Determination of picomolar dissolved free amino acids along a South Atlantic transect using reversed-phase high-performance liquid chromatography

• Published in: Marine Chemistry
• Main Authors: Sabadel, AJM, Browning, TJ, Kruimer, D, Airs, RL, Woodward, EMS, Van Hale, R, Frew, RD
• Format: Article in Journal/Newspaper
• Language: English
• Published: Science Direct 2017
• Subjects: Chemistry; Marine Sciences; Oceanography; South Atlantic Ocean
• Online Access: http://plymsea.ac.uk/id/eprint/7812/; http://plymsea.ac.uk/id/eprint/7812/7/Manuscript_Revised_22May2017.pdf; https://www.sciencedirect.com/science/article/pii/S0304420316302584?via%3Dihub; https://doi.org/10.1016/j.marchem.2017.09.008

Dissolved free amino acids (DFAA) in seawater are a form of nitrogen (N) available for marine microbes. In oligotrophic environments where N-containing nutrients are the limiting factor for microbial growth, N nutrition from DFAA could be crucial, but as yet it is poorly resolved. Measurements of individual DFAA are challenging as concentrations are typically in the low nmol L− 1 range. Here we report modifications to methodology using o-phthaldialdehyde (OPA) derivatization and reversed phase high performance liquid chromatography (HPLC) that provide a 30-fold improvement in sensitivity enabling the detection of 15 amino acids in seawater with a limit of detection as low as 10 pmol L− 1 with accuracy and precision of better than 10%. This analytical methodology is now suitable for the challenging quantitation of DFAA in oligotrophic seawaters. The method was successfully applied to a suite of seawater samples collected on a cruise crossing the South Atlantic Ocean, where concentrations of DFAAs were generally low (sub nmol L− 1), revealing basin-scale features in the oceanographic distributions of DFAA. This unique dataset implies that DFAAs are an important component of the N cycle in both near-coastal and open oceans. Further calculations suggest that the proportions of organic N originating from DFAA sources were significant, contributing between 0.2 and 200% that of NH4+ and up to 77% that of total inorganic nitrogen in the upper 400 m in some regions of the transect.