High molecular weight DOM

Free metal ions Inorganic ion pairs complexes Organic complexes chelates Metals species bound to high molecular weight DOM Metals in colloids Metals sorbed onto colloids Precipitates... 

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)...

One feature that has great relevance to the importance of DOM to the carbon cycle is its enrichment in carbon over the Redfield proportion. The C N P ratios of high molecular weight DOM are on the order of 350 20 1 (Kolowith et ai,... 

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. 

Humic substances High-molecular-weight organic compounds that are variable in composiUon, have complex structures, and are relaUvely inert. They comprise a large fracUon of the DOM. Found in sods, sediment, fresh, and seawater. 

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. 

Ultimately, the usefulness of tracer methods will partially depend upon how readily they can be incorporated into a field study. Methods that can be applied to filtered water samples are less labor intensive than those requiring some type of fractionation, such as the use of small-volume XAD-8 columns or ultrafiltration. However, column or ultrafiltration fractionation can be streamlined to make them practical for field studies, and the better resolution of DOM chemistry may make the extra effort worthwhile. If fulvic acid or high molecular weight fractions are isolated in a study, these can be saved for potential subsequent analysis of trace moieties as motivated by initial results. Finally, the overall question being addressed in a particular experimental or field study will determine which tracer methods, if any, are included. 

Relatively few biochemicals can be measured directly in natural waters because concentrations of individual compounds are low (nanomolar) and salts and other components often interfere with these analyses. DOM can be concentrated and isolated from natural waters for more thorough chemical characterization, and two approaches for DOM isolation, adsorption onto solid phases and ultrafiltration are now widely used. The adsorption of DOM onto XAD resins is used to isolate a fraction of DOM that is operationally defined as humic substances (Thurman, 1985). More recently, tangential-flow ultrafiltration with 1000 Da cutoff membranes has been used to isolate the high-molecular-weight or colloidal fraction of DOM (Benner et al., 1992, 1997). 

The retention of DOM in microbial biofilms involves several processes (A) sorption of a DOM molecule to the biofilm, (B) diffusion into the biofilm, (C) cleavage by extracellular enzymes (in the case of high-molecular-weight organic matter), and (D) uptake and microbial utilization of the DOM molecule. 

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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