Enhancement of metal bioremediation by use of microbial surfactants
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Breakthroughs and ViewsEnhancement of metal bioremediation by use of microbial surfactantsPooja Singh and Swaranjit Singh Cameotra *Institute of Microbial Technology,Sector 39-A,Chandigarh 160036,IndiaReceived 29March 2004Available online 18May 2004AbstractMetal pollution all around the globe,especially in the mining and plating areas of the world,has been found to have grave consequences.An excellent option for enhanced metal contaminated site bioremediation is the use of microbial products viz.mi-crobial surfactants and extracellular polymers which would increase the efficiency of metal reducing/sequestering organisms for field bioremediation.Important here is the advantage of such compounds at metal and organic compound co-contaminated site since microorganisms have long been found to produce surface-active compounds when grown on hydrocarbons.Other options capable of proving efficient enhancers include exploiting the chemotactic potential and biofilm forming ability of the relevant microor-ganisms.Chemotaxis towards environmental pollutants has excellent potential to enhance the biodegradation of many contami-nants and biofilm offers them a better survival niche even in the presence of high levels of toxic compounds.Ó2004Elsevier Inc.All rights reserved.Due to their extremely toxic nature even at trace concentrations and their non-biodegradability unlike organic pollutants,heavy metals are a persistent threat to our life and environment.Metal wastes are produced by a variety of sources including mines,tanneries,and electroplating facilities and through the manufacture of paints,metal pipes,batteries,and ammunition.Sulfide and oxidized ores exposed during mining operations have the potential to produce leachate solutions that contain high concentrations of dissolved metals.These leachate solutions can affect the quality of plant and animal health in surface water used for agriculture,recreation,and human consumption.Metal contami-nation has been linked to birth defects,cancer,skin le-sions,mental and physical retardation,learning disabilities,liver and kidney damage,and a host of other maladies.Rapid alteration in their toxicity hence as-sumes importance for a safe environment.Through the decades a number of microorganisms have been dis-covered to help in the reduction of metal contamination.However,their simple metabolic activity alone proves tobe inadequate especially at sites with metal and organic compound co-contamination.This review deals with the use of microorganisms and microbial products viz.biosurfactants,for the enhanced metal and metal co-contaminated site bioremediation.It also discusses the importance of the biofilm formation and chemotactic ability of the organisms in increasing the efficiency of the process.Strategies of metal bioremediationMetals are not degradable.Unlike hydrocarbons,biodegradation of metals into innocuous CO 2and water is not possible.Irrespective of the available reactions,the same metal will still be present but bacterial strains have been found to have the capacity to concentrate or remediate them into forms that are precipitated or vol-atilized from solution and hence less toxic and easily disposable.In other words,microorganisms can only alter the speciation of metal contaminants and convert them into non-toxic form [1].The various ways by which metal uptake can occur are depicted in Fig.1.Various excellent papers are available which deal with the dif-ferent ways of metal uptake in detail [2–8].Hence,this aspect has not been discussed here.However,what is*Corresponding author.Fax:+91-172-2690585;+91-172-2690632.E-mail addresses:ssc@imtech.res.in,swaranjitsingh@ (S.S.Cameotra).0006-291X/$-see front matter Ó2004Elsevier Inc.All rights reserved.doi:10.1016/j.bbrc.2004.04.155Biochemical and Biophysical Research Communications 319(2004)291–297BBRC/locate/ybbrcneeded at present are some new strategies for enhanced reduction in metal contamination at different sites.One such option gaining importance day by day is the use of surface-active compounds.Biosurfactants and metal remediationMicrobial compounds that exhibit particularly high surface activity and emulsifying activity are classified as biosurfactants.These are structurally diverse surface-active