Int. J. Environ. Sci. Technol. (2014) 11:241–250 DOI 10.1007/s13762-013-0387-9

REVIEW

Methanotrophs: promising for environmental remediation

V. C. Pandey • J. S. Singh • D. P. Singh • R. P. Singh

Received: 13 October 2011 / Revised: 16 June 2012 / Accepted: 1 December 2012 / Published online: 7 November 2013 Ó Islamic Azad University (IAU) 2013

Abstract Methanotrophs are unique and ubiquitous bac- Introduction teria that utilize methane as a sole source of carbon and energy from the atmosphere. Besides, methanotrophs may There is a growing concern about global warming world- also be targeted for bioremediation of diverse type of heavy wide. Methane (CH4) is one of the greenhouse gases metals and organic pollutants owing to the presence of (GHGs), which contributes to global warming. Methane is broad-spectrum methane monooxygenases enzyme. They about 23 times more effective as a greenhouse gas than are highly specialized group of aerobic bacteria and have a carbon dioxide (CO2) (Hasin et al. 2010). Methanotrophs unique capacity for oxidation of certain types of organic are the only known significant biological sink for atmo- pollutants like alkanes, aromatics, halogenated alkenes, etc. spheric CH4 and play a crucial role in reducing CH4 load Oxidation reactions are initiated by methane monooxyge- up to 15 % to the total global CH4 destruction (Singh et al. nases enzyme, which can be expressed by methanotrophs in 2010). Methanotrophs exist in a variety of habitats due to the absence of copper. The present article describes briefly having physiologically versatile nature and found in a wide the concerns regarding the unusual reactivity and broad range of pH, temperature, oxygen concentrations, salinity, substrate profiles of methane monooxygenases, which indi- heavy metal concentrations, and radiation (Barcena et al. cate many potential applications in bioremediation of heavy 2010; Dubey 2005; Durisch-Kaiser et al. 2005; Lindner metals and toxic organic compounds. Research is needed to et al. 2007; Tsubota et al. 2005). Broad-spectrum methane- develop understanding in plant–methanotrophs interactions oxidizing methane monooxygenase (MMO) enzyme is that optimize methanotrophs utilization in the field of envi- found only in methanotrophs, which possess two forms, ronmental remediation, while supporting other ecosystem namely membrane-associated or particulate form (pMMO) services. and soluble or cytoplasmic form (sMMO). The pMMO is found in all known methanotrophs except for the Keywords Bioremediation Á Heavy metals Á Methylocella (acidophilic) (Theisen et al. 2005), while the Methanotrophs Á Organic pollutants sMMO is present only in a few methanotroph strains (Murrell et al. 2000a, b). There are several sources that generate huge amount of V.C. Pandey and J.S. Singh contributed equally as first author. toxic heavy metals/metalloids (Cr, Cd, Pb, As, Cu, Zn, Ni, Hg, etc.) and organic pollutants into the environment V. C. Pandey (&) Á D. P. Singh Á R. P. Singh (Wijnhoven et al. 2007). Soils contaminated with heavy Department of Environmental Science, Babasaheb Bhimrao metals and/or organic pollutants are generally left aban- Ambedkar (Central) University, Lucknow 226025, Uttar Pradesh, India doned for several years and therefore may not be safe for e-mail: [email protected] agricultural production. Recently, microbial bioremedia- tion techniques have been found to alleviate the metal/ J. S. Singh organic pollutant toxicity of contaminated soils (Hasin Department of Environmental Microbiology, Babasaheb Bhimrao Ambedkar (Central) University, Lucknow 226025, et al. 2010; Shukla et al. 2009). Methanotrophs may be Uttar Pradesh, India promising bacteria for environmental bioremediation 123 242 Int. J. Environ. Sci. Technol. (2014) 11:241–250

