Nanomolar concentrations and rapid turnover of dissolved free amino acids in seawater: agreement between chemical and microbiological measurements

Abstract

Past studies of dissolved free amino acids (DFAAs) have reported inconsistencies between chemical (chromatographic) and microbiological (uptake kinetics) measures of concentrations. No systematic differences in this temperate zone study of summer coastal and winter continental shelf seawater were found. Concentrations of individual DFAAs were in the 1 to 15 nM range and turnover times averaged 1 h or less in summer and about 6 h in winter. Chemical assays by high pressure liquid chromatography (HPLC) employed collection and filtration techniques minimizing ubiquitous DFAA contamination. Kinetics assays utilized trace-metal clean techniques and extremely low additions (0.02 to 3 nM) of tritiated DFAAs. Kinetic data, corrected for respiration and based on HPLC-measured specific activities of added amino acids, indicated some evidence for multiphasic uptake kinetics, even at subnanomolar additions.