compounds capable of reducing surface and in-terfacial tension at the interfaces between liquids,solids, and gases,thereby allowing them to mix or disperse readily as emulsions in water or other liquids.In general, their structure includes a hydrophilic moiety consisting of amino acids,peptides,anions or cations;mono-,di-, or polysaccharides,and a hydrophobic moiety consist-ing of unsaturated or saturated fatty acids.Accordingly, the major classes of biosurfactants include glycolipids, lipopeptides and lipoproteins,phospholipids and fatty acids,polymeric surfactants,and particulate surfactants. Biosurfactant production,structure,and their various natural and industrial uses,including their role in pol-lution removal,have been extensively reviewed earlier [9–15].According to Miller[16],biosurfactants enhance the desorption of heavy metals from soils in2ways:plexation of the free form of the metal residingin solution.This decreases the solution phase activity of the metal and,therefore,promotes desorption ac-cording to Le Chatelier’s principle.2.Direct contact of biosurfactant to sorbed metal at so-lid solution interface under conditions of reduced in-terfacial tension,which allows biosurfactants to accumulate at solid solution interface.Not only are metals not degradable by microorgan-isms,but they also form strong bonds with soils,making it tough for microorganisms to even get access to the metals.Soil composition(type of soil),soil pH,cation exchange capacity(CEC),particle size,time and type of contamination,geological layout,etc.,all play a major role in establishing the effectiveness of biosurfactant action[17].Long-term contamination gives the metal ample time to stabilize and hence removal tends to be more difficult.Research has shown that metals such as lead and cadmium have stronger affinities for rhamnolipidsthan 292P.Singh,S.S.Cameotra/Biochemical and Biophysical Research Communications319(2004)291–297for many of the soil components to which they are bound in contaminated soils[18].Considerable work has been done on rhamnolipid biosurfactant produced by various Pseudomonas aeruginosa strains capable of selectively complexing cationic metal species such as Cd, Pb,and Zn.Studies have shown that this surfactant complexes preferentially with toxic metals such as cad-mium and lead than with normal soil metal cations such as calcium and magnesium,for which it has a much lower affinity[19,20].A5mM solution of rhamnolipid produced by P.aeruginosa ATCC9027was found to complex92%of cadmium,a complexation of22l g/mg rhamnolipid[21].The feasibility of using a biodegrad-able surfactant,surfactin from Bacillus subtilis,for the removal of heavy metals from a contaminated soil and sediments was evaluated by[22].After one andfive batch washings of the soil,25%and70%of the copper, 6%and25%of the zinc,and5%and15%of the cad-mium could be removed by0.1%surfactin along with 1%NaOH,respectively.Mulligan et al.[23]evaluated the feasibility of using surfactin,rhamnolipid,and sophorolipid for the removal of heavy metals,Cu and Zn,from sediments.A single washing with0.5%rhamnolipid removed65%of the copper and18%of the zinc,whereas,4%sophorolipid removed25%of the copper and60%of the zinc.Surfactin was less effective,removing15%of the copper and6%of the zinc.Sequential extraction of the sediments after washing with the various surfactants indicated that the biosurfactants,rhamnolipid and surfactin,could remove the organically bound copper and that the sophorolipid could remove the carbonate and oxide-bound zinc.They subsequently postulated that metal removal by the bio-surfactants occurs through sorption of the surfactant onto the soil surface and complexation with the metal, detachment of the metal from the soil into solution,and hence association with surfactant micelles.One way to apply biosurfactant for metal removal from soil is on a small part of contaminated soil in which soil is put in a huge cement mixer,treated with biosurfactant,biosurfactant–metal complexflushed out, soil deposited back,and biosurfactant–metal complex treated to precipitate out biosurfactant,leaving behind the metal(Fig.2).The bond formed between the positively charged metal and the negatively charged surfactant is so strong thatflushing water through soil removes the surfactant metal complex from the soil matrix.Similar action would be carried out for deeper subsurface contamination only with more pumping activities.In one study by Jeong Jin et al.