ISOTOPE-DILUTION OF INTRACELLULAR AMINO-ACIDS AS A TRACER OF CARBON AND NITROGEN-SOURCES OF MARINE PLANKTONIC BACTERIA

Isotope dilution (ID) of intracellular amino acids in bacterial assemblages in the subarctic Pacific was measured in order to examine the relationship between dissolved free amino acids (DFAA) and other sources of amino acids for protein synthesis. When H-3-amino acids were added at trace amounts to < 0.8-mu-m filtrates, ID of single amino acids varied greatly. Isotope dilution of serine, glycine, threonine, arginine, alanine, and tyrosine was 1- to 4-fold, indicating that 25 to 100 % of bacterial biosynthetic requirements was met by uptake of these amino acids. Valine, phenylalanine, isoleucine, and leucine exhibited 8- to 50-fold ID indicating that only a minor fraction of bacterial biosynthetic requirements was met by uptake of these amino acids. These results were consistent with measurements of the fraction of bacterial biomass production supported by uptake of DFAA (8 to 42 %). Intracellular isotope dilution appeared to be inversely related to the extracellular concentration of the respective DFAA. Glutamate ID was always high (13- to 95-fold) and independent of its concentration. This high ID was probably due to substantial ammonium uptake, indicating that in addition to DFAA, ammonium was an important N-source for amino acid de novo synthesis and thus for protein production.