Low molecular weight DOM

Fig. 2 Conceptual representation of the formation of marine gels from different pools of Phaeocystis organic matter (after Verdugo et al. 2004). (abbreviations LMW-DOM low-molecular-weight DOM, HMW-DOM high-molecular-weight DOM, POM particulate organic matter, TEP transparent exopolymeric particles)...

EDTA is a low-molecular-weight organic molecule, ethylenediaminetetraacetate, that is commonly used as a proxy for marine DOM. It is acts as buffer, maintaining constant pH and/or metal ion concentrations in experimental solutions. 

To aid in the characterization of the DOM pool, marine organic chemists have developed techniques fiar separating compounds into size fractions. Tangential flow ultrafiltration is used to isolate a high-molecular-weight (HMW) fraction from a low-molecular-weight (LMW) fraction. The size cutoff between these is approximately 1 tun, which equates... 

DOM is also released into seawater by phytoplankton fiar reasons that are as yet imclear. On average, 13% of the phytoplankton carbon is released as exudates, some of which are low-molecular-weight compounds, such as free amino acids and tricarboxylic acids. Other exudates are high-molecular-weight compoimds, such as the acylated heteropolysaccharides. These macromolecules are relatively chemically resistant and appear to form a large portion of the HMW DOC pool. 

Tracers of photochemical reactions include low-molecular-weight compounds, such as formaldehyde, pyruvate, and acetylaldehyde. The rates of these photochemical reactions are important to measure so that natural degradation of DOM can be quantified. Also, their variability due to increased ultraviolet radiation (from decreases in tropospheric ozone levels) should be studied. The ChemRawn IV conference had a major focus on photochemical reactions (Goldberg, 1988). 

FIGURE 1 Fate and major transformation pathways of phytoplankton- and macrophyte-derived DOM in aquatic systems. Arrows indicate fluxes POC denotes particulate organic matter LMW and HMW DOM refer to the monomeric (low molecular weight) and polymeric (high molecular weight) fractions, respectively. 

FIGURE 1 DOM pie diagram based on distribution of DOC in a typical river with a DOC concentration of 5 mg C/L (adapted from Thurman, 1985). Fulvic acids typically constitute the largest percentage of DOC, followed by low-molecular-weight organic acids. 

Table lb). There seem to be two different pools of DOM (1) a high-molecular-weight (HMW)-DOM pool with a high C/N ratio of over 12 and a carbohydrate content of over 67% and (2) a low-molecular-weight (LMW)-DOM pool with a low C/N ratio of 6.3. Since all studies used filtration techniques applying pressure, it is most likely that the DOM also contained mucopolysaccharides. The overall C/N ratio of the DOM is low, around 7 (Biddanda and Benner 1997 Solomon et al. 2003). Similar to mucopolysaccharides, extracolonial polymers self assemble in hydrated polymer gels on a time scale of approximately two days (Solomon et al. 2003). 

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