Microbial Metabolism: Importance for Environmental Biotechnology

Abstract

Microorganisms are the main agents responsible for biogeochemical transformations of carbon, nitrogen, sulfur, iron, and other elements. The prokaryotic world (domains Archaea and Bacteria) presents us with a far larger variety of metabolic types than are found among the eukaryotes (fungi, higher plants, protozoa and animals). The range of substrates used by prokaryotes as carbon sources for growth (assimilatory metabolism) is far greater than in the eukaryotic world. In addition, many groups of prokaryotes perform types of energy generation (dissimilatory reactions) that are altogether unknown among the eukaryotes. This chapter provides a general overview of the metabolism of microorganisms, with special emphasis on the prokaryotic world. Processes such as oxygenic and anoxygenic photosynthesis, aerobic and anaerobic respiration, and chemolithotrophic metabolism are discussed. Finally, it is shown how these processes together enable the functioning of the biogeochemical cycles of the elements on Earth.

Appendix: Compounds of Environmental Significance and the Microbial Processes Responsible for their Formation and Degradation

Below follows a nonexhaustive list of compounds formed during the dissimilatory metabolism of prokaryotic organisms (Bacteria as well as Archaea), with special emphasis on those compounds of importance in environmental engineering. Information is also provided on those microbial processes (assimilatory as well as dissimilatory) responsible for the disappearance of these compounds. Reference is made to the appropriate sections in the text above in which the nature of the respective processes was discussed in further depth.

1.1 Compounds of Carbon, Hydrogen, and Oxygen

1.1.1 Hydrogen (H₂)

Hydrogen is a characteristic end product of fermentation by anaerobic bacteria (representatives of the genus Clostridium and many others). It can be formed in ferredoxin-mediated reactions such as the oxidative decarboxylation of pyruvate to acetyl-CoA and/or by action of hydrogenase, using reducing equivalents from NADH (see Sect. 5.3.). Hydrogen is also excreted by syntrophic bacteria, such as Syntrophomonas and Syntrophobacter, in the course of the oxidation of organic acids and other compounds (see Sect. 5.7.). Minor amounts of hydrogen are formed also as a byproduct of nitrogenase activity in all nitrogen-fixing prokaryotes.

Hydrogen seldom accumulates at large concentrations in nature as it is effectively used by a variety of sulfate-reducing bacteria (Sect. 5.5.), methanogenic bacteria (Sect. 5.6.), and homoacetogenic bacteria (Sect. 5.6.) (all under anaerobic conditions), and by aerobic chemolithotrophic hydrogen oxidizers (“Knallgas bacteria”) (Sect. 6.4.).

1.1.2 Oxygen (O₂)

Molecular oxygen is formed as a byproduct of photosynthesis by oxygenic prokaryotes (Cyanobacteria) (see Sect. 4.1.), eukaryotic microalgae, macroalgae, and terrestrial plants.

Oxygen is the terminal electron acceptor in aerobic respiration, enabling degradation of about every biodegradable organic compound, as well as the chemoautotrophic oxidation of reduced nitrogen and sulfur compounds to nitrate and sulfate, respectively.

1.1.3 Carbon Dioxide (CO₂)

CO₂ is the end product of oxidation of organic material by organisms that perform aerobic respiration (animals, fungi, many bacteria) or anaerobic respiration with nitrate or sulfate as electron acceptor (see Sects. 5.1., 5.2., and 5.5.). CO₂ is also released in the course of many fermentative processes together with organic fermentation products (Sect. 5.3.), and in disproportionation reactions mediated by methanogenic Archaea, such as methanogenesis, from formate (Sect. 5.6.).

Most assimilation of carbon dioxide occurs through the Calvin cycle, with ribulose bisphosphate carboxylase (RuBisCO) as the key enzyme (Sect. 3.1.). This is true both for photoautotrophs and for chemoautotrophs. Alternative modes of autotrophic fixation exist in certain groups of microorganisms such as the green sulfur bacteria, the methanogenic Archaea, and others. Carboxylation reactions, such as the carboxylation of phosphoenolpyruvate to oxalacetate, incorporate carbon dioxide into cellular carbon also in heterotrophic organisms.

