Aldoses in various size fractions of marine organic matter: Implications for carbon cycling

Carbohydrates are major components of marine organic matter, but few molecular-level carbohydrate analyses in seawater have been undertaken owing to the low concentrations of individual compounds. This paper presents novel data on aldose compositions and concentrations in various size fractions of particulate and dissolved organic matter from the equatorial Pacific. Samples of high-molecular-weight (HMW >1 kDa) dissolved organic matter (DOM) and suspended particulate organic matter (POM, >0.1 mu m) were collected by tangential-flow ultrafiltration. Aldose content of low-molecular-weight (LMW) DOM was calculated as the difference between aldose content in unfiltered samples and HMW DOM + POM. Size-fractionated and unfiltered samples were hydrolyzed with sulfuric acid, and aldoses were separated and detected by high-performance anion-exchange chromatography and pulsed amperometric detection. A detailed description of the method is provided. The concentrations of fucose, rhamnose, arabinose, glucose, galactose, xylose, and mannose were determined. In general, the predominant sugars were glucose and galactose. The HMW DOM fraction was slightly enriched in deoxysugars and galactose. The average depth-integrated size distribution of aldoses indicated that 68% of aldoses were LMW DOM, 28% were HMW DOM, and 4% were POM. Concentrations and yields (normalized to organic carbon) of aldoses in POM and HMW DOM decreased with depth, indicating selective degradation of aldoses. The molecular compositions of the degraded components of POM and HMW DOM were similar to surface compositions but the mole % glucose increased with depth, implying that glucose was preferentially preserved in a refractory structure. The fraction with the highest aldose yield was POM, followed by HMW DOM and LMW DOM. In surface waters from both stations, the largest fractions of dissolved aldoses were found in HMW DOM, whereas in subsurface waters most dissolved aldoses were found in LMW DOM. Yields and concentrations of aldoses indicated a size-related diagenetic sequence where POM represents the most reactive material and LMW DOM the most refractory material. Aldoses represented 1.7-3.6% of DOC and 10-20% of total carbohydrates, indicating that a large fraction of the carbohydrate pool is still uncharacterized at the molecular level, Aldoses could be identified in all, presumably refractory, deep-water samples, and most of the aldoses in deep water were LMW, indicating that a factor other than molecular composition was important in determining biological availability of the aldoses.