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, >O. 1 pm) were collected by tangential-flow ultrafiltration. Aldose content of low-molecular-weight (LMW) DOM was calculated as the difference bctwecn aldose content in unfiltered samples and HMW DOM + POM. Size-fractionated and unfiltered samples were hydrolyzed with sulfuric acid, and aldoscs 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 fucosc, rhamnose, arabinose, glucose, galactose, xylose, and mannosc were determined. In general, the predominant sugars were glucose and galactose. The HMW DOM fraction was slightly enriched in deoxysugars and galactose. The average depthintegrated size distribution of aldoses indicated that 68% of aldoscs 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 compositiqns 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 aldoscs indicated a size-related diagenetic sequence where POM represents the most reactive material and LMW DOM the most refractory material, Aldoses represented 1.73.6% of DOC and IO-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 aldoscs. Carbohydrates are the largest identified fraction of organic matter in the ocean, accounting for 20-30% of organic matter in marine surface waters (Pakulski and Benner 1994; Benner ct al. 1992). Carbohydrates comprise lo-70% of the organic matter in the plankton cell (Romankevich 1984) and are known to be released directly to the water column by algae (Hcllebust 1965; Burney et al. 198 1; Tttekot et al. 1981; Mopper et al. 1995). Additionally, other plankton as well as grazing activities release carbohydrates to the water . column (Cowie and Hedges 1994, 1996; Strom et al. 1997). The major classes of carbohydrates have been identified in marine waters, including uranic acids (e.g. Mopper et al. 1995), amino sugars (e.g. Kerherve et al. 1995), and neutral I Corresponding author. Acknowledgments This research was supported by grants from the Department of Energy (DE-FG03-94ER61907) and the National Science Foundation (OCE 94-13843). Annelie Skoog is grateful for a scholarship from the Swedish Institute. We thank N. Borch and D. Kirchman for helpful discussions about the aldose analysis and M. McCarthy for assistance with sample collection. The manuscript improved from discussions with the Biogeochemistry Group at the University of Texas Marine Science Pnstitute and the MOG group at University of Washington. This is contribution 1005 from the University of Texas Marinc Science Institute. sugars (e.g. Mopper et al. 1995; Kerherve et al. 1995; Borch and Kirchman 1997). There have been several investigations of dissolved carbohydrates in marine waters (e.g. Handa 1970; Liebezeit et al. 19F9; Ittekot et al. 1981), but all of these studies relied on con sentration procedures that fractionate the sample and leave an unknown fraction of carbohydrates uncharacterized. Determination of bulk carbohydrate content is possible using the MBTH method (Pakulski and Benner 1992, 1994), but bulk measurements do not provide information on molecular composition that can provide clues about the origin and diagenetic state of the material. Recent application to marine samples of an efficient Chromatographic separation of aldoses and a sensitive detection method (Mopper et al. 1992, 1995) has made direct determinations in seawater possible without Preconcentration. The distribution and relatively high concentrations of combined carbohydrates in the upper ocean are indicative of a highly reactive substrate that supports heterotrophic metabolism in the surface ocean (Benner et al. 1992; Pakulski and Benner 1994). High-molecular-weight (HMW) carbohydrates seem to be rapidly consumed in surface waters whereas low-molecular-weight (LMW) carbohydrates persist in the deep ocean. These observations and recent direct comparisons of the bioreactivity of HMW and LMW components of dissolved organic matter (DOM) indicate that naturally occurring HMW DOM is of more recent origin and