1.1.4 Methane (CH₄)

Methane is formed only by a specialized group of Archaea as the end product of their energy-yielding reactions. The major precursors for methane are acetate, which is split into methane and carbon dioxide, and the reduction of carbon dioxide by molecular hydrogen (see Sect. 5.6.). Methane can also be formed from formate, from methanol, from methylated amines, and from dimethylsulfide.

Methane is oxidized aerobically by methanotrophic bacteria. Anaerobic methane oxidation is possible as well in a yet incompletely understood process performed by a consortium of Archaea and Bacteria in which methane oxidation is coupled with the reduction of sulfate to sulfide (see Sect. 5.7.).

1.1.5 Carbon Monoxide (CO)

No microorganisms are known that release CO into the environment. Carbon monoxide is an intermediate in the autotrophic fixation of CO₂ in certain autotrophs that do not use the reactions of the Calvin cycle (some sulfate-reducing bacteria, some methanogenic Archaea), and as such remains intracellular (see Sect. 3.1.).

Carbon monoxide can be metabolized by a variety of microorganisms, aerobic as well as anaerobic. Some aerobic chemoautotrophs can obtain their energy by the oxidation of CO to CO₂. Anaerobically, CO can be converted to methane. Another anaerobic energy-yielding pathway, performed by a number of thermophilic representatives of the Bacteria, is its oxidation to CO₂ with concomitant formation of hydrogen (see Sect. 6.4.).

1.1.6 Short-Chain Organic Acids

1.1.6.1 Formic Acid (HCOOH)

Formate is produced by pyruvate:formate lyase in a variety of fermentative processes, including, e.g., the anaerobic degradation of sugars by Escherichia coli under anaerobic conditions (see Sect. 5.3.).

Formate can be oxidized aerobically by a variety of bacteria. Anaerobically, it can be converted to a mixture of methane and carbon dioxide in a disproportionation reaction performed by methanogenic Archaea (see Sect. 5.6.). Alternatively, it may serve as electron donor for denitrifying bacteria or for certain sulfate-reducing bacteria.

1.1.6.2 Acetic Acid (${\mathrm{CH}}_{3}\mbox{ \textendash }\mathrm{COOH}$)

Acetate is formed as a major fermentation product by many carbohydrate- and amino acid-fermenting bacteria (see Sect. 5.3.) and by proton-reducing acetogens in syntrophic partnerships (see Sect. 5.7.). In addition, homoacetogenic bacteria form acetate under anaerobic conditions by reducing carbon dioxide with hydrogen as electron donor (see Sect. 5.6.). Moreover, acetate can be formed in incomplete oxidation processes, aerobically as well as anaerobically. Aerobic acetic acid bacteria, such as Acetobacter, oxidize ethanol to acetate with molecular oxygen as electron acceptor (Sect. 5.1.). Anaerobically, incomplete oxidation of lactate, propionate, and other organic acids by dissimilatory sulfate reducing bacteria using sulfate as electron acceptor leads to acetate formation (see Sect. 5.5.).

Acetate can be oxidized to carbon dioxide using oxygen, nitrate, sulfate, or trivalent iron as electron acceptors. Certain methanogenic Archaea split acetate into methane and carbon dioxide (see Sect. 5.6.). At high temperatures, acetate can be oxidized anaerobically to carbon dioxide with the release of molecular hydrogen in a process that has to be coupled with hydrogen consumption to be energetically feasible (see also Sect. 5.7.). Acetate is also used as an assimilatory carbon source by many aerobic bacteria, by facultative or obligatory photoheterotrophs, and by mixotrophic oxidizers of reduced sulfur compounds.

1.1.6.3 Propionic Acid (${\mathrm{CH}}_{3}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }\mathrm{COOH}$)

Propionate is a characteristic fermentation product, made from sugars or from lactate by a specialized group of propionic acid bacteria (Propionibacterium, Selenomonas, Megasphaera) (see Sect. 5.3.).

