Substrate humic molecules

Stimulation of active H+ extrusion from roots (Cesco, 1995 Pinton et al., 1997 Table 9.1) and transmembrane potential hyperpolarization (Slesak and Jurek, 1988) indicated the involvement of the PM H+-ATPase in the increased nutrient uptake generally observed in the presence of humic substances. Direct proof of an interaction between humic molecules and the PM H+-ATPase has been obtained by Vara-nini et al. (1993), who demonstrated that low-molecular-weight (<5kDa) humic molecules at concentrations compatible with those present in the rhizosphere can stimulate the phospho-hydrolytic activity of this enzyme in isolated PM vesicles (Table 9.1). Further proof of the action of humic molecules on PM FT-ATPase activity and on nutrient uptake mechanisms was obtained when studying the effect of these molecules on NO3 uptake. Transport of this nutrient is a substrate-inducible process and involves FT co-transport. At higher uptake rates, the levels and activity of root PM FT-ATPase increased (Santi et al., 1995). The short-term (4h) contact... 

Los ses ff dissolyed humic substances from the water column occur via adsorption onto surfaces of minerals and by cleavage upon exposure to UV radiation or ozone. Cleavage of humic molecules seems to be an important step in the decomposition of humic substances by microbes. Easily degradable substrates in the DOC pool (glucose, lactate, etc.) appear to stimulate microbial degradation of humic substances, either by a priming effect of an easily degradable substrate or by bacterial cometabolism. These... 

The DOC/enzyme/microbe interaction (DEMI) model divides bacterioplankton into two functional guilds, opportunists and decomposers, and DOC into two pools, labile and recalcitrant. In the context of the model, labile DOC is defined as directly assimilable monomers (saccharides, amino acids, and organic acids) and readily hydrolyzed polymers (polysaccharides, proteins, and nucleic acids). Because these substrates turn over rapidly, thus are unlikely to be transported far, most of the carbon in this pool will be autochthonous lysates and exudates, or allochthonous leachates from storms or seasonal litter fall. Recalcitrant DOC is defined as humic substances created by oxidative reactions among proteins, polysaccharides, hydrocarbons, and phenolic molecules. For inland waters, recalcitrant DOC is largely of allochthonous origin. 

The light-induced oxidation of humic or fiilvic acid by Mn02 [and by Fe(III) (hydr)oxides] produces, in addition to Mn(II) [and Fe(II)], low molecular weight organic molecules that can serve as substrates for microorganisms (Sunda, 1994). 

Thus, peroxidases may potentially effect direct free radical coupling reactions between aniline and humic substances or create additional substrate sites within the fiilvic or humic acid molecules for nucleophilic addition by aniline. A model for the latter pathway can be found in the work with guaiacol and 4-chloroaniline by Simmons et al. (19). Peroxidase catalyzed the coupling of guaiacol, itself not a substrate for nucleophilic addition, into the extended quinonoid dimer which subsequently underwent nucleophilic attack by the chloroaniline ... 

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