Changes in amino acid composition of Ulva intestinalis (Chlorophyceae) following addition of ammonium or nitrate

Abstract

M.W. Taylor, N.G. Barr, C.M. Grant and T.A.V. Rees. 2006. Changes in amino acid composition of Ulva intestinalis (Chlorophyceae) following addition of ammonium or nitrate. Phycologia 45: 270–276. DOI: 10.2216/05-15.1Nitrogen assimilation was investigated in the marine macroalga Ulva intestinalis. Following incubation of the alga in the presence of ammonium or nitrate, resulting changes in free amino acid content were determined. After 10 h, the largest changes by far occurred in the levels of glutamine and asparagine, which both increased more than 10-fold regardless of nitrogen source. Other amino acids increased slightly, but these two – together with their precursors glutamate and aspartate – comprised 83 and 76% of the total free protein amino acid-N pool upon addition of ammonium or nitrate, respectively. In subsequent time-course experiments, with ammonium as nitrogen source, glutamate initially decreased (with a concomitant increase in glutamine) before recovering to at least its original level. Asparagine levels began to increase after 1 h. Saturation of the glutamine and glutamate pools occurred after approximately 6 h, and coincided with the transition from the surge phase of ammonium uptake to the internally controlled uptake phase. This study represents the first investigation into how levels of specific amino acids change during ammonium assimilation in macroalgae, and as such extends our current knowledge of this poorly understood process.