Growth of a Marine Diatom and a Haptophycean Alga on Phenylalanine or Tyrosine Serving as Sole Nitrogen Source1

The marine diatom Navicula incerta Hustedt and the haptophyte Isochrysis galbana Parke, both accustomed to grow on nitrate, were tested for their adaptive capacity for growth in axenic culture on 0.025–2.0 mM l‐, d‐, and dl‐isomers of phenylalanine and tyrosine as sole nitrogen sources. The diatom showed immediate and excellent growth (superior to nitrate) on l‐phenylalanine and l‐tyrosine but required prolonged periods of adaptation before growing on the d‐isomers, which growth was considerably slower. Isochrysis showed good growth on l‐phenylalanine after short adaptation, fair growth on low but poor growth on high concentrations of l‐tyrosine and no growth on the d‐isomers of both amino acids. The racemate (dl‐mixture) of each amino acid produced growth responses, from both algae, inferior to those of the corresponding l‐isomer, indicating competitive inhibition from the d‐isomer in the presence of equal amounts of its counterpart. Increasing tyrosine concentration was beneficial to growth of Navicula but markedly inhibitory or toxic to that of Isochrysis. Phenylalanine concentration appeared to benefit growth of both algae. Ecological considerations suggest that N. incerta may be habituated to utilize both aromatic amino acids in its native milieu.