0.03 μmol · cm^-2 · d^-1 . 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^-2 · d^-1 , V_M = 2.26 ± 0.86 μmol · cm^-2 · d^-1 , and ISC_N = 22.90 ± 6.99 μmol · cm^-2 . Combining ISC and V_M 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.

Keywords: Ulva lactuca; nitrate uptake; phosphate uptake; storage capacity; uptake kinetics.