(Jiang et al. 2010). Methanotrophs have been shown to methanotrophs (Jiang et al. 2010; Kikuchi et al. 2002; degrade/co-oxidize diverse type of heavy metals and Shukla et al. 2009). The majority of methanotrophs are organic pollutants due to the presence of broad-spectrum known to produce particulate methane monooxygenase MMO (Lindner et al. 2005; McFarland et al. 1992; Smith (pMMO) except few strains like Methylcella palustris and Dalton 2004). The MMO has been shown to oxidize a (Dedysh et al. 2000)—a known producer of soluble wide range of substrates, including aromatic compounds methane monooxygenase (sMMO). The sMMO-expressing viz. halogenated benzenes, toluene, and styrene as well as methanotrophs, due to their relatively broad substrate range aliphatic hydrocarbons with up to eight carbons (Burrows (Shigematsu et al. 1999) and fast turnover kinetics, exhibit et al. 1984; Colby et al. 1977; Green and Dalton 1989). fast decline in the level of pollutants than that calculated Some of the comprehensive review articles provide for pMMO-expressing methanotrophs. In contrast to basic status and different perspectives of methanotrophs pMMO, which works on a very narrow spectrum of carbon viz. research history (Dalton 2005), extremophilic met- substrate (alkanes and alkenes), the sMMO is capable of hanotrophs (Trotsenko and Khmelenina 2002), oxidizing a wider range of organic compounds including and ecology (Hanson and Hanson 1996), methanotrophs aliphatic, aromatic hydrocarbons, and their halogenated and CH4 oxidation related to wetlands (Chowdhury and derivatives (Trotsenko and Murrell 2008). Dick 2013), properties of methane monooxygenase (MMO) In contrast to other microbes that are recognized to (Lieberman and Rosenzweig 2005), metabolic aspects degrade halogenated hydrocarbons via reductive pathways (Trotsenko and Murrell 2008), biochemistry (Anthony (Maymo-Gatell et al. 1999), the biodegradation of chlori- 1982; Hakemian and Rosenzweig 2007), gene regulation nated hydrocarbons by methanotrophs occurs in an oxida- (Murrell et al. 2000a, b), biotechnological applications tive manner (Lontoh et al. 2000). The oxidative (Dalton 2005; Trotsenko and Khmelenina 2005), molecular biodegradation carried out by MMO appears more signif- marker (Dumont and Murrell 2005; McDonald et al. 2007). icant than the reductive dechlorination of chlorinated eth- None of them paid attention to explain about bioremedia- enes, such as TCE and tetrachloroethylene, which often tion potential of methanotrophs. So, there is an urgent need results into accumulation of more toxic intermediates, e.g., to re-tabulate the real involvement and benefits of this vinyl chloride, a known potent carcinogen (Maymo-Gatell unique microbe in bioremediation of toxic pollutants. et al. 1999). On the contrary, the MMO-mediated oxidative Broad substrate profiles of MMOs and its unusual reac- mechanisms of degradation of halogenated compounds by tivity indicate diverse potential applications of methano- the methanotrophs do not accumulate hazardous interme- trophs in bioremediation. Advancement in our knowledge diates (McCue et al. 2002). Thus, the applicability of about methanotrophic bioremediation may facilitate their methanotrophic degradation of halogenated hydrocarbons wide applications for safe and sustainable environmental for in situ bioremediation of contaminated soil and water development. This article has been aimed to highlight the system can be a safer remediation technique for sustainable bioremediation potential of methanotrophs and provides a development of the environment. moderate review on the progress made in methanotrophic Earlier it has been reported that trichloroethylene research related to bioremediation and identifying the (haloalkenes), a volatile chlorinated organic pollutant, is critical research needs for developing and implementing generally resistant to biodegradation by microorganisms successful methanotrophic bioremediation as a model (Wilson and Wilson 1985), but methanotrophs have been worldwide. The possible ways to maximize its multiple shown to co-metabolize trichloroethylene by the potent uses for mitigating the various pollutants are also MMO enzyme (Van Hylckama Vlieg and Janssen 2001). suggested. Stockholm Convention banned the use of persistent organic pollutants (POPs) like lindane, which are highly carcino- genic, persistent, bio-accumulative and endocrine disruptor Remediation of hazardous organic pollutants (ATSDR 2005). Under aerobic conditions, the degradation by methanotrophs of lindane [c-hexachlorocyclohexane (C6H6Cl6)] by bac- teria occurs through repeated steps of dehydrochlorination Methanotrophs synthesize both particulate and soluble and dechlorination, and it gets converted to chlorobenzenes forms of methane monooxygenases (pMMO and sMMO, and the end-product is carbon dioxide, which can be taken respectively), which can co-metabolize diverse type of up by plants (Mathur and Saha 1975; Vonk and Quirijns hydrocarbons and halogenated organic compounds 1979). New, low cost and rapid screening tool for the including aromatics. The priority pollutants like trichloro- detection of microbes capable of degrading lindane has ethylene (TCE) can be easily degraded by application of been discovered (Phillips et al. 2001). The same assay

