Oceans humic substances

There are major differences in the chemical compositions of DOM isolated by XAD resins and ultrafiltration (Table I). In rivers and in the ocean, humic substances (XAD isolation) are depleted in N relative to UDOM. The C/N ratios of UDOM are more representative of bulk DOM than those of humic substances. Most of the functional groups identified by NMR are found in more than one class of compounds, so in most cases specific functional groups are not assigned to a particular group of biochemicals. However, in some circumstances it is possible to estimate the fraction of carbon associated with a biochemical class, such as carbohydrates. Carbohydrates are the most abundant polyalcohols in nature, and the ratio (4-5 1) of areas associated with NMR peaks at specific chemical shifts [e.g., 72 ppm (C—O) -102 ppm (O—C—O)] indicates that carbohydrates are their primary source (see Table I for references). In general, humic substances are depleted in carbohydrates (C—O and O—C—O) and enriched in aromatic and unsaturated C (C=C) relative to UDOM (Table I). As mentioned earlier, humic substances are relatively hydrophobic components of DOM, and it is consistent that they are depleted in N and carbohydrates and enriched in aromatic components. The UDOM fraction includes more hydrophilic components that are relatively enriched in N and carbohydrates. Humic substances from the ocean are enriched in aliphatic C (C—C) relative to UDOM, and this could reflect the more hydrophobic nature of the humic substances. 

The most striking characteristic of the dissolved humic substances is their chromophoric nature. As part of the DOM, they impart a yellow-brown cast to marine and freshwaters and, hence, are part of the CDOM pool. Terrestrial hiunic substances compose a significant fraction of the riverine DOM entering the ocean. In seawater, humic substances compose 5 to 15% of the HMW DOM. Differences exist in the bulk properties of marine and terrestrial humic substances. These are summarized in Table 23.6. They have been used to trace the fate of terrestrial organic matter in the ocean. 

Since most of the riverine DOM is comprised of humic substances, considerable attention has been fiacused on its fete in seawater. Little terrestrial DOM is detectable in seawater, suggesting the existence of an efficient removal process. This is surprising given the traditional view that humic substances are relatively refractory. Marine chemists are currently investigating the redox and photochemistry of humic substances to better understand its chemical fete in the oceans. 

Benner, R. H. 1998. Cycling of dissolved organic matter in the ocean. In Aquatic Humic Substances (D. O. Hessen and L. J. Tranvik, Eds.), pp. 317—331. Springer-Verlag, Berlin. 

Both of these methods have been used for DOM isolation from major rivers and the surface ocean, and the general characteristics of these fractions of DOM are presented in Table I. The major C functional groups of humic substances and ultrafiltered DOM (UDOM) have been characterized by solid-state, cross-polarization magic angle spinning 13C nuclear magnetic resonance (NMR) spectroscopy. The samples of humic substances that were characterized by NMR spectroscopy were collected from the Amazon River... 

Humic substances account for 40-70% of the DOC in rivers and 5-25% of the DOC in the ocean (Table I). It is important to note that recoveries of adsorbed humic substances from XAD resins are not quantitative, so the chemical characteristics of the recovered humic substances are not necessarily representative of all the humic substances retained by the resin. Tangential-flow ultrafiltration retains 45-80% of the DOC in rivers and 25-40% of the DOC in the surface ocean (Table I). Essentially all of the DOC retained during ultrafiltration is recovered for chemical characterization. In general, ultrafiltration recovers a larger fraction of the DOM from these systems. These methods also isolate DOM based on different mechanisms. Adsorption onto XAD resins at low pH chemically fractionates the DOM and isolates the more hydrophobic components, whereas ultrafiltration principally separates components of DOM on the basis of size and shape. 

There are also structural differences between humic substances or UDOM collected from rivers and oceans (Table I). Humic substances and UDOM from rivers are enriched in aromatic components compared with their counterparts from the ocean. Terrestrial vegetation is relatively rich in aromatic components, such as lignins and tannins, and this is reflected in the greater aromatic nature of DOM in rivers. These biopolymers are relatively resistant to microbial degradation and are important components of river DOM. Humic substances and UDOM from the ocean are enriched in carbohydrates compared with their counterparts from rivers. This is consistent with observations of higher C-normalized yields of neutral sugars in bulk DOM from the ocean compared with rivers (Table I). 

