Isotope dilution analysis of combined nitrogen in natural waters: I. Ammonium

Abstract

Dissolved free amino acids (DFAA) are an overlooked and potentially very important substrate for primary production. In the open ocean, an understanding of the flux of regenerated nitrogen is fundamental to our studies of carbon and nitrogen turnover. A major aim of these studies is to provide more accurate estimates of the drawdown of atmospheric carbon dioxide. We have estimated the concentration and regeneration rate of DFAA in seawater using a novel stable isotope dilution technique. Seawater is first spiked to an estimated 10% of the DFAA pool with a known quantity of 15N-labelled algal protein hydrolysate. One portion is filtered immediately and the DFAA concentration analysed by isotope dilution. A second portion is incubated in situ. After a set period this is filtered and DFAA regeneration is estimated by isotope dilution analysis of the filtrate. Combination with data from analysis of particulate nitrogen provides an assay of DFAA uptake by phytoplankton. DFAA are extracted from 50 mL seawater by ligand exchange chromatography. Amino acids are eluted, concentrated and derivatized prior to GC/MS anslysis. The new procedure is sensitive, precise, accurate and, as it is based on mass spectrometric detection, it has greater selectivity than methods based on high-performance liquid chromatography/optical detection. Furthermore, the method can provide concentration and flux data on all protein amino acids and is applicable in a number of fields