[24]biosurfactant based ultrafiltration was used to remove the divalent metal ions,Cuþ2,Znþ2,Cdþ2,and Niþ2,from aqueous solution containing either a single metal species or a mixture of metal ions.Polycarboxylic acid type bio-surfactant,sodium salt of2-(2-carboxyethyl)-3-decyl maleic anhydride(DCMA-3Na)derived from spicu-lisporic acid,was added to trap the metal ions through the binding onto the micelles and/or the formation of metal-biosurfactant precipitates.Apart from work on the use of biosurfactants for metal remediation,work has also been done on the use of other microbial prod-ucts like bacterial and algal exopolysaccharides and other bacterial polymers for metal removal from con-taminated sites[25].Jyh et al.[26]have also documented trace metal mobilization in soil by bacterial polymers.In yet another interesting study,not related to microbial activity,plant derived biosurfactant saponin from Quillaja bark was studied for the recovery of heavy metals from contaminated soils.Utilization of saponin was effective for the removal of heavy metals from soil attaining90–100%of Cd and85–98%of Zn extraction [27].However,biosurfactants have advantage over synthetic surfactants in their small size,less toxicity, biodegradability,specificity,and ease and manipulation of production process while they score over exopoly-saccharides in selectivity and small size.The effectiveness of the use of biosurfactants for metal remediation in-creases in terms of cost involved at sites co-contami-nated with organic compounds.Metal remediation in co-contaminated soilsIn US,37%of the organic compound polluted sites tested were found to be polluted also with metals such as arsenic,mercury,lead,and zinc[28].Bioremediation of such sites can be impaired by the presence of heavy metals,making cleanup of the contaminated site more anisms growing on hydrocarbons have long been shown to produce potent surface-active compounds that enhance the rate of degradation by emulsification or solubilization of the hydrophobic hy-drocarbon[29].Exploiting this property,Todd et al.[30] studied the effectiveness of rhamnolipid biosurfactants in the remediation of cadmium and naphthalene co-contaminated site.They observed reduced cadmium toxicity by P.aeruginosa rhamnolipid leading to an en-hanced naphthalene biodegradation by a Burkholderia species.They proposed that the mechanism bywhich P.Singh,S.S.Cameotra/Biochemical and Biophysical Research Communications319(2004)291–297293rhamnolipid reduces metal toxicity might involve a combination of rhamnolipid complexation of cadmium and rhamnolipid interaction with the cell surface to alter cadmium uptake resulting in enhanced rates of bio-remediation.In another co-contaminant study,it was observed that the inhibition of phenanthrene minerali-zation in the presence of cadmium was reduced by the pulsed addition of rhamnolipid[31].Anionic biosurf-actants were found to be more effective where metals are the agents to be sequestered.Surfactin,rhamnolipid, and sophorolipids,all anionic biosurfactants,were able to remove copper and zinc from a hydrocarbon-con-taminated soil[32].One advantage in case of co-con-taminated soil is that biosurfactants potentially can be produced in situ using the organic contaminants as substrates for their production,which subsequently would lead to remediation of both the contaminants along with greatly reducing the remediation cost.The efficiency of biosurfactants for stimulating bio-degradation of contaminants is uncertain given the specificity observed between biosurfactant and organ-ism.Addition of biosurfactant can stimulate some or-ganisms but also can inhibit some microorganisms.So, as mentioned earlier,a strategy suitable for effective remediation would be to stimulate biosurfactants pro-duced by indigenous population or use commercial biosurfactants produced by organisms found to be al-ready present at the contaminated site.Further,delivery of a biosurfactant into co-contaminated sites for in situ treatment may be more environmentally compatible and more economical than using modified clay complexes or metal chelators such as EDTA[33,34].However,an-other option whose potential is gaining importance is the remediation of pollutants in a biofilm at the con-taminated site.Metal remediation