Humics as an electron donor for anaerobic respiration

Abstract

The possibility that microorganisms might use reduced humic substances (humics) as an electron donor for the reduction of electron acceptors with a more positive redox potential was investigated. All of the Fe(III)- and humics-reducing microorganisms evaluated were capable of oxidizing reduced humics and/or the reduced humics analogue anthrahydroquinone-2,6,-disulphonate (AHODS), with nitrate and/or fumarate as the electron acceptor. These included Geobacter metallireducens, Geobacter sulphurreducens, Geothrix fermentans, Shewanella alga, Wolinella succinogenes and 'S. barnesii'. Several of the humics-oxidizing microorganisms grew in medium with AHQDS as the sole electron donor and fumarate as the electron acceptor. Even though it does not reduce Fe(III) or humics, Paracoccus denitrificans could use AHQDS and reduced humics as electron donors for denitrification. However, another denitrifier, Pseudomonas denitrificans, could not. AHODS could also serve as an electron donor for selenate and arsenate reduction by W. succinogenes. Electron spin resonance studies demonstrated that humics oxidation was associated with the oxidation of hydroquinone moieties in the humics. Studies with G. metallireducens and W. succinogenes demonstrated that the anthraquinone-2,6-disulphonate (AQDS)/AHQDS redox couple mediated an interspecies electron transfer between the two organisms. These results suggest that, as microbially reduced humics enter less reduced zones of soils and sediments, the reduced humics may serve as electron donors for microbial reduction of several environmentally significant electron acceptors.

Ferrihydrite-associated organic matter (OM) stimulates reduction by <i>Shewanella oneidensis</i> MR-1 and a complex microbial consortia

“…Previous studies have revealed that some Fe(III)-reducing microorganisms, such as S. alga, G. metallireducens, and Geothrix fermentans, have the ability to undergo dissimilatory humic reduction and to couple reduced humic oxidation with nitrate reduction. Moreover, Paracoccus denitrificans, a known denitrifier, is unable to utilize humics or Fe(III) as a terminal electrons acceptors; however, these microorganisms are capable of using reduced humics as an electron donor for denitrification (Coates et al, 2002;Lovley et al, 1999).…”

Electrical stimulation for enhanced denitrification in woodchip bioreactors: Opportunities and challenges

“…Without being degraded, these substrates can accept J.Y. Law et al Ecological Engineering 110 (2018) 38-47 electrons from the cathode, and then donate to the microorganisms for biodegradation of water pollutants (Lovley et al, 1996;Lovley et al, 1999;Thrash et al, 2007). These substrates include quinones, phenazines, and humic substances (Thrash and Coates, 2008).…”

Electron transfer capacity of soil dissolved organic matter and its potential impact on soil respiration

“…However, Cervantes et al (2000) pointed out that quinone respiration as a novel respiration pathway in an anoxic environment contributed greatly to CO 2 production. The fact that exogenous electroactive molecules can participate in electron transfer indicates that they may make a significant contribution to soil respiration, especially in iron(III) oxide-rich soils (Lovley et al 1999;Newman and Kolter 2000). In anoxic soil environments, conventional electron transfer is affected by limited electron acceptors with a more positive redox potential.…”