{{Short description|Type of anaerobic respiration}} {{More citations needed|date=October 2008}} '''Organohalide respiration (OHR)''' (previously named '''halorespiration''' or '''dehalorespiration''') is the use of halogenated compounds as terminal electron acceptors in anaerobic respiration.<ref name="Hollinger1998">{{Cite journal | last1 = Holliger | first1 = C. | last2 = Wohlfarth | first2 = G. | last3 = Diekert | first3 = G. | doi = 10.1111/j.1574-6976.1998.tb00377.x | title = Reductive dechlorination in the energy metabolism of anaerobic bacteria | journal = FEMS Microbiology Reviews | volume = 22 | issue = 5 | page = 383 | year = 1998 | url = https://doc.rero.ch/record/295119/files/22-5-383.pdf }}</ref><ref>{{Cite journal|last1=Jugder|first1=Bat-Erdene|last2=Ertan|first2=Haluk|last3=Bohl|first3=Susanne|last4=Lee|first4=Matthew|last5=Marquis|first5=Christopher P.|last6=Manefield|first6=Michael|date=2016|title=Organohalide Respiring Bacteria and Reductive Dehalogenases: Key Tools in Organohalide Bioremediation|journal=Frontiers in Microbiology|language=en|volume=7|page=249|doi=10.3389/fmicb.2016.00249|issn=1664-302X|pmc=4771760|pmid=26973626|doi-access=free}}</ref><ref name="sciencedirect.com">{{Cite journal|date=2015-10-01|title=Reductive Dehalogenases Come of Age in Biological Destruction of Organohalides|journal=Trends in Biotechnology|language=en|volume=33|issue=10|pages=595–610|doi=10.1016/j.tibtech.2015.07.004|pmid=26409778|issn=0167-7799|last1=Jugder|first1=Bat-Erdene|last2=Ertan|first2=Haluk|last3=Lee|first3=Matthew|last4=Manefield|first4=Michael|last5=Marquis|first5=Christopher P.}}</ref> 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,<ref name="Hiraishi2008">{{Cite journal | last1 = Hiraishi | first1 = A. | title = Biodiversity of Dehalorespiring Bacteria with Special Emphasis on Polychlorinated Biphenyl/Dioxin Dechlorinators | doi = 10.1264/jsme2.23.1 | journal = Microbes and Environments | volume = 23 | issue = 1 | pages = 1–12 | year = 2008 | pmid = 21558680| doi-access = free }}</ref> and ultramicrobacteria.<ref>{{cite journal | doi = 10.1134/S0026261712040054 | volume=81 | issue=4 | title=Ultramicrobacteria: Formation of the concept and contribution of ultramicrobacteria to biology | year=2012 | journal=Microbiology | pages=379–390 | last1 = Duda | first1 = V.I. | last2 = Suzina | first2 = N.E. | last3 = Polivtseva | first3 = V.N. | last4 = Boronin | first4 = A.M.| pmid=23156684 | s2cid=6391715 }}</ref>
== 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.<ref name=":22">{{Cite journal|last1=Futagami|first1=Taiki|last2=Goto|first2=Masatoshi|last3=Furukawa|first3=Kensuke|date=2008-01-01|title=Biochemical and genetic bases of dehalorespiration|journal=The Chemical Record|language=en|volume=8|issue=1|pages=1–12|doi=10.1002/tcr.20134|pmid=18302277|issn=1528-0691|doi-access=free}}</ref> Halogenated organic compounds are used as the terminal electron acceptor, which results in their dehalogenation.<ref name=":22" /> Reductive dehalogenation is the process by which this occurs.<ref name=":22" /> It involves the reduction of halogenated compounds by removing the halogen substituents, while simultaneously adding electrons to the compound.<ref name=":3">{{Cite journal|last1=Mohn|first1=W.W.|last2=Tiedje|first2=J.M.|date=September 1992|title=Microbial reductive dehalogenation|journal=Microbiological Reviews|volume=56|issue=3|pages=482–507|issn=0146-0749|pmid=1406492|pmc=372880|doi=10.1128/mmbr.56.3.482-507.1992}}</ref> Hydrogenolysis and vicinal reduction are the two known processes of this mechanism that have been identified.<ref name=":3" /> In both processes, the removed halogen substituents are released as anions.<ref name=":3" /> Reductive dehalogenation is catalyzed by reductive dehalogenases, which are membrane-associated enzymes.<ref name=":22" /><ref name=":5">{{Cite journal |last=Scheutz |first=Charlotte |last2=Durant |first2=Neal d. |last3=Dennis |first3=Philip |last4=Hansen |first4=Maria Heisterberg |last5=Jørgensen |first5=Torben |last6=Jakobsen |first6=Rasmus |last7=Cox |first7=Evan e. |last8=Bjerg |first8=Poul L. |date=2008-12-15 |title=Concurrent Ethene Generation and Growth of Dehalococcoides Containing Vinyl Chloride Reductive Dehalogenase Genes During an Enhanced Reductive Dechlorination Field Demonstration |url=https://pubs.acs.org/doi/10.1021/es800764t |journal=Environmental Science & Technology |language=en |volume=42 |issue=24 |pages=9302–9309 |bibcode=2008EnST...42.9302S |doi=10.1021/es800764t |issn=0013-936X|url-access=subscription }}</ref><ref name="sciencedirect.com"/> 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.