and biofilmsThe behavior,transport,and ultimate fate of con-taminants in aquatic environment may be significantly affected by the sorption and remobilization with biofilms [35].Many of the geochemical and biological processes mediated by microorganisms are localized by cells within a matrix of extracellular polymeric secretions (EPS),composed of polysaccharides,proteins,and nu-cleic acids collectively called a“microbial biofilm.”Protected within the matrix,cells have a better chance of adaptation and survival(especially during periods of stress)than when cells are present in solution[36].Due to close,mutually beneficial physical and physiological interactions among organisms in a biofilm,utilization of a xenobiotic is accelerated than when the same organ-isms are present individually in solution.Biofilm can sorb water,inorganic and organic solutes,and particles. EPS,cell walls,cell membranes,and cell cytoplasm:all can serve as sorption sites.Biofilms can affect the fate of other compounds in their vicinity by their physiological response to sorbed substance,e.g.,the uptake of toluene can lead to the formation of uronic acids in the EPS and thus to an increased sorption capacity for cations[37]. EPS are primarily composed of polysaccharides,uronic acid sugars,and proteins containing functional groups such as carboxylic acids and amino acid groups,which could be acidic and capable of binding metal ions.Various microbial biofilms have been found to sorb metals by varying activities.One major microbial com-munity involved in the formation of biofilms,especially in metal contaminated waters,is the group of sulfate reducing bacteria(SRB),which are highly efficient in the anaerobic degradation of many organic pollutants as well as in the precipitation of heavy metals from waste water as metal sulfide[38].In a study by White and Gadd[39],sulfate reducing bacterial biofilms,grown in continuous culture and exposed to the medium con-taining20/200l m Cu,were found to accumulate copper in the form of sulfide.A simultaneous increase in the EPS content of the biofilm was also observed suggesting the role of EPS and biofilm in the entrapment of metal precipitates.Similar results were obtained in a previous study with biofilms exposed to cadmium[40].Applica-bility and feasibility of treating acidic sulfate,zinc,and iron containing wastewater with SRB suspension and biofilm was studied.Supplementation of wastewater with lactate was found to enrich SRBs in suspension and biofilms and metals were found to precipitate and accumulate in biofilm as sulfides[41].Lion et al.[42]reviewed trace metal interactions with microbial biofilms in natural and engineered systems. Lead,one of the widely present aquatic contaminants, was found to adsorb strongly to the metal oxides in biofilms[43].Macaski and Dean[44]studied the use of immobilized biofilm of Citrobacter sp.formed under appropriate growth conditions,for the removal of ura-nium and lead from aqueousflow.Much of the exposed metals were found to accumulate in the form of insol-uble metal phosphate.Simultaneous sorption and bio-mineralization processes were found to be responsible for Pd(II)uptake by Burkholderia cepacia biofilms[45]. X-ray spectroscopic measurements and transmission electron microscopy(TEM)observations revealed that lead accumulation was due to the formation of nano-scale crystals of pyromorphite[Pb5(PO4)3(OH)]adja-cent to the outer membrane of a fraction of the total population of B.cepacia ngley and Beveridge [46]demonstrated an increased deposition of cytoplas-mic crystals of Au(III)and cell wall associated La(III)in biofilm relative to crystal formation during planktonic growth,suggesting that physiology and physical–chem-ical conditions around cells in biofilms are conducive for metal bioremediation.A recent study by Teitzel and Parsek[47]further consolidates the importance of294P.Singh,S.S.Cameotra/Biochemical and Biophysical Research Communications319(2004)291–297biofilm for use in metal bioremediation.They examined the relative toxicities of heavy metals Zn,Cu,and Pb for both biofilm and free-swimming cells of P.aeruginosa.It was found that biofilm was less susceptible to the toxi-city of metals than an equal number of free-swimming cells.However,the degree of increased resistance varied according to the heavy metal.There was a2-fold in-crease in the resistance in lead treated biofilm as com-pared to600-fold increase in the resistance for copper treated biofilm.Biofilms were observed to be more re-sistant to heavy metals than either stationary phase or logarithmically grown planktonic cells.A study by Von Canstein et al.