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 Paclflc was measured in order to examine the relat~onshlp between dissolved free anlino aclds (DFAA) and other sources of amino acids for protein synthesis When 3 ~-a n ~i n o acids were added at trace amounts to <0.8 pm filtrates, ID of single amino aclds vaned greatly.Isotope dilutlon of senne, glyclne, threonine, arginlne, alanine, and tyrosine was 1-to 4-fold, indicating that 25 to 100 O/o of bacterlal biosynthetlc requirements was met by uptake of these amlno acids Vallne, phenylalanine, isoleucine, and leucine exhibited 8-to 50-fold ID Indicating that only a minor fractlon of bacterlal biosynthetic requirements was met by uptake of these amlno acids.These results were consistent wlth measurements of the fractlon of bacterlal biomass productlon supported by uptake of DFAA (8 to 42 '%).Intracellular Isotope dllutlon appeared to be Inversely related to the extracellular concentration of the respective DFAA Glutamate ID was always hlgh (13-to 95-fold) and independent of its concentration.Thls high ID was probably due to substantial ammonium uptake, indicating that In addition to DFAA, ammonium was an Important N-source for amlno acld de novo synthesis and thus for proteln productlon.