<ref>{{Cite journal|last1=Jugder|first1=Bat-Erdene|last2=Ertan|first2=Haluk|last3=Wong|first3=Yie Kuan|last4=Braidy|first4=Nady|last5=Manefield|first5=Michael|last6=Marquis|first6=Christopher P.|last7=Lee|first7=Matthew|date=2016-08-10|title=Genomic, transcriptomic and proteomic analyses of Dehalobacter UNSWDHB in response to chloroform|journal=Environmental Microbiology Reports|volume=8|issue=5|pages=814–824|doi=10.1111/1758-2229.12444|pmid=27452500|bibcode=2016EnvMR...8..814J |issn=1758-2229|hdl=1959.4/unsworks_46701|hdl-access=free}}</ref> Most of these enzymes contain iron-sulfur (Fe-S) clusters, and a corrinoid cofactor at their active sites.<ref name=":22" /> Although the exact mechanism is unknown, research suggests that these two components of the enzyme may be involved in the reduction.<ref name=":22" />
=== 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.<ref name=":3" /> Many of these are persistent pollutants that can only be degraded anaerobically by organohalide respiration, either partially or completely.<ref name=":22" /><ref name=":3" /> Trichloroethylene (TCE) and tetrachloroethylene (PCE) are two examples of such pollutants, and their degradation has been a focus of research.<ref name=":22" /><ref name=":3" /><ref name=":4">{{Cite journal|last1=Maymó-Gatell|first1=X.|last2=Chien|first2=Y.|last3=Gossett|first3=J.M.|last4=Zinder|first4=S.H.|date=1997-06-06|title=Isolation of a bacterium that reductively dechlorinates tetrachloroethene to ethene|journal=Science|volume=276|issue=5318|pages=1568–1571|issn=0036-8075|pmid=9171062|doi=10.1126/science.276.5318.1568}}</ref> PCE is a chlorinated solvent that is widely used in dry cleaning, degreasing machinery and other applications.<ref name=":22" /><ref name=":3" /> It remains a common contaminant of groundwater.<ref name=":22" /><ref name=":3" /> Bacteria that are capable of completely degrading PCE to ethene, a gaseous chemical, have been isolated.<ref name=":4" /> They have been found to belong to the genus ''Dehalococcoides'' and to use H<sub>2</sub> as their electron donor.<ref name=":4" /> The process of organohalide respiration has been applied to ''in situ'' bioremediation of PCE and TCE in the past.<ref name=":22" /><ref name=":5" /> 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.<ref name=":5" /> In enhanced reductive dechlorination, the pollutants act as the electron acceptors and are completely reduced to ultimately produce ethene in a series of reactions.<ref name=":5" />
== Uses in Bioremediation == An ecologically significant aspect of bacterial organohalide respiration is the reduction of the two anthropogenic pollutants tetrachloroethylene (PCE) and trichloroethylene (TCE).<ref>{{Cite journal|last=Ruder|first=A.M.|date=September 2006|title=Potential health effects of occupational chlorinated solvent exposure.|journal=Annals of the New York Academy of Sciences|volume=1076|issue=1|pages=207–227|doi=10.1196/annals.1371.050|pmid=17119204|bibcode=2006NYASA1076..207R|s2cid=43678533}}</ref> Their presence as environmental pollutants arose from their common industrial use as metal-degreasing agents from the 1920s–1970.<ref>{{Cite journal|last1=Bakke|first1=Berit|last2=Stewart|first2=Patricia A.|last3=Waters|first3=Martha A.|date=November 2007|title=Uses of an Exposure to Trichloroethylene in U.S Industry: A Systematic Literature Review|journal=Journal of Occupational and Environmental Hygiene|volume=4|issue=5|pages=375–390|doi=10.1080/15459620701301763|pmid=17454505|bibcode=2007JOEH....4..375B |s2cid=32801149}}</ref> 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.<ref name=":0">{{Cite journal|last=Dugat-Bony|first=Eric|date=March 2012|title=In situ TCE degradation mediated by complex dehalorespiring communities during biostimulation processes|journal=Microbial Biotechnology|volume=5|issue=5|pages=642–653|doi=10.1111/j.1751-7915.2012.00339.x|pmid=22432919|pmc=3815876}}</ref>
