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 (1-50μmol·L-1 ) and a nonlimiting concentration of dissolved inorganic nitrogen (DIN; 5,000μmol·L-1 ) under fully controlled laboratory conditions in a "pulse-and-chase" assay over 10d. 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±0.10μmol·cm-2 ·d-1 and (ii) a steady state uptake (VM ) of 0.07±0.03μmol·cm-2 ·d-1 . Mean internal storage capacity (ISCP ) of 0.73± 0.13μmol·cm-2 was calculated for DIP. DIP uptake did not affect DIN uptake. Parameters of DIN uptake were also calculated: VS =12.54± 1.90μmol·cm-2 ·d-1 , VM =2.26±0.86μmol· cm-2 ·d-1 , and ISCN =22.90±6.99μmol·cm-2 . Combining ISC and VM values of P and N, nutrient storage capacity of U.lactuca was estimated to be sufficient for ~10d. Both P and N storage capacities were filled within 2d 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.