123 Int. J. Environ. Sci. Technol. (2014) 11:241–250 243 technique may be used for identifying lindane-degrading trichlorohydroquinone and dichlorohydroquinone, because methanotrophs. Thus, there is an urgent need to examine the activity of monooxygenase and dioxygenase was degradation of lindane by the potent MMO enzyme of the inhibited by the substituted chlorine (Rasul and Chapala- methanotrophs. Similarly, the methanotrophs-mediated madugu 1991). In anaerobic condition, the PCP is first environmental fate and degradation of polycyclic aromatic reductively dechlorinated and converted to tetra-, tri-, di-, hydrocarbons (PAHs) and polychlorinated biphenyls and mono-chlorophenol (TetCP, TCP, DCP, MCP). Nev- (PCBs) need to be explored. Biodegradation of potentially ertheless, these intermediates are more toxic than PCP and toxic PAHs has been examined by non-specific MMO are difficult to degrade (Yuancai et al. 2007). Since aerobic enzyme of marine methanotrophs (Rockne et al. 1998). The degradation of metabolites is more feasible, complete aerobic catabolism of a PAH molecule by bacteria occurs degradation of PCP is possible with the use of aerobic via oxidation of PAHs to a dihydrodiol by a multicompo- methanotrophs and anaerobic microorganisms. Gerritse nent enzyme system. The mechanism of bioremediation of et al. (1995) and Tartakovsky et al. (1998) have been able PAH involves ortho cleavage or meta cleavage type path- to demonstrate complete degradation of tetrachloroethyl- way, resulting in the formation of protocatechuates and ene (aromatic compound) by using anaerobic dechlorinat- catechols that are then converted to tricarboxylic acid ing and aerobic methanotrophic enrichment cultures or a cyclic intermediate (Kanaly and Harayama 2000). In the consortium of co-immobilized methanogenic and methan- biodegradation of PCBs, the ability of methanotrophic otrophic bacteria, respectively (Tables 1, 2). bacterium Methylosinus trichosporium OB3b, expressing soluble methane monooxygenase, has been shown to oxi- dize a range of ortho-halogenated biphenyls (2-chloro-, Remediation of heavy metals by methanotrophs 2-bromo-, and 2-iodobiphenyl) although at lower rates than the unsubstituted biphenyl (Lindner et al. 2000). Hydrox- Methanotrophic bacteria have considerable potential for ylation is the dominant reaction during the degradation of use in biotechnology and in bioremediation due to the the aromatic ring, followed by dehalogenation. This type of amenability of these bacteria to large-scale cultivation study provides a platform on how methanotrophs can be (Jiang et al. 2010; Overland et al. 2010; Semrau et al. used in conjunction with anaerobic degraders for the 2010). It has been suggested that methanotrophs also removal of highly chlorinated biphenyls. Additional influence the speciation and bioavailability of metals in the research is still needed to determine how the products of environment (Choi et al. 2006; Jenkins et al. 1994). Hasin methanotrophic oxidation of ortho-substituted biphenyls et al. (2010) have reported the reductive transformation of are further oxidized by heterotrophic microorganisms to soluble and more toxic Cr(VI) into a less toxic ensure complete mineralization (Lindner et al. 2000). Some Cr(III)species by methanotrophic bacteria (Methylococcus of the reports emphasize that polar regions are interesting capsulatus Bath), as the Cr(III) is insoluble and tends to get because of the presence of POPs, transported by a complex precipitated at high pH. mechanism involving successive volatilization and depo- The importance of a toxicity reducing, Cu-carrier mol- sition steps from warmer areas toward cooler regions ecule is mainly linked to methanotrophs given their typical including Antarctica (Bengtson 2011). Barcena et al. habitat, i.e., geochemically distinct microaerophilic zones. (2010) have reported methanotrophic methane oxidation in In such sites, intense redox cycling leads to active pre-

