Real-time determination of dissolved free amino acids and primary amines in seawater by time-resolved fluorescence

Abstract

Dissolved free amino acids (DFAAs) and ammonium are important nutrients in marine microbial food webs. DFAA concentrations are most often studied by high-pressure liquid chromatography of fluorescent derivatives with ortho-phthaldialdehyde (OPA). This procedure is laborious, requires extensive sample handling, and yields results hours or days after sampling. A fluorescent monitoring system for DFAAs and other primary amines was developed that is automated and continuous, and provides results in real time. A gently pumped stream of seawater is reacted with an OPA-mercaptoethanol-buffer reagent and passed over the tip of a fiber optic probe which supplies pulsed laser light at 337 nm. The fluorescent signal, with a maximum at 455 nm, is transmitted by the probe to an optical multichannel analyzer which uses a linear photodiode array. A plot of fluorescent intensity vs. time is recorded. The signal is linear in the range of 1–500 nM alanine equivalents. The signals with ammonium and urea are weak, not linear with respect to concentration, and not expected to contribute significantly to the total signal of well-oxygenated seawater. Ammonium can be resolved from a mixture of amino acids by time-resolved fluorescence. The fluorescent lifetime of the ammonium derivative is 9 ns; that of all amino acids tested is 21 ns, as is that of the bulk seawater fluorescence. A surface transect from San Francisco to Hawaii showed hundreds of nanomoles per liter of alanine equivalents in the California Current, declining to ∼45 nmol l −1 in the central gyre, and rising sharply at Pearl Harbor. Loose correlation was observed with chlorophyll and bioluminescence. The system has demonstrated utility in monitoring bulk amino acid concentrations while under way at sea.