Humic acids electron acceptor

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

The microbial degradation of contaminants under anaerobic conditions using humic acids as electron acceptors has been demonstrated. These included the oxidations (a) chloroethene and 1,2-dichloroethene to CO2 that was confirmed using C-labeled substrates (Bradley et al. 1998) and (b) toluene to CO2 with AQDS or humic acid as electron acceptors (Cervantes et al. 2001). The transformation of l,3,5-trinitro-l,3,5-triazine was accomplished using Geobacter metallireducens and humic material with AQDS as electron shuttle (Kwon and Finneran 2006). 

Bradley PM, FH Chapelle, DR Lovley (1998) Humic acids as electron acceptors for anaerobic microbial oxidation of vinyl chloride and dichloroethene. Appl Environ Microbiol 68 3102-3103. 

Anaerobic conditions often develop in hydrocarbon-contaminated subsurface sites due to rapid aerobic biodegradation rates and limited supply of oxygen. In the absence of O, oxidized forms or natural organic materials, such as humic substances, are used by microorganisms as electron acceptors. Because many sites polluted by petroleum hydrocarbons are depleted of oxygen, alternative degradation pathways under anaerobic conditions tend to develop. Cervantes et al. (2001) tested the possibility of microbially mediated mineralization of toluene by quinones and humus as terminal electron acceptors. Anaerobic microbial oxidation of toluene to CO, coupled to humus respiration, was demonstrated by use of enriched anaerobic sediments (e.g., from the Amsterdam petroleum harbor). Natural humic acids and... 

DOM can also act as an electron acceptor for biotically mediated oxidation reactions. Many active microorganisms, particularly phototrophs, produce reductants in excess of metabolic needs that must be regenerated by transfering electrons to acceptors in the environment via membrane-spanning reductases (Price and Morel, 1990). It has been discovered that some iron-reducing bacteria use humic and fulvic acids as terminal electron acceptors for their respiratory transport systems (Coates et al., 1998). 

There are several reasons why the ability to reduce humic substances could be an advantage to metal-respiring microorganisms. Higher rates of reduction should be possible with humic acids and other soluble oxidants than with solid-phase oxidants that require the organism to continually establish physical contact in order to access fresh oxide surfaces. Humic substances are ubiquitous and accumulate to levels that could make them significant alternative electron acceptors in some ecosystems. As with other anaerobic electron acceptors, the capacity to support respiration is enhanced by processes that regenerate humic... 

A more recent addition to the list of terminal electron acceptors is humic substances (Section 8.08.6.4.1). Cervantes et al. (2000) demonstrated that a humic acid analogue (AQDS) inhibited methanogenesis due to a combination of toxic and... 

Sears, P. G., Wolford, R. K., and Dawson, L. R. (1956). Conductances of some acids, bromides, and picrates in dimethylformamide at 25°C. J. Electrochem. Soc. 103, 633-636. Senesi, N. (1981). Free radicals in electron donor-acceptor reactions between a soil humic acid and photosynthesis inhibitor herbicides. Z. Pflanzenernahr Dung. Bodenkd. 144, 580-586. Senesi, N., Chen, Y., and Schnitzer, M. (1977a). Hyperfine splitting in electron spin resonance spectra of fulvic acid. Soil Biol. Biochem. 9, 371-372. 

Phosphorus content of organic substrate or C P ratios Microbial activities and synthesis of hydrolytic enzymes Supply of electron acceptors and redox conditions Humic and fulvic acid content of soil organic matter Presence of metallic cations Type of clay minerals Soil and water column pH Temperature... 

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