Antarctica. There are several reports about the existence of cipitation of Mn and Fe oxides (Ferris et al. 1999). CH4 different psychrophilic methanotrophs from Antarctica oxidation requires Cu (due to its high reactivity), which, in (Bengtson 2011). But very little information is available turn, demands a strong Cu defense system. There is a about the nature of psychrophilic methanotrophs diversity molecular carrier for Cu, termed as methanobactin (mb)—a in contaminated sites, the genes that confer them the 1,216-Da fluorescent metal-binding chromopeptide (Kim capability for bioremediation as well as survival in the et al. 2004), which confers protection to the cells both from extreme low temperature. external and internal Cu toxicity. The study of Knapp et al. Pentachlorophenol (PCP), a highly substituted aromatic (2007) provides a strong evidence about the mb-mediated compound, is extensively used as wood preservative, bac- Cu release from the mineral stage, which changes the Cu tericide, fungicide, and herbicide (Yuancai et al. 2007). availability and allows pMMO gene expression in met- PCP has been listed as a pollutant by the US Environmental hanotrophs. Therefore, mb might be particularly critical for Protection Agency owing to its toxicity. Under aerobic ecological success of methanotrophs in such metal-polluted condition, PCP is first converted to tetrachloro-p-hydro- environments where proteins like methanobactin (mb) quinone by hydroxylation, and daughter products are allow the selective acquisition of Cu, while protecting the

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Table 1 Methanotrophic bacteria involved in the bioremediation of various toxic hydrocarbon and heavy metal pollutants Methanotrophic species Targeted organic pollutants References

Methylosinus trichosporium OB3b Halogenated hydrocarbons Hanson et al. (1990), Oldenhuis et al. (1991) Methylomonas albus BG8, Methylocystis parvus OBBP, and Polynuclear aromatic hydrocarbons and Jenkins et al. (1994) Methylosinus trichosporium OB3b transition metals Methylosinus trichosporium OB3b TCE Lontoh and Semrau (1998) Type II methanotrophs Phenanthrene, anthracene, and fluorene Rockne et al. (1998) Methylocystis sp. M, Methylococcus capsulatus (Bath), Methylosinus TCE-degradation Kikuchi et al. (2002) trichosporium OB3b, strain 5, and unidentified strains of methanotrophs (MP18, MP20, P14) Type II methanotrophs TCE Shukla et al. (2009) Methylosinus trichosporium OB3b and Methylocystis daltona SB2 TCE, DCE, and VC Yoon (2010) Methylocystis strain SB2 Vinyl chloride (VC), dichloroethylene (DCE), Im and Semrau (2011) trichloroethylene (TCE), and chloroform (CF) Methanotrophic mixed culture Biotransformation of three Chang and Criddle hydrochlorofluorocarbons (HCFCs) and one (1995) hydrofluorocarbon (HFC) Methanotrophic species Targeted inorganic pollutants References

