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

The general agreement between the HPLC and microbial techniques was consistent with rapid DFAA turnover and bacterial uptake rates approaching the maximum rate of the highest affinity systems, i.e. the uptake affinity constants (K,) were smaller than the ambient concentrations.

Abstract

Dissolved free amino acids (DFAAs) are important sources of carbon and nitrogen for marine bacteria.Past studies of DFAAs have reported inconsistencies between chemical (chromatographic) and microbiological (uptake h e t i c s ) measures of concentrations.We found no systematic differences in this temperate zone study of summer coastal and winter continental shelf seawater.Concentrations of individual DFAAs were in the 1 to 15 nM range and turnover hmes 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.Rnetic 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.The general agreement between the HPLC and microbial techniques was consistent with rapid DFAA turnover and bacterial uptake rates approaching the maximum rate of the highest affinity systems, i.e. the uptake affinity constants (K,) were smaller than the ambient concentrations.