Propionate can be degraded aerobically by aerobic respiration, and anaerobically by denitrifying bacteria or sulfate-reducing bacteria such as Desulfobulbus, which oxidizes propionate incompletely to acetate + carbon dioxide (see Sect. 5.5.). The proton-reducing acetogen, Syntrophobacter, converts propionate to acetate + carbon dioxide as well as hydrogen, which has to be efficiently removed for the process to be energetically favorable (see Sect. 5.7.).

1.1.6.4 Butyric Acid (${\mathrm{CH}}_{3}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }\mathrm{COOH}$)

Butyrate is a fermentation product excreted by many fermentative prokaryotes growing on sugar or amino acids (see Sect. 5.3.).

Butyrate can be oxidized to carbon dioxide by many aerobic bacteria. Under anaerobic conditions, butyrate can be converted to carbon dioxide by denitrification (Sect. 5.2.) or by certain sulfate-reducing bacteria (Sect. 5.5.). When no electron acceptors are available, Syntrophomonas converts butyrate to acetate and hydrogen in process that is thermodynamically favorable only if the hydrogen formed is efficiently removed by a syntrophic partner (see Sect. 5.7.).

1.1.6.5 Lactic Acid (${\mathrm{CH}}_{3}\mbox{ \textendash }\mathrm{CHOH}\mbox{ \textendash }\mathrm{COOH}$)

Lactate is a product of fermentation by specialized lactic acid bacteria: homolactic organisms such as Streptococcus and many Lactobacillus species, and heterolactic species such as Leuconostoc, which produce a mixture of lactate, ethanol, and carbon dioxide. It is formed during other fermentations as well, such as the mixed acid fermentation of Escherichia coli and relatives under anaerobic conditions (see Sect. 5.3.).

Lactate can be degraded by aerobic respiration, by anaerobic respiration with nitrate as electron acceptor, as well as by sulfate-reducing bacteria, such as Desulfovibrio, that degrade lactate incompletely to acetate + carbon dioxide (see Sect. 5.5.). The sulfate-reducing thermophile Archaeoglobus performs complete oxidation of lactate to carbon dioxide using sulfate as electron acceptor. Lactate can also be fermented further under anaerobic conditions to a mixture of propionate, acetate, and carbon dioxide (Sect. 5.3.).

1.1.6.6 Succinic Acid ($\mathrm{COOH}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }\mathrm{COOH}$)

Succinate is formed as a minor fermentation product in the mixed acid fermentation of enteric bacteria such as Escherichia coli, and is also formed by anaerobic bacteria such as Bacteroides, Ruminobacter, and Succinomonas that live in the digestive system of animals (see Sect. 5.3.). Succinate can also be formed anaerobically as the product of anaerobic reduction of fumarate used as electron acceptor in respiration.

Succinate can be oxidized aerobically and anaerobically (by denitrification) to carbon dioxide. Moreover, it can be fermented to propionate + carbon dioxide by certain propionic acid bacteria (Propionigenium, Schwartzia) (Sect. 5.3.).

1.1.7 Ethanol (${\mathrm{CH}}_{3}\mbox{ \textendash }{\mathrm{CH}}_{2}\mathrm{OH}$)

Ethanol is formed in many fermentation processes, both in eukaryotes (the alcohol fermentation of yeasts) and prokaryotes (Zymomonas, heterolactic fermenters such as Leuconostoc, and also as a minor product in the fermentation of enteric bacteria and some clostridia; see Sect. 5.3.).

Ethanol can be oxidized aerobically (complete oxidation to CO₂ or incomplete oxidation to acetate by acetic acid bacteria; see Sect. 5.1.). Under anaerobic conditions, ethanol can be oxidized to CO₂ while using nitrate as electron acceptor, to acetate by sulfate-reducing bacteria such as Desulfovibrio (see Sect. 5.5.), or by proton-reducing acetogens under the excretion of molecular hydrogen (see Sect. 5.7.).

1.1.8 Isopropanol (${\mathrm{CH}}_{3}\mbox{ \textendash }\mathrm{CHOH}\mbox{ \textendash }{\mathrm{CH}}_{3}$)

Isopropanol is formed as a minor product during carbohydrate fermentation by certain species of Clostridium and related organisms (see Sect. 5.3.).