Methylophilus methylotrophus EHg7 Cadmium (Cd) De Marco et al. (2004) Methylophilus methylotrophus ECr4 Chromium (Cr) De Marco et al. (2004) Methylococcus capsulatus Bath Chromium (Cr) Hasin et al. (2010)

Table 2 Methanotrophs that have been isolated from a wide variety model of methanotroph Methylococcus capsulatus (Bath), of habitats/environments capable of bioremediation of chromium (VI) pollution over -1 Variety of habitats References a wide range of concentrations (1.4–1,000 mg L of Cr6?). The genome sequence of M. capsulatus (Bath) Soils Dubey (2005) suggested at least five genes for the chromium (VI) Sediments Tavormina et al. (2008) reductase activity in this bacterium. Study of De Marco Landfills Ait-Benichou et al. (2009) et al. (2004) was the first attempt to systematically analyze Groundwater Lindner et al. (2007) the capability of methylotrophic strains to develop toler- Seawater Durisch-Kaiser et al. (2005) ance against heavy metal pollutants. These workers iso- Peat bogs/peat lands Larmola et al. (2010) lated thirty-one novel methylotrophic bacterial strains from Hotsprings Tsubota et al. (2005) a range of soil and sediment sources (both pristine and Plant rhizosphere Qiu et al. (2008) polluted). Furthermore, they noted that some of the isolates Salt reservoirs Heyer et al. (2005) exhibited interesting characteristics of resistance to heavy Antarctic Barcena et al. (2010) metals, arsenate, or organic pollutants. Among them, four strains were considered as real ‘super-bugs’ for their ability to withstand extremely high concentrations of a variety of methanotrophs against other similar potentially toxic met- heavy metal pollutants. The toxic mercury (II) ion is usu- als. Microbial-based bioremediation of heavy metals, pro- ally detoxified by bacteria via its reduction to elemental duced from metal plating, tanning, paper-making industries mercury, catalyzed by an NAD(P)H-dependent mercuric (Cervantes et al. 2001; Hasin et al. 2010; Zayed and Terry reductase enzyme (EC 1.16.1.1). It has been proved that 2003), can be used for detoxification of metals through Methylococcus capsulatus (Bath)—a methanotrophic their conversion to less toxic and less soluble form like member of the Gammaproteobacteria—uses this enzyme Cr(III). Hasin et al. (2010) reported a well characterized to detoxify mercury (De Marco et al. 2004). In radio

