Close coupling between release and uptake of dissolved free amino acids in seawater studied by an isotope dilution approach

Dissolved free amino acids (DFAAs) play a major role in the f l u of organic carbon and nitrogen in marine biotic systems.In t h s study, DFAA release and uptake rates in samples from Long Island Sound and the Atlantic continental shelf were measured simultaneously using a tritium isotope dilution approach in conjunction with h g h pressure liquid chromatography (HPLC).Numerous measurements over a 20 mo period showed that release and uptake rates were usually similar, and net changes in DFAA concentration were much slower than (typically less than 30 '10 of) the gross uptake or release rates; this indicates close coupling between these 2 processes.DFAA turnover was rapid, with summer turnover times typically 0.5 h or less, and concentrations of individual DFAAs were usually a few nanomolar.Comparisons of total uptake rates with estimates of bacterial heterotrophic produchon confirm that DFAAs represent a significant source ( > l 0 %) of carbon and nitrogen for bacterial growth.DFAA release mediated by copepods, either through 'sloppy feeding' or by excretion, can be of comparable magnitude to direct release by microplankton.