Redox gradient

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Depiction of common redox reactions in the environment. Adapted from figures by Zhang and Gorny. Redox pairs are listed with the oxidizer (electron acceptor) in red and the reducer (electron donator) in black. Redox Tower (2).png
Depiction of common redox reactions in the environment. Adapted from figures by Zhang and Gorny. 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 (DG) for the reaction where a higher DG is more energetically favorable (Adapted from Libes, 2011). 2Reduction reaction energetics.png
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).

A redox gradient is a series of reduction-oxidation (redox) reactions sorted according to redox potential. [4] [5] The redox ladder displays the order in which redox reactions occur based on the free energy gained from redox pairs. [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. [5] [4] Common environments where redox gradients exist are coastal marshes, lakes, contaminant plumes, and soils. [1] [4] [5] [6]

Contents

The Earth has a global redox gradient with an oxidizing environment at the surface and increasingly reducing conditions below the surface. [4] Redox gradients are generally understood at the macro level, but characterization of redox reactions in heterogeneous environments at the micro-scale require further research and more sophisticated measurement techniques. [5] [1] [7] [6]

Measuring redox conditions

Redox conditions are measured according to the redox potential (Eh) in volts, which represents the tendency for electrons to transfer from an electron donor to an electron acceptor. Eh can be calculated using half reactions and the Nernst equation. [1] An Eh of zero represents the redox couple of the standard hydrogen electrode H+/H2, [8] a positive Eh indicates an oxidizing environment (electrons will be accepted), and a negative Eh indicates a reducing environment (electrons will be donated). [1] In a redox gradient, the most energetically favorable chemical reaction occurs at the “top” of the redox ladder and the least energetically favorable reaction occurs at the “bottom” of the ladder. [1]

Eh can be measured by collecting samples in the field and performing analyses in the lab, or by inserting an electrode into the environment to collect in situ measurements. [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. [5] [1] Collecting a high number of samples can produce high spatial resolution, but at the cost of low temporal resolution since samples only reflect a singular a snapshot in time. [8] [1] [5] In situ monitoring can provide high temporal resolution by collecting continuous real-time measurements, but low spatial resolution since the electrode is in a fixed location. [1] [5]

Redox properties can also be tracked with high spatial and temporal resolution through the use of induced-polarization imaging, however, further research is needed to fully understand contributions of redox species to polarization. [6]

Environmental conditions

Redox gradients are commonly found in the environment as functions of both space and time, [9] [8] particularly in soils and aquatic environments. [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. [4]

The following is a list of common reactions that occur in the environment in order from oxidizing to reducing (organisms performing the reaction in parentheses): [1]

  1. Aerobic respiration (aerobes: aerobic organisms)
  2. Denitrification (denitrifiers: denitrifying bacteria)
  3. Manganese reduction (Manganese reducers)
  4. Iron reduction (iron reducers: iron-reducing bacteria)
  5. Sulfate reduction (sulfate reducers: Sulfur-reducing bacteria)
  6. Methanogenesis (methanogens)

Aquatic environments

Redox gradients form in water columns and their sediments. Varying levels of oxygen (oxic, suboxic, hypoxic) within the water column alter redox chemistry and which redox reactions can occur. [10] Development of oxygen minimum zones also contributes to formation of redox gradients.

Benthic sediments exhibit redox gradients produced by variations in mineral composition, organic matter availability, structure, and sorption dynamics. [5] Limited transport of dissolved electrons through subsurface sediments, combined with varying pore sizes of sediments creates significant heterogeneity in benthic sediments. [5] Oxygen availability in sediments determines which microbial respiration pathways can occur, resulting in a vertical stratification of redox processes as oxygen availability decreases with depth. [5]

Terrestrial environments

Soil Eh is also largely a function of hydrological conditions. [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. [1] [8] Moreover, small anoxic hotspots may develop within soil pore spaces, creating reducing conditions. [6] With time, the starting Eh of a soil can be restored as water drains and the soil dries out. [1] [8] Soils with redox gradients formed by ascending groundwater are classified as gleysols, while soils with gradients formed by stagnant water are classified as stagnosols and planosols.

