Uptake of urea and amino acids by the macroalgae Ulva lactuca (Chlorophyta) and Gracilaria vermiculophylla (Rhodophyta)

Dissolved organic nitrogen (DON) makes up a large fraction of the total dissolved nitrogen pool in coastal waters, but is often ignored as a potential nitrogen source for primary producers. In laboratory experiments, we measured the uptake of small, labile DON compounds, urea and a variety of different amino acids, by the common estuarine macroalgae Ulva lactuca and Gracilaria vermiculophylla. Urea uptake was measured based on its disappearance from solution; amino acid uptake was measured using this method as well as by assimilation of 15Nand 13C-labeled amino acids. The Michaelis-Menten uptake parameters (Km, Vmax, Vmax/Km) were calculated for all compounds. Whereas both species were capable of assimilating urea and amino acids, U. lactuca consistently exhibited significantly higher uptake rates than G. vermiculophylla. There were 2 distinct phases of uptake for urea, an initially rapid ‘surge’ uptake phase and a slower, ‘sustained’ phase. This suggests that both species can take advantage of pulsed urea availability. Vmax rates for urea for U. lactuca were lower than published values for ammonium uptake, but were still high enough to be a significant factor at natural urea concentrations. We did not observe surge uptake of amino acids by either species and the uptake rates varied substantially among the 6 amino acids studied. The differential uptake of 15N and 13C by U. lactuca suggested that both alanine and glycine are decarboxylated prior to uptake. However, following decarboxylation, the residual of the glycine molecule is assimilated, while the amine group on alanine is likely removed prior to assimilation. Glycine uptake rates by N-starved or NH4-fertilized macroalgae were higher than uptake rates by NO3-fertilized macroalgae, which suggests that the induction of glycine and ammonium uptake may be similar. The low half-saturation constant (Km) and the high affinity at low concentrations (Vmax/Km) that we measured for amino acids suggests that macroalgae can take advantage of the low concentration of amino acids found in estuarine waters. This study shows that macroalgae are capable of utilizing DON and that under conditions of low inorganic nitrogen availability, organic nitrogen may provide a significant portion of the total N demand.