Halorespiration

Key takeaways

Halorespiration belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Halorespiration to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Halorespiration from memory before moving on to harder problems.

Reference excerpt

Organohalide respiration (OHR) (previously named halorespiration or dehalorespiration) is the use of halogenated compounds as terminal electron acceptors in anaerobic respiration. Organohalide respiration can play a part in microbial biodegradation. The most common substrates are chlorinated aliphatics (PCE, TCE, chloroform) and chlorinated phenols. Organohalide-respiring bacteria are highly diverse. This trait is found in some Campylobacterota, Thermodesulfobacteriota, Chloroflexota (green nonsulfur bacteria), low G+C gram positive Clostridia, and ultramicrobacteria.

Process of organohalide respiration The process of organohalide respiration uses reductive dehalogenation to produce energy that can be used by the respiring microorganism to carry out its growth and metabolism. Halogenated organic compounds are used as the terminal electron acceptor, which results in their dehalogenation. Reductive dehalogenation is the process by which this occurs. It involves the reduction of halogenated compounds by removing the halogen substituents, while simultaneously adding electrons to the compound. Hydrogenolysis and vicinal reduction are the two known processes of this mechanism that have been identified. In both processes, the removed halogen substituents are released as anions. Reductive dehalogenation is catalyzed by reductive dehalogenases, which are membrane-associated enzymes. A number of not only membrane-associated but also cytoplasmic hydrogenases, in some cases as part of the protein complexes, are predicted to play roles in the organohalide respiration process. Most of these enzymes contain iron-sulfur (Fe-S) clusters, and a corrinoid cofactor at their active sites. Although the exact mechanism is unknown, research suggests that these two components of the enzyme may be involved in the reduction.

Substrates Used and Environmental Significance Common substrates that are used as terminal electron acceptors in organohalide respiration are organochloride pesticides, aryl halides and alkyl solvents. Many of these are persistent pollutants that can only be degraded anaerobically by organohalide respiration, either partially or completely. Trichloroethylene (TCE) and tetrachloroethylene (PCE) are two examples of such pollutants, and their degradation has been a focus of research. PCE is a chlorinated solvent that is widely used in dry cleaning, degreasing machinery and other applications. It remains a common contaminant of groundwater. Bacteria that are capable of completely degrading PCE to ethene, a gaseous chemical, have been isolated. They have been found to belong to the genus Dehalococcoides and to use H2 as their electron donor. The process of organohalide respiration has been applied to in situ bioremediation of PCE and TCE in the past. For example, enhanced reductive dechlorination has been used to treat contaminated groundwater by introducing electron donors and dehalorespiring bacteria into the contaminated site, to create conditions that stimulate bacterial growth and organohalide respiration. In enhanced reductive dechlorination, the pollutants act as the electron acceptors and are completely reduced to ultimately produce ethene in a series of reactions.

Uses in Bioremediation An ecologically significant aspect of bacterial organohalide respiration is the reduction of the two anthropogenic pollutants tetrachloroethylene (PCE) and trichloroethylene (TCE). Their presence as environmental pollutants arose from their common industrial use as metal-degreasing agents from the 1920s–1970. These xenobiotic compounds tend to form partially insoluble layers called dense non-aqueous phase liquids (DNAPLs) at the bottom of groundwater aquifers, which solubilize in a slow, reservoir-like manner, making TCE and PCE among the most common groundwater pollutants. A commonly used strategy for the removal of TCE and PCE from groundwater is the use of bioremediation via enhanced reductive dechlorination (ERD). ERD involves in situ injections of dehalorespiring bacteria, among fermentable organic substrates serving as electron donors, while the two pollutants, TCE and PCE, act as the electron acceptors. This facilitates the sequential dechlorination of PCE and TCE into noxious cis-1,2-Dichloroethylene (DCE) and Vinyl chloride (VC), which then suit as electron acceptors for the full dechlorination into ethylene. A wide array of bacteria across different genera have the capacity to partially dechlorinate PCE and TCE into cis-DCE and VC. One such example of this is the Magnetospirillum bacterium, strain MS-1, which can reduce PCE into cis-DCE under aerobic conditions. However, these daughter substrates have higher toxicity profiles than their parent compounds. As such, effective dechlorination of cis-DCE and VC into innocuous ethene is crucial for bioremediation of PCE and TCE-contaminated aquifers. Currently, bacteria of the Dehalococcoides genera are the only known organisms that can fully dechlorinate PCE into ethylene. This is due to their specific transmembrane reductive dehalogenases (RDases) that metabolize the chlorine atoms on the xenobiotic pollutants for cellular energy. In particular, Dehalococcoides isolates VS and BAV1 encode Vinyl Chloride RDases, which metabolize VC into innocuous ethene, making them required species in ERD systems used in bioremediation of PCE and TCE.

See also Bioaugmentation Reductive dechlorination Chloroflexota Dehalococcoides Dehalobacter

References

Worked examples

Example 1 — a first encounter with Halorespiration

Start with the simplest possible case. Write down what Halorespiration claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Halorespiration before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Halorespiration ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Halorespiration

In research: Halorespiration appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Halorespiration in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Halorespiration is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrochemistry, Respiration, so understanding it makes those chapters shorter.
In everyday life: Look for Halorespiration outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Halorespiration in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Halorespiration means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Halorespiration out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Halorespiration in simple terms?

Organohalide respiration (OHR) (previously named halorespiration or dehalorespiration) is the use of halogenated compounds as terminal electron acceptors in anaerobic respiration. Organohalide respiration can play a part in microbial biodegradation.

Why does Halorespiration matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Halorespiration?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Halorespiration.