123 Int. J. Environ. Sci. Technol. (2014) 11:241–250 245 respirometry studies, it has been found that cells exposed to potential in the presence of various stress conditions mercury dissimilated 100 % of [14C]-methane and that including the presence of organic and inorganic pollutants. provided reducing equivalents to fuel mercury (II) reduc- To examine the impact of these environmental stresses on tion (Boden and Murrel 2011). Thus, methanotrophs not methanotrophs in relation to bioremediation potential, only degrade the organic moiety but also aid in remediation there is an urgent need for detailed studies to address these of inorganic elements. There are reports that few methan- questions. otrophic bacteria produce extracellular polymers with the potential for industrial applications as well as for metal bioremediation. It is an important point to examine the Genetic engineering in methanotrophs for enhanced broad-spectrum enzyme MMO of methanotrophs for its bioremediation capability to detoxify/transform other toxic elements into a less toxic form as it depends upon the involvement of With the aid of biotechnology and genetic engineering, enzyme-mediated redox reactions (Valls and de Lorenzo bacteria have been exploited for in-situ bioremediation of a 2002). Thus, the use of methanotrophic bacteria in the wide range of pollutants (Barac et al. 2004; Liu et al. 2011; remediation of such toxic elements from contaminated sites Villacieros et al. 2005). Genetic engineering of indigenous could be an emerging tool in more sustainable way. bacteria, well adapted to local conditions, offers more efficient bioremediation of polluted sites (Singh et al. 2011). Genetic engineering in methanotrophs may provide Environmental stresses and methanotrophic opportunities to exploit the unusual reactivity and broad remediation substrate profile of MMO for maximum benefit in the field of remediation technologies and to manipulate the toler- Adaptation mechanisms of methanotrophs at molecular ance, degradation potential of methanotrophs against var- level under various stresses viz. temperature, pH, salinity, ious organic and inorganic pollutants through introduction sodicity, drought, and different types of chemicals are still of desired genes. Thus, the development and application of not known (Jiang et al. 2010). Some other environmental genetic engineering of the native methanotrophs will defi- factors well known to influence the population of met- nitely offer more efficient and enhanced bioremediation of hanotrophs are the ammonium and/or nitrite ions as they the pollutants viz. heavy metals, organics or co-contami- act as competitive substrates for MMO (Dunfield and nants, making the bioremediation more viable for envi- Knowles 1995; Hanson and Hanson 1996). The exact ronment remediation (Fig. 1). Some points have to be put mechanism of enzyme inhibition by nitrogen compounds in mind such as bio-safety assessment, risk mitigation, and and methane oxidation still needs to be understood. The factors of genetic pollution before using the genetically main mechanism by which nitrogen inhibits methane oxi- engineered bacteria at field level (Singh et al. 2011) dation is through ammonium, which competes with meth- including methanotrophs. However, the future application ane for binding site on MMO in methanotrophic bacteria. of genetically engineered methanotrophs for pollution Although the affinity of MMO for methane is many folds remediation will not be free from the risks associated with higher than its affinity for ammonium, excessively high their release in the environment. The future risk regarding concentrations of ammonium is known to substantially use of such engineered bacteria is still unclear. The path- inhibit methane oxidation (Bedard and Knowles 1989). way of safe application of efficient bioremediation through However, recent studies with rice plants have revealed that genetically engineered methanotrophs: laboratory ? bench nitrogen fertilization increases CH4 oxidation in densely scale ? pilot scale ? finally field scale testing ? com- rooted soils because rhizosphere methanotrophs face mercial field application, etc. is not a remote dream. There intense plant and microbial competition for nitrogen is need to develop novel methanotrophs through the bio- (Macalady et al. 2002; Eller et al. 2005). The information technological and genetic engineering approach to protect about MMO-mediated methanotrophic remediation affec- the environment. ted by nitrogenous compounds is almost lacking. Hence, impact of nitrogenous compounds on methanotrophic remediation needs to be investigated. However, modern Limitations to methanotrophic bioremediation research in ‘omics’ technologies (proteomic, metabolomic, genomic, soil metagenomic, and transcriptomic) may boost High cell density cultivation, number of culture conditions, our understanding on the adaptation potential of methan- and fermentation technologies for methanotrophs have otrophs in various habitats and their bioremediation been studied extensively. However, there are still several

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Fig. 1 A hypothetical model showing the application of Capping of methane from methanotrophic bacteria for diverse fields environmental remediation

Remediation of organic Remediation of toxic pollutants elements

Harnessing methanotrophs for environmental Genetically engineered Does it show the activity of plant remediation indigenous methanotrophs for growth promoting rhizobacteria enhanced bioremediation in rice fields?

