Five characteristics of plant phenolics ensure that these compounds strongly influence system-level processes in soils and freshwater aquatic ecosystems: (1) the sheer abundance of phenolics produced by land plants; (2) the relative recalcitrance of detrital lignin and tannins to degradation by microbes; (3) the moderate solubility of phenolics in water; (4) the prevalence of carbon-to-carbon bonds and aromatic ring structures, which makes phenolics efficient at absorbing certain wavelengths of light; and (5) their chemical energy content, potentially available to decomposers. Characteristics (1) through
(3) were described earlier in this article; characteristics
The valence electrons of the carbon atoms of phenolic structures can delocalize over distances of several atoms. This tendency is referred to as resonance. Resonance is important because it explains how electrons shift about in the aromatic ring, when electron-withdrawing or electron-rich substituent groups are added to or removed from a phenolic structure. Essentially, the ability of the phenolic structure to move electrons about permits the structure to react with other dissolved materials, including cations (Zn2+, Fe2+, Ca2+, etc.) and polar materials such as chlorinated pesticides.
Two classes of naturally occurring phenolic-based materials - humic and fulvic substances - are especially noteworthy with respect to ecosystem-level function. Humic and fulvic substances are chemically heterogeneous and arise from plant-derived partially degraded lignin, tannins, and other phenolics. Humics and fulvics are defined operationally by the procedure used to extract them from an environmental sample. Humic and fulvic acids are similar in that they dissolve in water under basic (pH 10) conditions. In water at pH 10, humic and fulvic materials are small enough to pass through a 0.5-p.m-pore-size glass fiber filter: thus, they are said to be dissolved. Humic and fulvic acids differ in that humic acids (but not fulvic acids) precipitate from water at pH 2.
Humic substances are especially abundant in the humus-rich soils (humus, humic, human: a common etiology, stemming from ''of the Earth''). Humic and fulvic acids have a net negative charge and readily complex metal ions. Humics become less soluble when complexed with common divalent cations that nullify the humic's net negative charge. The toxicities of metals such as Cu, Zn, and Cd are reduced dramatically by even low (<1 to 5mgl concentrations of humic substances. Many highly chlorinated pesticides also become less biologically available to aquatic organisms when they bind to humic substances: the pesti-cide-humic complex is too large, apparently, to pass through cellular membranes. Humic materials also bind to carbohydrates, amino acids, and proteins, making these compounds less available to bacteria and fungi.
Sphagnum bogs are good examples of humic-dominated aquatic systems. The brown-colored water of bogs is acidic (pH 4-5) and contains high concentrations of plant-derived organic acids. When exposed to light containing UV wavelengths, dissolved humic and fulvic materials can react with dissolved oxygen in an iron-mediated process involving singlet oxygen. Thus, the surface waters of bogs typically are undersaturated with respect to dissolved oxygen. Blackwater streams and rivers in tropical lowland forested areas carry water the color of well-steeped tea due to high concentrations of humic and fulvic materials.
See also: Antipredation Behavior; Bioaccumulation; Bioavailability; Biodegradability; Biodegradation; Chemical Communication; Composting and Formation of Humic Substances; Detritus; Endocrine Disruptor Chemicals: Overview; Photolysis; Plant Competition; Soil Formation; Swamps.
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