Recalcitrant dissolved organic matter in the ocean: major contribution of small amphiphilics
For the bulk of dissolved organic carbon (DOC) in the ocean, chemical structure and basic biochemical parameters are largely unknown. To reveal new structural and physicochemical information on DOC, we applied gel-permeation chromatography in conjunction with a high-sensitivity DOC detector on samples from the Arctic Rivers and Ocean. Carbon-normalized data on molecular size, polarity, charge and aromaticity were obtained for marine-derived and terrigenous refractory compounds in the ocean. The main fraction (60-70%) of riverine DOC exhibited characteristic features of terrigenous humics, in terms of aromaticity and molecular mass. Molar UV-absorption coefficients (OC at 254 nm) of 72-78 m2∙mol-1 indicated high aromaticity and nominal molecular masses (Mn) were 870-1050 g∙mol-1. The remainder was composed mainly of subunits of humics, and of diagenetically fresh biopolymers. In the coastal mixing zone, the hydrodynamic diameter of humics decreased probably due to intramolecular contraction or coiling induced by increases of ion strength and concentration of divalent ions. Aromaticity and the proportion of chromatographic DOC-fractions, on the other hand, behaved conservatively, which indicates compositional stability of terrigenous DOC during mixing in the Arctic Ocean. In the deep-sea, a main fraction (~60%) of DOC showed characteristic features of aquatic fulvic acids and their subunits, with low-molecular masses (Mn≈550 g∙mol-1) and low aromaticities (OC≈13 m2∙mol-1). The remainder (~40% of DOC) was composed of aliphatic and very small (<300 g∙mol-1) neutral molecules, largely with hydrophobic functional groups. These molecules have escaped from the analytical window so far, because they cannot be isolated via ultrafiltration or XAD-resins and do not absorb UV-light to be seen by conventional detection. Therefore, a considerable part of dissolved organic matter has very different chemical structures and physicochemical features than previously thought. Most mechanisms that have been suggested to describe the formation of recalcitrant organic matter in the ocean cannot be applied to a major fraction of DOC in the Arctic Ocean.
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