Role of plant–methanotrophs interactions in bioremediation

limitations for methanotrophic study such as slow growing potent greenhouse gas methane. There is an urgent need strains, low solubility of methane, and oxygen in aqueous to optimize the effect of agricultural practices on this phase (Jiang et al. 2010). It is general problem that toxic microbe in different bio-geographical regions of the intermediates may be formed during the aerobic degrada- world. The population dynamic and diversity of the tion of chlorinated compounds which cause damage to the methanotrophs need to be studied with respect to edaphic bacterial cells. In this regard, utilizing a dichloromethane and climatic conditions of environment. A few citations (DCM)-degrading microbe which has an important DNA- on these aspects have emerged recently (Vishwakarma repairing mechanism in response to DNA damage caused et al. 2009; Zheng et al. 2008; Singh et al. 2010; Singh by formation of toxic intermediate during the degradation and Pandey 2013). Recently, plant–microbe interactions of chlorinated compounds (Kayser and Vuilleumier 2001). have been suggested as a promising technology to This bacterium utilizes DCM as a sole source of carbon and enhance phytoremediation (Rajkumar et al. 2012), which energy. An aerobic bacterium was reported to degrade offers a future tool for potential application of methano- mono- to trichlorinated dioxins with a newly discovered trophs for exploitation in other ecosystem services. Sev- enzyme, an angular dioxygenase known as carbazole 1, 9 a eral plants involved in interactions with rhizosphere- dioxygenase (Habe et al. 2001). For characterization of associated microbes can be exploited to remediate toxic microbial populations, a new screening assay based on environments (Weyens et al. 2009). But plant–methano- quinone profiling (a culture-independent lipid biomarker trophs relations studied till date have mainly focused on approach) has been developed for the analyses of in situ rice fields and wetlands because of their importance as microbial populations in dioxin-polluted soils (Hiraishi major areas of methane production (DeBont et al. 1978). et al. 2001). Knowledge of DNA repair mechanism But the role of plant–methanotrophs interactions in the (important physiological tool) in dioxin-degrading bacte- field of remediation of toxic elements from contaminated rial strain and bacterial screening assay in the dioxin-pol- sites is still little explored. Plants can also take benefit luted soil may help us in our understanding and prediction from these associations. Since methanotrophs can excrete of the methanotrophs-based degradation of chlorinated di- or release phytohormones (cytokinins and auxins) after oxins. Further more research is needed to eliminate these cell lysis and other bioactive compounds (Doronina et al. limitations for exploitation of these promising methano- 2004), the plants can benefit from their association. trophs for human welfare. Additionally, different aspects of methanotrophs were examined in the rice fields (Horz et al. 2001; Mohanty et al. 2007; Singh et al. 2010; Singh and Pandey 2013; Other research needs related to methanotrophs Wu et al. 2009), but none of these were related to plant growth-promoting methanotrophs. So, there is still need to It is well known that methanotrophs are ubiquitous in the examine the plant growth-promoting activity of methan- environment and globally important in oxidizing the otrophs in agricultural fields. Similarly, methanotrophs

