A novel protocol for determining the concentration and composition of sugars in particulate and in high molecular weight dissolved organic matter (HMW-DOM) in seawater

Abstract

A method is described to simultaneously determine the neutral, amino, and acidic sugar content of combined carbohydrates in high molecular weight (HMW, > 1 kDa) dissolved organic matter and in particles from seawater samples. Monomeric sugars are determined after acid hydrolysis and neutralization through acid evaporation using high performance anion exchange chromatography (HPAEC) coupled with pulsed amperometric detection (PAD). The separation of sugars during chromatography is achieved in two steps, an isocratic elution (18 mM NaOH) followed by a gradient course of two mobile eluent phases (NaOH and CH 3COONa). HPAEC-PAD has previously been applied to measure neutral and amino sugars in marine samples. Since salt anions interfere with the measurement, some of the earlier studies used ion exchange resins for seawater desalting. Thereby, variable losses of neutral and amino sugars, and the complete removal of acidic sugars have been reported. Here, we show that desalting by membrane dialysis (1 kDa) is an efficient alternative to ion exchange resins and yields recoveries of > 90% for HMW carbohydrates. We conducted several tests to determine the accuracy and reproducibility of the method. Sugar concentrations determined with our protocol were compared to results obtained with the colorimetric TPTZ-method, and with earlier HPAEC-PAD protocols using cation/anion exchange resins. Applications of our protocol to field samples indicated that acidic sugars can comprise a substantial fraction (30–50%) of HMW dissolved carbohydrates in seawater. The simultaneous analysis of the three classes of sugars appears promising to detect a larger fraction of marine combined carbohydrates, and thus to improve our understanding of organic matter cycling in the ocean.