Inhibition of microbial mineralization

Available data suggest that the supra-CMC inhibitory effect on biodegradation is reversible and not a specific toxic effect. Partial inhibition of microbial degradation of phenanthrene was observed for nonionic surfactants at sub-CMC doses. It is not clear whether these effects result from surfactant interactions with microorganisms or from preferential use of surfactant as substrate or source of carbon. The effects of surfactant monomers and micelles on microbial cell surfaces and constituents, and effects related to preferential substrate utilization and mineralization of degradation products, must be better understood in order to evaluate whether synthetic surfactants may be employed advantageously to enhance bioremediation in soil-water systems. 

Other studies use soil or sediment samples for a more accurate indication of microbial activity in natural environments. In these samples, organic matter and clay particles play a role in metal toxicity.76112113 Both organic material and clay particles in soil can bind metals and reduce their bioavailability. For example, Pardue et al.87 demonstrated that much less solution-phase cadmium was required to inhibit trichloroaniline (TCA) dechlorination in a mineral-based soil than in a soil containing a higher concentration of organic matter. Other studies have shown that adding clay minerals to a medium mitigates toxicity. Clay minerals, such as kaolinite, montmorillonite, bentonite, and vermiculite, can bind to metals to decrease the amount that is bioavailable.112 115... 

Microbially produced exopolysaccharides have been directly implicated in weathering of silicates and aluminosilicates through either complexation of cationic constituents of these minerals or, in the case of add mucopolysaccharides, through acidolysis (Barker Banfield, 1996). In some instances, however, exopolysaccharides may inhibit weathering, as in the case of plagi-oclase by gluconate at circumneutral pH (Welch Vandevivere, 1995). 

A series of related experiments investigated nonionic surfactant sorption onto soil, mechanisms of nonionic surfactant solubilization of polycyclic aromatic hydrocarbon (PAH) compounds from soil, and microbial mineralization of phenanthrene in soil-aqueous systems with nonionic surfactants. Surfactant solubilization of PAH from soil at equilibrium can be characterized with a physicochemical model by using parameters obtained from independent tests in aqueous and soil-aqueous systems. The microbial degradation of phenanthrene in soil-aqueous systems is inhibited by addition of alkyl ethoxylate, alkylphenyl ethoxylate, or sorbitan- (Tween-) type nonionic surfactants at doses that result in micellar solubilization of phenanthrene from soil. Available data suggest that the inhibitory effect on phenanthrene biodegradation is reversible and not a specific, toxic effect. 

The evidence from field studies is somewhat contrary to the predictions based on equilibrium chemistry. Abiological precipitation of CaC03 seems to be very limited, restricted to geographically and geochemically unusual conditions. The reasons why carbonate minerals are reluctant to precipitate from surface seawater are still poorly understood, but probably include inhibiting effects of other dissolved ions and compounds. Even where abiological precipitation is suspected—for example, the famous ooid shoals and whitings of the Bahamas (Box 6.5)—it is often difficult to discount the effects of microbial involvement in the precipitation process. 

Several mechanisms have been proposed to explain the antimicrobial properties of tannins. The antimicrobial effects of phenolic compounds are probably related to the inhibition of bacterial enzymes, alterations in cell wall permeability, an increase in the hydrogen ion activity of the microbial environment, a reduction in the surface and/or interfacial tension and perhaps chelation of essential minerals, particularly iron with a concomitant impairment of the microbial oxidative metabolic system (Chung et al. 1998). The antimicrobial activities of tannins are ascribed to the interactions of goats tannins with the extracellular enzymes secreted and... 

Tannins can also affect the nitrogen and carbon cycles they can complex proteins or metallic ions and can present toxicity for microorganisms and inhibit enzymatic activities. In the majority of studies, mineralization of the nitrogen is decreased by the addition of tannins, while the effects on carbon mineralization remain more variable. By their characteristic link to proteins, tannins inhibit the microbial extracellular enz5mies and can also limit the available substrates for microbial growth. The difference in tannin content of the leaves seems to act on the aquatic macroinvertebrate communities using... 

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