Characterisation of dissolved combined amino acids in marine waters

Dissolved combined amino acids (DCAA) are important constituents of the dis solved organic nitrogen (DON) pool in marine environments, although little is known about their sources, dynamics and sinks. The DCAA pool consists of various compounds including proteins and peptides, proteins linked to suga rs and amino acids adsorbed to humic and fulvic acids, clays and other mate rials. The proportions of each of these components and the extent to which they are used by microplankton living within the photic zone are not known. An investigation was carried out, using N-15 isotope dilution techniques, to determine the concentration and composition of dissolved amino acid pool s in the marine environment. A near-shore seawater sample was collected and split into fractions to determine the concentrations of dissolved free ami no acids (DFAA), DCAA and a <3 kDa dissolved peptide fraction (DPEP; obtain ed by ultrafiltration). DCAA and DPEP fractions were hydrolysed to yield fr ee amino acids and all samples were analysed by gas chromatography/mass spe ctrometry (GC/MS) as isobutyloxycarbonyl/tert-butyldimethylsilyl derivative s. The DFAA was the smallest fraction representing similar to1% of total di ssolved amino acids. The majority of DCAA was contained in the low molecula r weight DPEP fraction (90%) and was probably as a result of release from p hytoplankton and degradation by heterotrophic bacteria. Copyright (C) 2001 John Wiley & Sons, Ltd.