ABSTRACT
Dissolved free (DFAA) and combined amino acids (DCAA) are important substrates for growth of heterotrophic planktonic bacteria since protein comprises 60% of their biomass. If the carbon and nitrogen requirements are not met by DFAA and DCAA de novo synthesis from precursors such as ammonium and a carbon source occurs. Measuring uptake rates of DFAA is attractive because it is a sensitive method by which a major fraction of substrates for growth of planktonic bacteria is often assessed. If glutamic acid dominates the pool and the isotope dilution (ID) of glutamic acid is significantly higher than the ID of the most abundant DFAA, ammonium and a carbon source are the predominant substrates for bacterial growth besides DFAA. If glutamic acid is not dominating the bacterial pool and ID of the most abundant amino acids are equally high, DCAA are the most important substrates for bacterial growth.
