Redox gradient

[4][5][6] These redox gradients form both spatially and temporally as a result of differences in microbial processes, chemical composition of the environment, and oxidative potential.

[6][5][1] Typical environments to measure redox potential are in bodies of water, soils, and sediments, all of which can exhibit high levels of heterogeneity.

[8][6] Gradients are caused by varying physiochemical properties including availability of oxygen, soil hydrology, chemical species present, and microbial processes.

[1][4][9][8] Specific environments that are commonly characterized by redox gradients include waterlogged soils, wetlands,[8] contaminant plumes,[9][4] and marine pelagic and hemipelagic sediments.

Benthic sediments exhibit redox gradients produced by variations in mineral composition, organic matter availability, structure, and sorption dynamics.

[1][8][6] In the event of a flood, saturated soils can shift from oxic to anoxic, creating a reducing environment as anaerobic microbial processes dominate.

[8] Redox gradients form based on resource availability and physiochemical conditions (pH, salinity, temperature) and support stratified communities of microbes.

[5] Due to this heterogeneity, gradients of reducing and oxidizing chemical species do not always overlap enough to support electron transport needs of niche microbial communities.

[5] Biofilms, found in tidal flats, glaciers, hydrothermal vents, and at the bottoms of aquatic environments, also exhibit redox gradients.

Depiction of common redox reactions in the environment. Adapted from figures by Zhang [ 1 ] and Gorny. [ 2 ] Redox pairs are listed with the oxidizer (electron acceptor) in red and the reducer (electron donator) in black.
Relative favorability of redox reactions in marine sediments based on energy. Start points of arrows indicate energy associated with half-cell reaction. Lengths of arrows indicate an estimate of Gibb's free energy (ΔG) for the reaction where a higher ΔG is more energetically favorable (Adapted from Libes, 2011). [ 3 ]