A commonly used strategy for the removal of TCE and PCE from groundwater is the use of bioremediation via enhanced reductive dechlorination (ERD).<ref name=":1">{{Cite journal|last=Scheutz|first=Charlotte|date=November 2008|title=Concurrent ethene generation and growth of Dehalococcoides containing vinyl chloride reductive dehalogenase genes during an enhanced reductive dechlorination field demonstration.|journal=Environmental Science & Technology|volume=42|issue=24|pages=9302–9309|doi=10.1021/es800764t|pmid=19174908|bibcode=2008EnST...42.9302S}}</ref> 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.<ref name=":1" /> 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.<ref name=":1" />
A wide array of bacteria across different genera have the capacity to partially dechlorinate PCE and TCE into ''cis''-DCE and VC.<ref name=":1" /> One such example of this is the ''Magnetospirillum'' bacterium, strain MS-1, which can reduce PCE into ''cis-''DCE under aerobic conditions.<ref>{{Cite journal|last=Sharma|first=Pramod K.|date=March 1996|title=Isolation and Characterization of a Facultatively Aerobic Bacterium That Reductively Dehalogenates Tetrachloroethene to cis-1,2-Dichloroethene|journal=Applied and Environmental Microbiology|volume=62|issue=3|pages=761–765|pmc=1388792|pmid=16535267|doi=10.1128/aem.62.3.761-765.1996|bibcode=1996ApEnM..62..761S}}</ref> However, these daughter substrates have higher toxicity profiles than their parent compounds.<ref name=":1" /> As such, effective dechlorination of ''cis''-DCE and VC into innocuous ethene is crucial for bioremediation of PCE and TCE-contaminated aquifers.<ref name=":1" /> 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.<ref name=":2">{{Cite journal|last=Khoshnood|first=Behrang|date=August 2015|title=Genome Closing and Transcription Kinetics for RDase Genes in Dehalococcoides and Their Prevalence in a Wastewater Treatment Plant|url=http://scholarbank.nus.sg/handle/10635/122574|journal=National University of Singapore Libraries|id={{ProQuest|<!-- add ProQuest data here -->}}|access-date=2017-11-19|archive-date=2017-12-01|archive-url=https://web.archive.org/web/20171201042115/http://scholarbank.nus.sg/handle/10635/122574}}</ref> 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.<ref name=":2" />
==See also== * Bioaugmentation * Reductive dechlorination * Chloroflexota * Dehalococcoides * Dehalobacter
==References== {{reflist}}
==Further reading== {{refbegin|}} * {{cite journal|last1=Jugder|first1=J.|title=Reductive dehalogenases come of age in biological destruction of organohalides|journal=Trends in Microbiology|volume=33|issue=10|year=2015|pages=595–610|doi=10.1016/j.tibtech.2015.07.004|pmid=26409778}} * {{cite journal|last1=Leys|first1=D.|last2=Adrian|first2=L.|last3=Smidt|first3=H.|title=Organohalide respiration: microbes breathing chlorinated molecules|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=368|issue=1616|year=2013|article-number=20120316|issn=0962-8436|doi=10.1098/rstb.2012.0316|pmid=23479746|pmc=3638457}} * {{cite book|last1=Futagami|first1=Taiki|last2=Goto|first2=Masatoshi|last3=Furukawa|first3=Kensuke|title=Biodegradative Bacteria |chapter=Genetic System of Organohalide-Respiring Bacteria |year=2014|pages=59–81|doi=10.1007/978-4-431-54520-0_4|isbn=978-4-431-54519-4}} * {{cite journal|last1=Hug|first1=L. A.|last2=Maphosa|first2=F.|last3=Leys|first3=D.|last4=Loffler|first4=F.E.|last5=Smidt|first5=H.|last6=Edwards|first6=E. A.|last7=Adrian|first7=L.|title=Overview of organohalide-respiring bacteria and a proposal for a classification system for reductive dehalogenases|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=368|issue=1616|year=2013|article-number=20120322|issn=0962-8436|doi=10.1098/rstb.2012.0322|pmid=23479752|pmc=3638463}} * {{cite journal|last1=Maphosa|first1=Farai|last2=de Vos|first2=Willem M.|last3=Smidt|first3=Hauke|title=Exploiting the ecogenomics toolbox for environmental diagnostics of organohalide-respiring bacteria|url=https://zenodo.org/record/3422980|journal=Trends in Biotechnology|volume=28|issue=6|year=2010|pages=308–316|issn=0167-7799|doi=10.1016/j.tibtech.2010.03.005|pmid=20434786}} {{refend}}
Category:Electrochemistry Category:Respiration