Oxygenated Hydrocarbons in Coastal Waters

Abstract

Low molecular weight (LMW) carbonyl compounds (also termed oxygenated hydrocarbons) are produced photochemically from chromophoric dissolved organic matter (CDOM) in natural waters (Kieber and Mopper, 1987; Kieber et al, 1990; de Bruyn et al., 2011). CDOM refers to the optically-active (i.e. light absorbing and hence colored) portion of dissolved organic matter (DOM). DOM is the largest organic carbon reservoir in the ocean (700 x 1015 g C), of comparable size to the atmospheric carbon dioxide (CO2) pool (Hedges, 2002). Studying the distribution and dynamics of marine DOM is of interest to oceanographers because oceanic DOM is a significant component of the global carbon cycle, in addition to affecting the bioavailability of chemical species through absorption, contributing to the spectral properties of seawater, acting as a water source tracer and a microbial food source (Hedges, 2002). DOM is primarily derived from decaying terrestrial plant matter in fresh and coastal waters, with production in oceanic waters from marine organisms via viral or bacterial lysis, grazing and microbial degradation (Perdue, 1998; McKnight and Aiken, 1998; Hessen and Tranvik, 1998). DOM is a complex macromolecular humic-type material, which has been extensively studied by marine scientists over the last 4 decades using analytical techniques including chromatographic separation, size fractionation, radioisotopes, elemental analyses, mass spectroscopy and nuclear resonance mass spectrometry (Perdue, 1998; Benner, 2002; Sharp, 2002; Whitehead, 2008). Optical techniques like UV-VIS absorption and fluorescence spectroscopy are used to study CDOM, including the identification of components via 3 dimensional excitation-emission matrix spectroscopy (EEMs) and parallel factor analysis (PARAFAC) computational methods (see for example Yamashita et al., 2008). CDOM absorbs sunlight to produce photo-excited states which undergo a series of primary and secondary reactions in natural waters that produce reactive species like peroxides and singlet oxygen, in addition to low molecular weight carbon-containing compounds (Miller, 1994). These photochemical processes play a significant role in the global carbon cycle through remineralization of dissolved organic carbon (DOC) to carbon dioxide (Bano et al., 1998). As a major energy source for microorganisms, LMW carbonyls also influence CO2 levels indirectly (Clark et al., 2004). LMW carbonyls in seawater also have the potential to influence atmospheric chemistry via air-sea exchange between surface waters and the atmosphere (Zhou and Mopper, 1990; Jacob et al., 2002). Oxygenated hydrocarbons are