Isopropanol can be oxidized aerobically to CO₂. In the absence of oxygen, it can serve as electron donor for the reduction of carbon dioxide to methane in certain methanogenic Archaea.

1.1.9 n-Butanol (${\mathrm{CH}}_{3}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mathrm{OH}$)

n-Butanol is often formed during fermentation of carbohydrates by Clostridium species (see Sect. 5.3.).

Butanol can be oxidized aerobically and anaerobically to CO₂ with oxygen, nitrate, or sulfate as electron acceptors.

1.1.10 Acetone (${\mathrm{CH}}_{3}\mbox{ \textendash }\mathrm{CO}\mbox{ \textendash }{\mathrm{CH}}_{3}$)

Acetone is a minor product of some fermentation processes, e.g., the fermentation of carbohydrates by Clostridium acetobutylicum (see Sect. 5.3.). Furthermore, it can be formed by certain methanogenic bacteria from isopropanol that may serve as electron donor for methanogenesis.

Acetone can be oxidized aerobically by oxidation to hydroxyacetone by means of a monooxygenase, followed by oxidation to pyruvate. Anaerobic degradation by certain denitirying bacteria is possible in a pathway initiated by carboxylation to acetoacetate.

1.2 Nitrogen-Containing Compounds

1.2.1 Ammonium (NH₄ ⁺ )

Ammonium ions are generated as the result of aerobic as well as anaerobic degradation of amino acids and other organic compounds containing reduced nitrogen such as the purine and pyrimidine bases present in nucleic acids (ammonification, see Sect. 6.1.). Ammonium ions can also be formed in the dissimilatory process of nitrate reduction, but nitrate ammonification is less common than denitrification with the formation of dinitrogen and nitrous oxide.

Ammonium can be used both aerobically and anaerobically for assimilatory purposes as nitrogen source, and can also serve as energy source in dissimilatory processes: nitrification (under aerobic conditions, where it is oxidized to nitrite), or anaerobic ammonium oxidation (the “anammox” process) in which nitrite serves as electron acceptor (see Sect. 6.1.).

1.2.2 Nitrite (NO₂ ⁻ )

Nitrite can be formed aerobically as the product of the oxidation of ammonium ions in the first step of autotrophic nitrification by Nitrosomonas and related organisms as well as by ammonium-oxidizing Archaea (see Sect. 6.1.). Anaerobically, nitrite can accumulate as an intermediate in denitrification processes during the reduction of nitrate. Certain bacteria, such as Escherichia coli, anaerobically reduce nitrate to nitrite as end product. Minor amounts of nitrite may also originate from –NO₂ residues during the aerobic breakdown of organic nitro compounds.

Nitrite can be used as nitrogen source for assimilatory purposes by a variety of photosynthetic and nonphotosynthetic microorganisms. Furthermore, it serves as energy source for autotrophic nitrifiers such as Nitrobacter (see Sect. 6.1.). Anaerobically, it is reduced via nitric oxide and nitrous oxide to dinitrogen in the process of denitrification (see Sect. 5.2.), or it may be used as the electron acceptor in anaerobic oxidation of ammonium ions (the “anammox” reaction, see Sect. 6.1.).

1.2.3 Nitrate (NO₃ ⁻ )

Nitrate is the end product of autotrophic nitrification in which ammonium ions are aerobically oxidized via nitrite to nitrate. Minor amounts of nitrate may be formed anaerobically by the “anammox” bacteria, which use nitrite as electron donor to provide electrons for autotrophic fixation of carbon dioxide (see Sect. 6.1.).

Nitrate can be consumed both in assimilatory processes when it serves as nitrogen source to plants, microalgae, and many aerobic bacteria (Sect. 3.2.), as well as in dissimilatory processes: nitrate respiration – denitrification with the formation of more reduced products: nitrite, nitric oxide, nitrous oxide, dinitrogen, or ammonium ions (Sect. 5.2.).

1.2.4 Dinitrogen (N₂)

Nitrogen is the major end product of denitrification – the dissimilatory reduction of nitrate and nitrite under anaerobic conditions (see Sect. 5.2.) – as well as the product of anaerobic oxidation of ammonium ions with nitrite as electron acceptor in the “anammox” process (see Sect. 6.1.).