123 Int. J. Environ. Sci. Technol. (2014) 11:241–250 247 should be collected from different agro-climatic zones of References the world for assessing their bioremediation potential. The collected methanotrophs should be cultured and used in Ait-Benichou S, Jugnia LB, Greer CW, Cabral AR (2009) Methan- different parts of the world for detailed morphological, otrophs and methanotrophic activity in engineered landfill biocovers. Waste Manage 29:2509–2517 genomic, and molecular characterizations. The fast-grow- Anthony C (1982) The biochemistry of methylotrophs. Academic ing methanotrophs could be used for remediation of dif- Press, London ferent types of pollutants. The database knowledge ATSDR (2005) Toxicological profile for hexachlorocyclohexanes. exchange program should be facilitated at the global level. U.S. Department of Health & Human Services. Public Health Service. Agency for Toxic Substances and Disease Registry. Remediation of soil pollutants by methanotrophs is http://www.atsdr.cdc.gov/toxprofiles/tp43.html emerging fast. Methanotrophic bacteria in forest soil Barac T, Taghavi S, Borremans B, Provoost A, Oeyen L, Colpaert JV, exhibit the highest methane sink activity on a global scale Vangronsveld J, van der Lelie D (2004) Engineered endophytic (Dalal and Allen 2008), but their methane sink capacity bacteria improve phytoremediation of water-soluble, volatile, organic pollutants. Nat Biotechnol 22:583–588 decreases when natural land use pattern is altered (Dorr Barcena TG, Yde JC, Finster KW (2010) Methane flux and high- et al. 2010). Methane sink activity (kg ha-1 year-1)by affinity methanotrophic diversity along the chronosequence of a methanotrophs in different ecosystems is increased in receding glacier in Greenland. 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FEMS Microbiol Lett widespread salt-affected denuded wasteland (about 324(2):106–110 955 9 106 ha) in arid and semi-arid regions (Szabolcs Burrows KJ, Cornish A, Scott D, Higgins IJ (1984) Substrate 1994), there is a great potential of methane sink activity specificities of the soluble and particulate methane mono- through afforestation program on these wasteland for better oxygenases of Methylosinus trichosporium OB3b. J Gen Micro- biol 136:335–342 growth of these bacteria (Pandey et al. 2011). Cervantes C, Campos-Garcıa J, Devars S, Gutierrez-Corona F, Loza- Tavera H, Torres-Guzman JC, Moreno-Sanchez R (2001) Interactions of chromium with microorganisms and plants. Conclusion FEMS Microbiol Lett 25:335–347 Chang WK, Criddle CS (1995) Biotransformation of HCFC-22, HCFC-142b, HCFC-123, and HFC-134a by methanotrophic Methanotrophs were discovered over a century ago; how- mixed culture MM1. Biodegradation 6:1–9 ever, methanotrophs have not been explored well. The Choi DW, Do YS, Zea CJ, McEllistrem MT, Lee SW, Semrau JD, research related to bioremediation potential of methano- Pohl NL, Kisting CJ, Scardino LL, Hartsel SC, Boyd ES, Geesey trophs is still in infancy stage. For better harnessing of GG, Riedel TP, Shafe PH, Kranski KA, Tritsch JR, Antholine WE, DiSpirito AA (2006) Spectral and thermodynamic proper- methanotrophs in industrial application and bioremedia- ties of Ag(I), Au(III), Cd(II), Co(II), Fe(III), Hg(II), Mn(II), tion, a number of limitations need to be worked out such as Ni(II), Pb(II), U(IV), and Zn(II) binding by methanobactin from lack of suitable cultivable methanotrophs and isolation Methylosinus trichosporium OB3b. J Inorg Biochem techniques, competitive inhibition of methane mono-oxy- 100:2150–2161 Chowdhury TR, Dick RP (2013) Ecology of aerobic methanotrophs in genase by ammonium, low solubility of methane, produc- controlling methane fluxes from wetlands. Appl Soil Ecol tion of toxic intermediates. With the combination of 65:8–22 biotechnology and genetic engineering, methanotrophs can Colby J, Stirling DI, Dalton H (1977) The soluble methane mono- be exploited for in situ bioremediation of a wide range of oxygenase of Methylococcus capsulatus (bath), its ability to oxygenate n-alkanes, n-alkenes, ethers, and alicyclic, aromatic inorganic and organic pollutants. and heterocyclic compounds. Biochem J 165:395–402 Dalal RC, Allen DE (2008) Greenhouse gas fluxes from natural Acknowledgments VC Pandey is thankful to University Grants ecosystems (Turner review no. 18). Aust J Bot 56:369–407 Commission (UGC) for awarding Dr. DS Kothari Post-Doctoral Fel- Dalton H (2005) The Leeuwenhoek lecture 2000. The natural and lowship (Award Letter No: F.4-2/2006(BSR)/13-209/2008(BSR)). unnatural history of methane-oxidizing bacteria. Philos Trans R Thanks are also due to Vice Chancellor, Babasaheb Bhimrao Ambedkar Soc Lond B Biol Sci 360:1207–1222 University (Central) for providing facilities and encouragements. De Marco P, Pacheco CC, Figueiredo AR, Moradas-Ferreira P (2004) Anonymous reviewers are thanked for their constructive comments and Novel pollutant-resistant methylotrophic bacteria for use in suggestions on the previous version of this article. bioremediation. FEMS Microbiol Lett 234:75–80

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