[48]demonstrates the fact that species diversity improves the efficiency of biofilms in metal remediation.Six mercury resistant environmental pro-teobacterial isolates and one genetically modified mer-cury resistant Pseudomonas putida strain were analyzed for physiological traits of adaptive relevance in an en-vironment of packed bed bioreactors designed for the decontamination of mercury polluted chlor-alkali wastewater.Mixed culture biofilms displayed a high mercury retention efficiency that was not affected by rapid increases in mercury or continuously high mercury concentrations.An aspect of practical and commercial importance is the use of granular activated carbon for biofilm development.Both the factors entrap metals and also adsorb not only metals but also polluting organic residues and hence can be used at metal and hydrocar-bon co-contaminated sites[49].All the above-mentionedfindings establish the fact that formation and use of biofilm can be an effective way of treatment of metal contaminated wastewaters.It would indeed be interesting to fully study the interaction of microorganisms within the biofilm and how it relates to metal remediation as compared to organisms in sus-pension.Important to note here is that biosurfactants have been established as compounds which play a vital role in regulating the attachment–detachment of mi-croorganisms to and from surfaces,in quorum sensing and in biofilm formation and hence this aspect opens yet another avenue to be worked upon with respect to metal bioremediation.Apart from better survival in the nat-ural environment,another advantage of biofilms is that they can be processed to collect the sorbed metal and thus recover some process cost.A number of algal biofilms have been studied for metal sorption.However, discussion of algal biofilms for use in metal contami-nation bioremediation is beyond the scope of this review and hence they have not been mentioned here.Metals and chemotaxisAnother interesting aspect related to metal bioreme-diation,which also has been missed until now is chemo-taxis towards metals.Bacterial chemotaxis,movement under the influence of a chemical gradient,either towards (positive chemotaxis)or away(negative chemotaxis)from it,helps bacteria tofind optimum conditions for their growth and survival and this is an integral feature of biodegradation.Negative chemotaxis in response to ac-ids,bases,salts,alcohol,hydrocarbons,heavy metals, oxygen,and phenol has been reported and various other aspects dealt with in a recent review[50].No confirmatory evidence is available however to explain this phenome-non.In the absence of any direct evidence such a survival value of negative chemotaxis can be expected to influence biodegradation by:(i)keeping biodegradation competent cells away from high toxic concentrations and allowing access to only low concentrations of pollutants and(ii) keeping non-degrading cells away from the toxic pollu-tants,thus allowing access only to degrading microor-ganisms so that competition for other nutrients is eliminated.Thus,negative chemotaxis too may have potential in bioremediation application.A number of pollutants have been found to be more rapidly degraded by the wild type strain than by a non-motile mutant or a mutant specifically non-chemotactic to the pollutant[51].In a single study on chemotaxis w.r.t.metals,Childers et al.[52]studied the movement by chemotaxis of Geobacter metallireducens to access insoluble Fe(III)oxide as a means of an adaptive strategy for survival.It would be indeed very efficient if we have metal bioremediating species with a strong chemotactic ability and also an ability to form biofilm. This would make them ideal choice for use in metal bioremediation in wastewater reactors or in soil pro-viding them with better adaptive and survival properties. ConclusionApart from direct microbial metabolism,other mi-crobial activities need to be exploited for enhanced and more efficient metal and metal–organic pollutant co-contaminated site remediation.An excellent option discussed above is the use of microbial products viz. biosurfactants and