Nitrogen can be used as a nitrogen source for assimilatory purposes by a limited number of prokaryotes, many of them living in symbiotic associations with higher organisms, in an energy-expensive process catalyzed by the enzyme nitrogenase (see Sect. 3.2.).

1.2.5 Nitrous Oxide (N₂O)

Nitrous oxide is a product of dissimilatory nitrate respiration – denitrification (see Sect. 5.2.), and is generally found as a minor end product besides dinitrogen. There are also indications that activity of nitrifying bacteria may be responsible for the formation of part of the nitrous oxide present in the marine environment.

Nitrous oxide can be further reduced to dinitrogen during denitrification.

1.2.6 Trimethylamine [(CH₃)₃N] and Other Methylated Amines

Trimethylamine and other methylated amines can be formed during degradation of choline (a component of the lipid phosphatidylcholine) or glycine betaine, a compound found as an intracellular osmotic stabilizer in many halophilic and halotolerant microorganisms inhabiting hypersaline environments.

Methylated amines can be oxidized aerobically by a variety of methylotrophic bacteria. Anaerobically, they can be used as energy source by many methanogenic Archaea with the production of methane, carbon dioxide, and ammonium ions.

1.2.7 Putrescine (${\mathrm{NH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{NH}}_{2}$), Cadaverine (${\mathrm{NH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{NH}}_{2}$), Agmatine (${\mathrm{NH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }{\mathrm{CH}}_{2}\mbox{ \textendash }\mathrm{NH}\mbox{ \textendash }\mathrm{C}({\mathrm{NH}}_{2}) = \mathrm{NH}$), and Related Organic Amines

Putrescine, cadaverine, agmatine, and related bad-smelling compounds can be formed by decarboxylation of amino acids (ornithine, lysine, and arginine, respectively) by a variety of anaerobic fermentative bacteria.

Little is known about the further metabolism of these compounds in the absence of molecular oxygen. Putrescine can be fermented to acetate, butyrate, hydrogen, and ammonium ions. The amines can all be oxidized to carbon dioxide and ammonium ions under aerobic conditions.

1.3 Sulfur-Containing Compounds

1.3.1 Hydrogen Sulfide (H₂S)

Hydrogen sulfide may be formed by desulfurylation during anaerobic degradation of amino acids (cysteine, methionine) and other organic sulfur compounds. In addition, major amounts of sulfide are produced as the end product of dissimilatory reduction of sulfate, elemental sulfur, and other oxidized inorganic sulfur compounds under anaerobic conditions (see Sect. 5.5.).

Sulfide is unstable under aerobic conditions and is oxidized abiotically in the presence of molecular oxygen. Moreover, it serves as the energy source for chemolithotrophic aerobic sulfide oxidizers such as Thiobacillus and Beggiatoa (see Sect. 6.2.). Anaerobically, sulfide can be oxidized by green and purple phototrophic sulfur bacteria, in which it serves as electron donor for autotrophic fixation of carbon dioxide (Sect. 4.2.), or by certain denitrifying sulfide oxidizers, in which it acts both as energy source and as electron donor for autotrophic growth (Sect. 6.2.).

1.3.2 Sulfate (SO₄ ^{2 −} )

Sulfate is formed as the end product of both photosynthetic (Sect. 4.2.) and chemosynthetic (Sect. 6.2.) oxidation of sulfide and other reduced sulfur compounds. Photosynthetic sulfide oxidation occurs in anaerobic environments in which sufficient light is available to serve as energy source. Chemoautotrophic sulfur oxidation occurs aerobically, but can also proceed anaerobically in the presence of nitrate as electron acceptor.

Sulfate can be used as source of sulfur for assimilatory purposes by plants, algae, and many bacteria (see Sect. 3.4.). Sulfate is also used as terminal electron acceptor for anaerobic respiration by sulfate-reducing bacteria (Sect. 5.5.).

