Comparison of fluorescamine and o‐phthaldialdehyde methods for measuring amino acid exchange in marine organisms

Two methods routinely used to quantify total primary amine concentrations in seawater are the fluorescamine bulk assay and o‐phthaldialdehyde derivatization with chromatographic separation (OPA‐HPLC). Investigators using OPA‐HPLC have observed net amino acid uptake by marine invertebrates; those using fluorescamine observe excretion of primary amines. We systematically compared the two methods with amino acid and ammonium standards and also conducted two excretion experiments with two species of bivalve molluscs. Mytilus edulis exhibits both uptake and excretion of primary amines and the flux is correlated with the initial concentration of amino acids. The two methods give comparable results, particularly after correcting the fluorescamine data for differences in relative fluorescence among amino acids. Mercenaria mercenaria exhibits net uptake of amino acids when flux is measured by OPA‐HPLC and net excretion of fluorescamine‐positive substances. Small peptides may contribute to the discrepancy between assay methods.