Dissolved organic matter (DOM) was isolated with XAD-2 and 4 resins in different water masses of the Greenland Sea and Fram Strait. The contribution of XAD-extractable dissolved organic carbon (DOC), operationally defined as recalcitrant or humic substances, to total DOC ranged between 45±9% in surface waters and 60±6% in deep waters. The concentration and composition of carbohydrates was determined using the L-tryptophan / sulphuric acid method (for the bulk carbohydrate concentration, TCHO) and high-performance anion-exchange chromatography with pulsed amperometric detection after sulphuric acid hydrolysis (for the distribution of total hydrolysable neutral sugars, THNS). Carbohydrates contributed up to 6.8% to both total and recalcitrant DOC. TCHO contribution to total DOC decreased with depth from on average 4.1±1.2% in surface waters to 2.2±1.0% in deep waters, whereas the THNS contribution was similar in both layers accounting for 2.5±1.6% (surface) and 2.4±0.2% (at depth). TCHO contribution to XAD-extractable DOC also decreased with depth from 4.5±1.7% to 2.1±1.0%, whereas THNS contribution was almost constant with yields of 0.5±0.3% in surface samples and 0.6±0.1% at depth. The molecular size distribution of the recalcitrant DOM derived from selective resin fractionation of XAD-2 and XAD-4 showed in all fractions a clear trend towards small molecules in the deep sea. More than half of the XAD-extractable carbohydrates of surface samples and more than 70% of deep sea samples were found in the unpolar fraction of XAD, which was eluted with methanol. Glucose was the dominant carbohydrate in the surface water samples, whereas in the deep sea the composition was more uniform. In the XAD extracts the compositions were less variable than in the original samples. The neutral sugar composition, in particular glucose and the deoxysugars, are indicatives of the diagenetic state of the extracted DOM. The molar ratio (fucose+rhamnose) / (arabinose+xylose) was lowest for deep sea extractable DOM indicating a high contribution of material modified by microorganisms. The THNS composition and distribution reveals that recalcitrant carbohydrates are heteropolysaccharides, carbohydrate units incorporated into a framework of a highly unpolar structure with a lack of functional groups.
Helmholtz Research Programs > MARCOPOLI (2004-2008) > POL5-Autecology of planktonic key species and groups