The catabolism of L-phenylalanine, L-tyrosine, and some related aromatic compounds by two marine species of phytoplankton

Abstract

Phytoplankton catabolism of L-phenylalanine and L-tyrosine was investigated with axenic cultures of the marine haptophyte Isochrysis galbana and the marine diatom Navicula incerta. Metabolic conversion and ¹⁴C-tracer studies indicated that phenylalanine is catabolized through pbenylpyruvate, phenylacetate, mandelate, benzoylformate and benzoate to p-hydroxybenzoate and its m-bromo derivative as major end-products, with marginal formation of CO₂ from aromatic-ring degradation. Tyrosine is similarly metabolized through the corresponding p-hydroxylated derivatives to p-hydroxybenzoate. The side chains of these amino acids are degraded stepwise and each C₁-fragment was identified as CO₂. Cinnamate or p-coumarate were not detected as intermediates, nor could the ammonia-lyases for phenylalanine and tyrosine be demonstrated. However, externally supplied cinnamic acids were metabolized via hydracrylate intermediates to the corresponding benzoic acids. It was concluded that the phytoplankters catabolize the aromatic amino acids primarily in order to assimilate their nitrogen via trans-aminase(s), while deriving incidental nutritional benefits from the phenolic intermediates formed. Aromatic halogenation by members of the marine phytoplankton was recorded for the first time.