doi:10.1016/j.gca.2005.02.027 Molecular formulae of marine and terrigenous dissolved organic matter detected by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry BORIS P. KOCH, 1, *MATTHIAS WITT, 2 RALPH ENGBRODT, 1 THORSTEN DITTMAR, 3 and GERHARD KATTNER 1 1 Alfred-Wegener-Institut für Polar- und Meeresforschung, Am Handelshafen 12, 27570 Bremerhaven, Germany 2 Bruker Daltonik GmbH, Fahrenheitstrasse 4, 28359 Bremen, Germany 3 Florida State University, Department of Oceanography, OSB 311, Tallahassee, FL 32306-4320, USA (Received July 6, 2004; accepted in revised form February 21, 2005) Abstract—The chemical structure of refractory marine dissolved organic matter (DOM) is still largely unknown. Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT- ICR-MS) was used to resolve the complex mixtures of DOM and provide valuable information on elemental compositions on a molecular scale. We characterized and compared DOM from two sharply contrasting aquatic environments, algal-derived DOM from the Weddell Sea (Antarctica) and terrigenous DOM from pore water of a tropical mangrove area in northern Brazil. Several thousand molecular formulas in the mass range of 300 – 600 Da were identified and reproduced in element ratio plots. On the basis of molecular elemental composition and double-bond equivalents (DBE) we calculated an average composition for marine DOM. O/C ratios in the marine samples were lower (0.36  0.01) than in the mangrove pore-water sample (0.42). A small proportion of chemical formulas with higher molecular mass in the marine samples were characterized by very low O/C and H/C ratios probably reflecting amphiphilic properties. The average number of unsaturations in the marine samples was surprisingly high (DBE = 9.9; mangrove pore water: DBE = 9.4) most likely due to a significant contribution of carbonyl carbon. There was no significant difference in elemental composition between surface and deep-water DOM in the Weddell Sea. Although there were some molecules with unique marine elemental composition, there was a conspicuous degree of similarity between the terrigenous and algal-derived end members. Approximately one third of the molecular formulas were present in all marine as well as in the mangrove samples. We infer that different forms of microbial degradation ultimately lead to similar structural features that are intrinsically refractory, independent of the source of the organic matter and the environmental conditions where degradation took place. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Marine dissolved organic matter (DOM) represents one of the largest active pools of organic carbon in the global carbon cycle (Hedges, 1992). Its amount (700  10 15 g C) is compa- rable to that of carbon in atmospheric CO 2 (750  10 15 g C). An annual net oxidation of 1% of dissolved organic carbon (DOC) would create a CO 2 flux larger than the anthropogenic (fossil-fuel derived) flux (Hedges, 2002). Most of marine DOC (90%–95%) is present in the deep-sea and represents a refrac- tory background with low concentrations of 35– 45 M DOC (e.g., Kähler et al., 1997; Hopkinson et al., 1997; Hansell and Carlson, 1998; Ogawa et al., 1999) and average residence times of several thousand years (Williams and Druffel, 1987). Sur- face DOM in the ocean is characterized by higher DOC con- centrations and significant proportions of more labile com- pounds (e.g., Hopkinson et al., 2002). Despite the importance of DOM, the composition, sources, diagenesis and preservation mechanisms are largely unknown and represent a missing link in models of global elemental cycles. The fate of terrestrial-derived DOM in the oceans is still a conundrum in organic geochemistry. Global discharge of riv- erine DOC is sufficient alone to sustain turnover of the entire pool of organic carbon dissolved in seawater (Williams and Druffel, 1987; Hedges et al., 1997). However, analyses of lignin phenols as unequivocal terrestrial markers suggest that probably only 0.7 to 2.4% of total DOM in the oceans is of terrigenous origin (Opsahl and Benner, 1997). Significant re- moval of terrigenous DOM via sorption to mineral particles and subsequent sedimentation is unlikely because the discharge of riverine particulate organic carbon alone is sufficient to account for all organic carbon preserved in marine sediments (Berner, 1989). While photodegradation is known to reduce aromaticity of terrestrial humic substances (e.g., Opsahl and Benner, 1998), this process alone cannot account for the almost complete loss of terrigenous compounds in the ocean (Hedges et al., 1992). It is possible that the disappearance of terrestrial DOC in the deep oceans might be due to some combined effects of photodegra- dation and efficient decomposition and reworking by marine microbes. The ultimate source of most DOM in the ocean is probably marine primary production. Ogawa et al. (2001) showed that marine bacteria themselves “produce” refractory DOM out of different simple substrates within days. This material persisted for more than 1 yr, and only 10 to 15% of the DOM was identified as amino acids and sugars, the principal building blocks of freshly produced, labile DOM. The chemical identity of DOM can be the key to the most urgent questions regarding the cycling of DOM in the ocean. It is known that both seawater and freshwater humic substances have predominantly carboxyl and hydroxyl functional groups (Thurman, 1985) and that freshwater substances have higher aromatic contents and more phenolic hydroxyl groups. Molec- * Author to whom correspondence should be addressed (bkoch@awi- bremerhaven.de). Geochimica et Cosmochimica Acta, Vol. 69, No. 13, pp. 3299 –3308, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 3299