MACROALGAL RESPONSES TO NITROGEN-SOURCE AND AVAILABILITY - AMINO-ACIDMETABOLIC PROFILING AS A BIOINDICATOR USING GRACILARIA-EDULIS (RHODOPHYTA)

The use of macroalgae as biological indicators of dissolved nutrient s ource and availability ill the water column was investigated. Total ti ssue nitrogen (N) content, pigments, and amino acids of the red alga G racilaria edulis (Ginelin) Silva were compared to N source and availab ility in laboratory and field incubations to identify responses that w ould serve as bioindicators of N. Field-collected algae were preincuba ted (6-8 wk) in low-nutrient seawater to deplete their luxury reserves of N. Incubations were then conducted for periods of 3 d in laborator y aquaria (N-spiked seawater) and in the field using macroalgal incuba tion chambers. After incubation in different N sources (NH4+, NO3-, an d urea) in laboratory aquaria, photosynthetic pigments (phycoerythrin and chlorophyll a) and total tissue N increased, in response to increa sing [NH4+] but not to [NO3-] or [urea]. Incubation in two ranges of [ NH4+], one from 0 to 80 mu M and the other from 0 to 800 mu M, in labo ratory aquaria increased the total amino acid pool. Citrulline concent rations were the most responsive to [NH4+] (r(2) = 0.84). NH4+ source treatments produced increases in citrulline, phenylalanine, serine, an d free NH4+ and decreases in alanine; NO3- treatments produced increas es in glutamic acid, citrulline, and alanine; and urea treatments prod uced increases in free NH4+ and decreases in phenylalanine and serine. The observed variations in amino acid content facilitated the develop ment of an index for each N source based on relative concentrations of various amino acids (i.e. metabolic profiling). Gracilaria edulis was incubated along a field N gradient in the Brisbane River (three sites ) and Moreton Bay (four sites), Queensland, Australia. Both phycoeryth rin and tissue N appeared to respond equally to NH4+ and NO3- availabi lity in the field. N source indices, based on amino acid concentration , were effective predictors of both [NH4+] and [NO3-] over a wide rang e of concentrations along the field gradient. Macroalgal physiological responses, particularly amino acid content, to changes in source and availability of N appear to be useful as sensitive bioindicators of N.