1.3.3 Elemental Sulfur (S⁰)

Sulfur can be formed both by the abiotic oxidization of hydrogen sulfide and as an intermediate during the oxidation of sulfide to sulfate by green and purple photosynthetic bacteria (see Sect. 4.2.).

Under aerobic conditions, elemental sulfur is used as electron donor and energy source by chemolithotrophic bacteria (Bacteria of the Thiobacillus group; at high temperatures Archaea such as Sulfolobus), causing acidification of the medium (see Sect. 6.2.). Under anaerobic conditions, elemental sulfur can be an electron donor to photosynthetic green and purple bacteria, which oxidize it to sulfate (Sect. 4.2.), or as an electron acceptor in anaerobic respiration by Bacteria such as Desulfuromonas or a variety of thermophilic Archaea (Sect. 5.5.).

1.3.4 Dimethylsulfide (${\mathrm{CH}}_{3}\mbox{ \textendash }\mathrm{S}\mbox{ \textendash }{\mathrm{CH}}_{3}$) and Methylmercaptan (${\mathrm{CH}}_{3}\mbox{ \textendash }\mathrm{SH}$)

Dimethylsulfide and methylmercaptan (methylsulfide) can be produced during the anaerobic degradation of the amino acid methionine and other organic compounds that contain reduced sulfur. A major source of dimethylsulfide in the marine environment is the degradation of DMSP, an intracellular osmotic stabilizer of many marine algae. Dimethylsulfide can also be formed as the product of anaerobic respiration processes with dimethylsulfoxide as electron acceptor. Finally, anaerobic degradation of methoxylated aromatic compounds in the presence of hydrogen sulfide can lead to the formation of dimethylsulfide.

Under aerobic conditions, dimethylsulfide can be oxidized by chemolithotrophic sulfur oxidizers and by methylotrophs, leading to the formation of carbon dioxide and sulfate. In the absence of molecular oxygen, dimethylsulfide can be used as energy source by certain methanogenic Archaea.

1.3.5 Other Elements

1.3.5.1 Iron Oxides

Oxidized forms of iron (Fe^{3 +} ) are formed as the result of the chemolithotrophic oxidation of divalent iron by bacteria such as Acidithiobacillus ferrooxidans (see Sect. 6.3.). Massive accumulations of iron hydroxides [Fe(OH)₃ and other forms] are often found in mine drainage waters, accompanied by low pH caused by autotrophic oxidation of reduced sulfur compounds (pyrite and others) present in many ores. Another organism that deposits trivalent iron is the autotrophic Gallionella, which produces iron oxide stalks. An intermediate state of oxidation as magnetite (Fe₃O₄) is found intracellularly in magnetotactic bacteria (see Sect. 3.5.).

Under anaerobic conditions, trivalent iron can be reduced to divalent iron by iron-reducing bacteria such as Geobacter and Shewanella (see Sect. 5.4.).

1.3.5.2 Manganese (Mn^{2 +} , Mn^{4 +} )

Oxidized forms of manganese (Mn^{4 +} ) are formed as the result of the chemolithotrophic oxidation of divalent manganese (see Sect. 6.3.).

Under anaerobic conditions, tetravalent manganese can be reduced to the divalent form in anaerobic respiration processes.

1.3.5.3 Selenate (SeO₄ ^{2 −} ), Selenite (SeO₃ ^{2 −} ), and Elemental Selenium (Se^o)

Selenate can be used as an electron acceptor for anaerobic respiration, and is respired to selenite (SeO₃ ^{2 −} ) or to a mixture of selenite and elemental selenium. Furthermore, it can be taken up for assimilatory use by many microorganisms and used in the biosynthesis of selenocysteine, an unusual amino acid that is incorporated into some proteins.

1.3.5.4 Arsenate (AsO₄ ^{3 −} ) and Arsenite (AsO₃ ^{3 −} )

Arsenate can be used as an electron acceptor for anaerobic respiration by a number of bacteria, who reduce it to arsenite (AsO₃ ^{3 −} ).

Arsenite can be used as an electron donor for chemoautotrophic arsenite oxidizers, both under aerobic conditions and anaerobically, using nitrate as electron acceptor, causing its oxidation to arsenate (AsO₄ ^{3 −} ).