Carbohydrates associated with humic substances

Humic substances can play an important role in aquatic ecology by several mechanisms which have been discussed. Humic substances may also control the bioavailability of dissolved proteinaceous material and carbohydrates. In field and laboratory studies, Stabel (1977) and Steinberg (1977a) showed that carbohydrates associated with humic substances had a greater microbial availability than proteinaceous matter associated with humic substances. 

This structure does not account for the nitrogen and sulfur content of humic substances. It has been suggested that these elements are derived from parts of other types of molecule, for example, proteins, which are associated with the humic substances. Indeed it has been proposed that humic substances consist of an aromatic core to which peptides, carbohydrates, metals, and phenolic acids are chemically or physically attached. It can be seen that the structure in Fig. 5-9 is an "open" network. In fact, there have been suggestions that organic and inorganic materials associated with humic substances are trapped inside these "holes" in the humic substance structure. " ... 

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. 

---