Amino acid utilization by marine phytoplankton: A novel mechanism

An enzyme on the cell surface of some marine phytoplankton, particularly the genus Pleurochrysis ( Hymenomonas), oxidizes many l ‐amino acids to produce H 2 O 2 , NH 3 and an α ‐keto acid extracellularly. The NH 4 + is subsequently taken up and used for growth. The enzyme synthesized by Pleurochrysis carterae (clone CoccoII) has a half‐saturation constant of 0.25 µ M assayed with l ‐ α‐ aminobutyric acid as a model substrate. The enzyme activity is absent in NH 4 + ‐grown (logarithmic phase) cells, is present in NO 3 ‐grown (logarithmic phase) cells, and is enhanced 3.5‐fold in N‐limited cells. Some Pleurochrysis species show different patterns of regulation. The existence of this novel, high‐affinity mechanism for amino acid utilization—one that would not be detected by typical field methods—suggests that amino acids may be a more important source of N for phytoplankton than is currently believed.