Characterization of methyl sugars, 3-deoxysugars and methyl deoxysugars in marine high molecular weight dissolved organic matter

Abstract

Nuclear magnetic resonance spectroscopy of marine high molecular weight dissolved organic matter (HMWDOM) in surface waters show that >50% of the carbon is a compositionally well-defined family of acylated-polysaccharides that are conserved across ocean basins. However, acid hydrolysis of HMWDOM followed by chromatographic analyses recover only 10-20% of the carbon as neutral, amino, and acidic sugars. Most carbohydrate in HMWDOM therefore remains uncharacterized. Here we use acid hydrolysis followed by Ag+ and Pb2+ cation exchange chromatography to separate HMWDOM hydrolysis products for characterization by 1-D and 2-D NMR spectroscopy. In addition to neutral sugars identified in past studies, we find 3-Omethylglucose, 3-O-methylrhamnose, 2-O-methylrhamnose and 2-O-methylfucose. We also find 3-deoxysugars to be present, although their complete structures could not be determined. Methyl sugars are widely distributed in plant and bacterial structural carbohydrates, such as cell wall polysaccharides, and their presence in HMWDOM suggests that structural carbohydrates may contribute to DOM in surface seawater. We find most HMWDOM carbohydrate is not depolymerized by acid hydrolysis, and that the nonhydrolyzable component includes 6-deoxysugars.