Microbially Driven Redox Reactions in Anoxic Environments: Pathways, Energetics, and Biochemical Consequences

Abstract

After consumption of molecular oxygen, anaerobic microbial communities can use a continuum of alternative electron acceptors such as nitrate, manganese oxides, iron oxides, sulfate or CO 2 , with decreasing spans of available free energy. The electron transfer to insoluble metal oxides or to partner organisms such as methanogens may require the employment of electron carrier systems such as soil organic matter or sulfur compounds, with consequences for the reaction kinetics. The redox potentials of the electron acceptor systems do not only influence the reaction energetics but may also determine the pathways of degradation, especially in the degradation of organic contaminants. These aspects are discussed with examples of aromatic compounds, in particular phenols and cresols. The results demonstrate that beyond the mere lack of oxygen availability also the redox potential of the electron acceptor system in play determines to a large extent the kinetics, energetics and biochemistry of anaerobic transformation processes.

Inhibition of Cellulase-Catalyzed Lignocellulosic Hydrolysis by Iron and Oxidative Metal Ions and Complexes

“…Redox chemistry not directly related to microbial lignocellulolysis might also impact biodegradation. Redox-active Fe and Mn species and other diffusible redox mediators are involved in Fe(III) and Mn(IV) reducers' anaerobic respirations and anoxygenic photosynthesis (4,34). Such redox agents from nonlignocellulolytic cohabitants of local microbiota could affect anaerobic or even aerobic (depending on the local O 2 distribution) lignocellulolytic microbes and their (hemi)cellulases, either "unintentionally" or competitively.…”

Dehalococcoides mccartyi gen. nov., sp. nov., obligately organohalide-respiring anaerobic bacteria relevant to halogen cycling and bioremediation, belong to a novel bacterial class, Dehalococcoidia classis nov., order Dehalococcoidales ord. nov. and family Dehalococcoidaceae fam. nov., within the phylum Chloroflexi

“…A plausible explanation is the use of naturally occurring organohalides, which have been observed in a variety of habitats including marine ecosystems, sediments, subsurface environments and soils (Ö berg, 2002;Gribble, 2003;Krzmarzick et al, 2012). The production of organohalogens in soils has been demonstrated (de Jong et al, 1994;Hoekstra et al, 1999), and anoxic microenvironments occur in unsaturated soils (Schink, 2006), suggesting that the habitat range of Dehalococcoides probably extends beyond anoxic sediments, sludges and saturated aquifers, which have been the source materials for enrichment efforts to date. Moreover, the presence of 12-36 sets of genes predicted to encode RDases indicates that this genus is highly adapted to use naturally occurring as well as anthropogenic organohalogens.…”

Subsurface Petroleum Microbiology

“…Anaerobic microbes gain considerably less energy from substrate biotransformation processes and, as a result, form much smaller amounts of biomass compared to aerobic microbes (Widdel and Rabus 2001). Aerobic degradation usually proceeds more rapidly and efficiently; consequently aerobic reactions require less free energy for initiation and yield more energy per reaction (Schink 2006;Wentzel et al 2007). Laboratory experiments reveal that the cultivation of microorganisms with hydrocarbons as growth substrates under anaerobic conditions is more demanding with slower growth rates than cultivation of conventional anaerobes that grow along with facultative aerobic microorganisms (Widdel 2010).…”