Soil Eh generally ranges from −300 to +900 mV. [8] The table below summarizes typical Eh values for various soil conditions: [1] [8]

Soil conditionsTypical Eh range (mV) [1] [8]
WaterloggedEh < +250
Aerated – moderately reduced+100 < Eh < +400
Aerated – reduced−100 < Eh < +100
Aerated – highly reduced−300 < Eh < −100
Cultivated+300 < Eh < +500

Generally accepted Eh limits that are tolerable by plants are +300 mV < Eh < +700 mV. [8] 300 mV is the boundary value that separates aerobic from anaerobic conditions in wetland soils. [1] Redox potential (Eh) is also closely tied to pH, and both have significant influence on the function of soil-plant-microorganism systems. [1] [8] The main source of electrons in soil is organic matter. [8] Organic matter consumes oxygen as it decomposes, resulting in reducing soil conditions and lower Eh. [8]

Role of microorganisms

Redox gradients form based on resource availability and physiochemical conditions (pH, salinity, temperature) and support stratified communities of microbes. [1] [5] [9] [8] [7] Microbes carry out differing respiration processes (methanogenesis, sulfate reduction, etc.) based on the conditions around them and further amplify redox gradients present in the environment. [9] [1] [8] However, distribution of microorganisms cannot solely be determined from thermodynamics (redox ladder), but is also influenced by ecological and physiological factors. [6] [5]

Redox gradients form along contaminant plumes, in both aquatic and terrestrial settings, as a function of the contaminant concentration and the impacts it has on relevant chemical processes and microbial communities. [1] [9] The highest rates of organic pollutant degradation along a redox gradient are found at the oxic-anoxic interface. [1] In groundwater, this oxic-anoxic environment is referred to as the capillary fringe, where the water table meets soil and fills empty pores. Because this transition zone is both oxic and anoxic, electron acceptors and donors are in high abundance and there is a high level of microbial activity, leading to the highest rates of contaminant biodegradation. [1] [9]

Benthic sediments are heterogeneous in nature and subsequently exhibit redox gradients. [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] Cable bacteria have been characterized as sulfide-oxidizing bacteria that assist in connecting these areas of undersupplied and excess electrons to complete the electron transport for otherwise unavailable redox reactions. [5]

Biofilms, found in tidal flats, glaciers, hydrothermal vents, and at the bottoms of aquatic environments, also exhibit redox gradients. [5] The community of microbes—often metal- or sulfate-reducing bacteria—produces redox gradients on the micrometer scale as a function of spatial physiochemical variability. [5]

See sulfate-methane transition zone for coverage of microbial processes in SMTZs.

See also

Related Research Articles

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Redox is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a decrease in the oxidation state. The oxidation and reduction processes occur simultaneously in the chemical reaction.

Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.

<span class="mw-page-title-main">Denitrification</span> Microbially facilitated process

Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denitrification as a type of respiration that reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favorable include nitrate (NO3−), nitrite (NO2−), nitric oxide (NO), nitrous oxide (N2O) finally resulting in the production of dinitrogen (N2) completing the nitrogen cycle. Denitrifying microbes require a very low oxygen concentration of less than 10%, as well as organic C for energy. Since denitrification can remove NO3−, reducing its leaching to groundwater, it can be strategically used to treat sewage or animal residues of high nitrogen content. Denitrification can leak N2O, which is an ozone-depleting substance and a greenhouse gas that can have a considerable influence on global warming.

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The pedosphere is the skin of the Earth and only develops when there is a dynamic interaction between the atmosphere, biosphere, lithosphere and the hydrosphere. The pedosphere is the foundation of terrestrial life on Earth.

<span class="mw-page-title-main">Purple bacteria</span> Group of phototrophic bacteria

Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll a or b, together with various carotenoids, which give them colours ranging between purple, red, brown, and orange. They may be divided into two groups – purple sulfur bacteria and purple non-sulfur bacteria. Purple bacteria are anoxygenic phototrophs widely spread in nature, but especially in aquatic environments, where there are anoxic conditions that favor the synthesis of their pigments.

<span class="mw-page-title-main">Sulfate-reducing microorganism</span> Microorganisms that "breathe" sulfates

Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO2−
4) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S). Therefore, these sulfidogenic microorganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration.

<span class="mw-page-title-main">Sulfur cycle</span> Biogeochemical cycle of sulfur

The important sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. It is important in geology as it affects many minerals and in life because sulfur is an essential element (CHNOPS), being a constituent of many proteins and cofactors, and sulfur compounds can be used as oxidants or reductants in microbial respiration. The global sulfur cycle involves the transformations of sulfur species through different oxidation states, which play an important role in both geological and biological processes. Steps of the sulfur cycle are:

<span class="mw-page-title-main">Sediment–water interface</span> The boundary between bed sediment and the overlying water column

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<span class="mw-page-title-main">Iron-oxidizing bacteria</span> Bacteria deriving energy from dissolved iron

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Beggiatoa is a genus of Gammaproteobacteria belonging to the order Thiotrichales, in the Pseudomonadota phylum. These bacteria form colorless filaments composed of cells that can be up to 200 μm in diameter, and are one of the largest prokaryotes on Earth. Beggiatoa are chemolithotrophic sulfur-oxidizers, using reduced sulfur species as an energy source. They live in sulfur-rich environments such as soil, both marine and freshwater, in the deep sea hydrothermal vents, and in polluted marine environments. In association with other sulfur bacteria, e.g. Thiothrix, they can form biofilms that are visible to the naked eye as mats of long white filaments; the white color is due to sulfur globules stored inside the cells.

Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. Anoxic waters can be contrasted with hypoxic waters, which are low in dissolved oxygen. This condition is generally found in areas that have restricted water exchange.

Microbial metabolism is the means by which a microbe obtains the energy and nutrients it needs to live and reproduce. Microbes use many different types of metabolic strategies and species can often be differentiated from each other based on metabolic characteristics. The specific metabolic properties of a microbe are the major factors in determining that microbe's ecological niche, and often allow for that microbe to be useful in industrial processes or responsible for biogeochemical cycles.

Sulfur is metabolized by all organisms, from bacteria and archaea to plants and animals. Sulfur can have an oxidation state from -2 to +6 and is reduced or oxidized by a diverse range of organisms. The element is present in proteins, sulfate esters of polysaccharides, steroids, phenols, and sulfur-containing coenzymes.

Arsenate-reducing bacteria are bacteria which reduce arsenates. Arsenate-reducing bacteria are ubiquitous in arsenic-contaminated groundwater (aqueous environment). Arsenates are salts or esters of arsenic acid (H3AsO4), consisting of the ion AsO43−. They are moderate oxidizers that can be reduced to arsenites and to arsine. Arsenate can serve as a respiratory electron acceptor for oxidation of organic substrates and H2S or H2. Arsenates occur naturally in minerals such as adamite, alarsite, legrandite, and erythrite, and as hydrated or anhydrous arsenates. Arsenates are similar to phosphates since arsenic (As) and phosphorus (P) occur in group 15 (or VA) of the periodic table. Unlike phosphates, arsenates are not readily lost from minerals due to weathering. They are the predominant form of inorganic arsenic in aqueous aerobic environments. On the other hand, arsenite is more common in anaerobic environments, more mobile, and more toxic than arsenate. Arsenite is 25–60 times more toxic and more mobile than arsenate under most environmental conditions. Arsenate can lead to poisoning, since it can replace inorganic phosphate in the glyceraldehyde-3-phosphate --> 1,3-biphosphoglycerate step of glycolysis, producing 1-arseno-3-phosphoglycerate instead. Although glycolysis continues, 1 ATP molecule is lost. Thus, arsenate is toxic due to its ability to uncouple glycolysis. Arsenate can also inhibit pyruvate conversion into acetyl-CoA, thereby blocking the TCA cycle, resulting in additional loss of ATP.

Dissimilatory nitrate reduction to ammonium (DNRA), also known as nitrate/nitrite ammonification, is the result of anaerobic respiration by chemoorganoheterotrophic microbes using nitrate (NO3−) as an electron acceptor for respiration. In anaerobic conditions microbes which undertake DNRA oxidise organic matter and use nitrate (rather than oxygen) as an electron acceptor, reducing it to nitrite, and then to ammonium (NO3− → NO2− → NH4+).

Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified; have an oxic, highly productive, thin surface layer; and have anoxic, sulfidic bottom water. The word "euxinia" is derived from the Greek name for the Black Sea (Εὔξεινος Πόντος (Euxeinos Pontos)) which translates to "hospitable sea". Euxinic deep water is a key component of the Canfield ocean, a model of oceans during part of the Proterozoic eon (a part specifically known as the Boring Billion) proposed by Donald Canfield, an American geologist, in 1998. There is still debate within the scientific community on both the duration and frequency of euxinic conditions in the ancient oceans. Euxinia is relatively rare in modern bodies of water, but does still happen in places like the Black Sea and certain fjords.

<span class="mw-page-title-main">Microbial oxidation of sulfur</span>

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An oxygen minimum zone (OMZ) is characterized as an oxygen-deficient layer in the world's oceans. Typically found between 200 m to 1500 m deep below regions of high productivity, such as the western coasts of continents. OMZs can be seasonal following the spring-summer upwelling season. Upwelling of nutrient-rich water leads to high productivity and labile organic matter, that is respired by heterotrophs as it sinks down the water column. High respiration rates deplete the oxygen in the water column to concentrations of 2 mg/L or less forming the OMZ. OMZs are expanding, with increasing ocean deoxygenation. Under these oxygen-starved conditions, energy is diverted from higher trophic levels to microbial communities that have evolved to use other biogeochemical species instead of oxygen, these species include nitrate, nitrite, sulphate etc. Several Bacteria and Archea have adapted to live in these environments by using these alternate chemical species and thrive. The most abundant phyla in OMZs are Pseudomonadota, Bacteroidota, Actinomycetota, and Planctomycetota.

References

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