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

Abstract

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 15 N-and 13 C-labeled amino acids. The Michaelis-Menten uptake parameters (K m , V max , V max /K m ) 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. V max 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 15 N and 13 C 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 NH 4 + -fertilized macroalgae were higher than uptake rates by NO 3 --fertilized macroalgae, which suggests that the induction of glycine and ammonium uptake may be similar. The low half-saturation constant (K m ) and the high affinity at low concentrations (V max /K m ) 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.KEY WORDS: Marine macroalgae · Organic nitrogen · Nitrogen uptake · Urea · Amino acids · Ulva · Gracilaria Resale or republication not permitted without written consent of the publisher

Selection of effective macroalgal species and tracing nitrogen sources on the different part of Yantai coast, China indicated by macroalgal δ15N values

“…Without the δ 15 N value of each nitrogen source along the Yantai coast, we could only discuss the dominant nitrogen sources in our study. Besides, the seawater DIN concentration together with the algal TN and C:N analysis indicated no DIN depletion at the four sites, so the preferred use of other dissolved forms is not likely to occur (Tyler et al, 2005). The nitrogen sources based on the analysis of macroalgal tissue δ 15 N in our study are mainly about the DIN sources.…”

“…2), indicating the high nitrogen uptake and demand (Pedersen and Borum, 1996;Raven and Taylor, 2003;Teichberg et al, 2008Teichberg et al, , 2010. C. pilulifera, Scytosiphon lomentaria and Petalonia fascia all showed lower TN values and higher C:N ratios compared to other macroalgal species, might be related to the low ability of DIN absorption and/or quick nitrogen release (Tyler et al, 2005;Tyler and McGlathery, 2006). Tissue δ 15 N values were also affected by seawater nutrient and metallic factors.…”

Growth and biomass development of the introduced red alga Gracilaria vermiculophylla is unaffected by nutrient limitation and grazing

“…Gracilaria may thus have a high affinity for inorganic nutrients, organic nutrients may serve as additional nutrient sources (Tyler et al 2005), and/or it may have a large storage capacity for nutrients. The storage capacity of Gracilaria vermiculophylla should potentially be larger than that of true fastgrowing algae (i.e.…”

“…In situ conditions and nutrient availability may well vary among these studies, but the results nevertheless indicate that members of the Gracilariales seem able to gain sufficient nutrients even at low substrate concentrations. Gracilaria may thus have a high affinity for inorganic nutrients, organic nutrients may serve as additional nutrient sources (Tyler et al 2005), and/or it may have a large storage capacity for nutrients. The storage capacity of Gracilaria vermiculophylla should potentially be larger than that of true fastgrowing algae (i.e.…”

Tissue Nutrient Content in Ulva spp. (Chlorophyceae) as Bioindicator for Nutrient Loading Along the Coast of East Africa

“…: +46-8-16 17 44; Fax: + 46-8-158417; E-mail: gustafls@ecology.su.se of both nitrogen (N) (e.g. urea and amino acids) [14] and phosphorous (e.g. organophosphates) [15], it may even be misleading to use water column concentrations of inorganic nutrients (e.g.…”