Influence of Nitrogen Source and Availability on Amino Acids, Pigments and Tissue Nitrogen of Gracilaria edulis (Rhodophyta)

Increasing eutrophication of coastal marine environments has led to the development of nutrient sampling programs to monitor water quality. Various shortcomings of chemical analyses have identified the need to develop biological indicators (bioindicators) that can be used to detect available nutrient concentrations. Macroalgal tissue nutrient content, pigments, and amino acids appear to be responsive to water column nutrient availability. The responses of the red alga, _Gracilaria edulis_ (Gmelin) Silva, were related to nitrogen (N) source and availability in laboratory and field incubations to identify characteristics that would serve as bioindicators of N. The amino acid, pigment, and tissue N composition of _G. edulis_ was analysed after incubation in different N sources (NH[sub:4][super:+], NO[sub:3][super:-], and urea) and a range of [NH[sub:4][super:+]] in laboratory aquaria. These results were compared to field responses after incubation of _G. edulis_ along a N gradient in the Brisbane River (3 sites) and Moreton Bay (5 sites), Queensland, Australia. Photosynthetic pigments (phycoerythrin and chlorophyll _a_) increased in laboratory experiments, in response to increasing [NH[sub:4][super:+]], but not [NO[sub:3][super:-]] or [urea]. Phycoerythrin was observed to be the more responsive of the two. Total tissue N increased linearly with increasing [NH[sub:4][super:+]] in the laboratory but did not respond to [NO[sub:3][super:-]] or [urea]. In the field both phycoerythrin and tissue N appeared to respond equally to NH[sub:4][super:+] and NO[sub:3][super:-] availability. The amino acid composition provided the best representation of the concentration and source of available N. Citrulline was the most responsive of all amino acids to changes in concentrations of available N, with citrulline concentrations increasing linearly (r[super:2] = 0.84) with [NH[sub:4][super:+]] in laboratory experiments. NH[sub:4][super:+] treatments produced increases in citrulline, phenylalanine, serine and free NH[sub:4][super:+], and decreases in alanine; NO[sub:3][super:-] treatments produced increases in glutamic acid, citrulline and alanine; urea treatments produced increases in free NH[sub:4][super:+] and decreases in phenylalanine and serine. The observed variations in amino acid content facilitated the development of an index for each N source based on relative concentrations of various amino acids. The N source index was used to predict the dominant source of N being assimilated by the macroalgae (i.e., metabolic profiling). Results demonstrated strong correlations between the N source index value and water column nitrogen concentrations (r[super:2] = 0.99 for NO[sub:3][super:-] and r[super:2] = 0.79 for NH[sub:4][super:+]) in Brisbane River field trials. In Moreton Bay where dissolved inorganic nitrogen values were low (<2 mM), N source indices proved valuable in identifying predominant N sources. Physiological responses to N by this macroalga could be related to N source and availability in both laboratory and field situations, thus providing a sensitive bioindicator of N. In particular, the analysis of macroalgal amino acid content to infer the source and availability of N in a variety of field situations, appears to be a very sensitive biological indicator.

Keywords: amino acids, bioindicator, Gracilaria edulis, macroalgae, nutrients, pigments, Rhodophyta, tissue nitrogen, water quality, metabolic profiling