Jones, R. I. 1998. Phytoplankton, primary production and nutrient cycling. In Aquatic Humic Substances (D. O Hessen and L. J. Tranvik, Eds.), pp. 145-176. Springer-Verlag, Berlin. Kirchman, D. L. 2000. Uptake and regeneration of inorganic nutrients by marine heterotrophic bacterial. In Microbial Ecology of the Oceans (D. L. Kirchman, Ed.), pp. 261-288. Wiley, New York. 

Black C, produced by wild fires and humic substances (HS), the natural by products of SOM decomposition in soil and water systems, are certainly the classes of organic compounds that most closely approximate this recalcitrant behavior. HS occur widely, being found in large amounts not only in the soil and sediments but also in lakes, rivers, ground waters, and even the open ocean (Stevenson, 1994). Besides these relatively refractory substances, more labile compounds can persist in soil for a much longer time than would be predicted from their inherent recalcitrance to decomposition. SOM stabilization (Figure 5.2) is generally considered to occur by three main mechanisms (i) physical protection, (ii) chemical stabilization, and (iii) biochemical stabilization (Six et al., 2002). 

Bussmann, I. (1999). Bacterial utilization of humic substances from the Arctic Ocean. Aquat. Microb. Ecol. 19, 37-45. 

While specific compounds such as siderophores have been found in natural waters, the bulk of the dissolved organic matter (DOM) is made up of relatively refractory compounds known as humic substances. These substances exhibit complex, ill-defined structures with the actual structure depending markedly on the source of the organic material. DOM in the open ocean is almost entirely authochthonous and formed by condensation, polymerisation and partial oxidation of smaller molecules such as triglycerides, sugars and amino acids and exhibits very little aromatic character [78]. In contrast, the DOM in fresh and coastal waters is largely allochthonous and derived... 

Humic substances occurring in natural waters in the form of stable negatively charged sols exhibit shielding aetion with respect to colloidal solutions of Si02, Fe(OH)j and Al(OH)3 (Kul skiy, 1960). Colloids shielded by humus do not coagulate and, remaining in the state of sols, can be transported by the waters of rivers, seas, and oceans (Kuznetsov, 1964) for considerable distances, and the Schultz-Hardy rule becomes inapplieable to shielded colloids. 

Only occasionally has the N content of solid phase extracts been reported. At a site in the Atlantic Ocean the carbon to nitrogen ratio (C N) of XAD 8 and XAD 2 extracts fell in the range of 40-57 (57 0.9 and 41.1 3.3, respectively DrufFel et ai, 1992). In contrast, at the same site XAD 4, when used as the second resin in series with XAD 8 or XAD 2, extracted compounds with lower C N ratios - 19—24 (21.0 2.4). These values are only slighdy higher than ratios reported for total DOM (see below). McKnight and Aiken (1998) reported a C N value of 37 for DOM extracted by XAD 8 at one site in the Pacific Ocean at other sites in the N. Pacific Ocean XAD 2 was found to extract DOM with a C N ratio between 32 and 46.5 (Druffel et al, 1992 Meyers-Schulte and Hedges, 1986). Bronk (2002, Table III) compiled various literature values and arrived at an average C N ratio of 32.8 19.5 for total humic substances isolated from a variety of aqueous environments (see McCarthy and Bronk, this volume). 

Proton and C-NMR data compare well with each other and suggest that surface ocean HMWDOM has a H C ratio of approximately 1.8—1.9 (Aluwihare, 1999 Benner et al, 1992) and an 0 C ratio between 1 and 1.1. These H C and 0 C ratios are very close to those of a pure carbohydrate with a general hexose structure (e.g., C6H12O6). In comparison, humic substances isolated from seawater have an H C ratio between 1.2 (direct elemental analyses) and 1.4 (based on NMR estimates) and are therefore, relatively C-rich (Hedges et al, 1992). The H C and 0 C composition of phytoplankton as estimated by NMR spectroscopy is approximately 1.7 and 0.3, respectively (Hedges et al, 2002). In comparison to phytoplankton... 

In the Arctic and Antarctic Ocean amino acids were also found in humic substances isolated from DOM by XAD-2 resins (Hubberten et ai, 1995). The concentration of THAA in humic substances was between 233—246 nM, with aU hydrolysable amino acids in the deep ocean and 60% of amino acids in the surface ocean residing in this fraction. Glycine was by far the most abundant amino acid detected in the humic fraction. These authors concluded that amino acids in the XAD-2 extracts represent a refractory protein background that is present throughout the ocean. The dominance of this refractory protein background in the surface and deep ocean could explain the relatively stable amino acid distribution observed by Yamashita and Tanoue (2003) at their open ocean sites. 

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