extracellular polymers which would increase the efficiency of the metal reducing/sequestering organisms infield bioremediation These substances are specific,environmentally more friendly,easily biode-gradable,and easily manageable.Another option is the exploitation of biofilms by metal remediating organisms. Use of mono-/multi-species biofilm offers the organisms a better survival niche and enhances their metabolic capabilities even in the presence of high levels of toxic compounds.The chemotactic potential of the relevant microorganisms needs to be worked upon to enhance the rate of remediation of metal contaminated sites.The use of microorganisms and development of new pro-cesses for the treatment of soils and ground water pol-luted by heavy metals is of economic and environmentalP.Singh,S.S.Cameotra/Biochemical and Biophysical Research Communications319(2004)291–297295interest.Since microorganisms have evolved an enor-mous catabolic potential for numerous natural and synthetic compounds,they are considered infallible. Bioremediation alone,or in combination with microbial activities,hence is an environmentally friendly,cost-effective,and specific alternative to various physical and chemical methods of remediating a polluted site. AcknowledgmentWe thank the Director of the Institute of Microbial Technology (IMTECH)for his encouragement and support.This is IMTECH communication No.022/2003.References[1]D.R.Lovely,J.D.Coates,Bioremediation of metal contamina-tion,Curr.Opin.Biotechnol.8(1997)285–289.[2]D.R.Lovely,E.J.P.Phillips,Novel mode of microbial energymetabolism:organic carbon oxidation coupled to dissimilatory reduction of iron or manganese,Appl.Environ.Microbiol.54 (1988)1472–1480.[3]K.J.Blackwell,I.Singleton,J.M.Tobin,Metal cation uptake byyeast:a review,Appl.Microbiol.Biotechnol.43(1995)579–584.[4]H.L.Ehrlich,Microbes and metals,Appl.Microbiol.Biotechnol.48(1997)687–692.[5]M.D.Geoffrey,Bioremediation potential of microbial mecha-nisms of metal mobilization and immobilization,Curr.Opin.Biotechnol.11(2000)271–279.[6]T.Barkay,J.Schaefer,Metal and radionuclide bioremediation:issues,considerations and potential,Curr.Opin.Microbiol.4(2001)318–323.[7]B.P.Rosen,Transport and detoxification systems for transitionmetals,heavy metals and metalloids in eukaryotic and prokaryotic microbes,Comp.Biochem.Physiol.-Part A Mol.Integr.Physiol.133(2002)689–693.[8]M.Valls,V.D.Lorenzo,Exploiting the genetic and biochemicalcapacities of bacteria for the remediation of heavy metal pollution, FEMS Microbiol.Rev.26(2002)327–338.[9]I.M.Banat,Characterization of biosurfactants and their use inpollution removal—state of the art(review),Acta Biotechnol.3(1995)251–267.[10]I.M.Banat,R.S.Makkar,S.S.Cameotra,Potential commercialapplication of microbial surfactants,Appl.Microbiol.Biotechnol.53(2000)495–508.[11]Z.R.Eliora, E.Rosenberg,Natural role of biosurfactants,Environ.Microbiol.3(2001)229–236.[12]Z.R.Eliora,E.Rosenberg,Biosurfactant and oil bioremediation,Curr.Opin.Biotechnol.13(2002)249–252.[13]R.S.Makkar,S.S.Cameotra,An update on the use of uncon-ventional substrates for biosurfactants production and their new applications,Appl.Microbiol.Biotecnol.58(2002)428–434. [14]L.Siegmund,Biological amphiphiles(microbial biosurfactants),Curr.Opp.Coll.Inter.Sci.7(2002)12–20.[15]S.S.Cameotra,J.M.Bollag,Biosurfactant enhanced bioremedi-ation of PAH,CRC Crit.Rev.Environ.Sci.Technol.30(2003) 111–126.[16]ler,Biosurfactant facilitated remediation of contami-nated soil,Environ.Health Perspect.103(Suppl.1)(1995)59–62.[17]C.N.Mulligan,R.N.Yong, B.F.Gibbs,Surfactant enhancedremediation of contaminated soil:a review,Eng.Geol.60(2001) 371–380.[18]L.Frazer,Lipid lather removes metals,Environ.Health Perspect.108(2000)320–323.[19]D.C.Herman,J.F.Artiola,ler,Removal of cadmium,lead and zinc from soil by a rhamnolipid biosurfactant,Environ.Sci.Technol.29(1995)2280–2285.[20]J.L.Torrens,D.C.Herman,er,Biosurfactants(rhamn-olipid)sorption and the impact on rhamnolipid-facilitated re-moval of cadmium from various soils under saturatedflow conditions,Environ.Sci.Technol.32(1998)776–781.[21]H.Tan,J.T.Champion,J.F.Artiola,M.L.Brusseau,ler,Complexation of cadmium by a rhamnolipid biosurfactant, Environ.Sci.Technol.28(1994)2402–2406.[22]C.N.Mulligan,C.N.Yong,B.F.Gibbs,Removal of heavy metalsfrom contaminated soil and sediments using the biosurfactant surfactin,J.Soil Contam.8(1999)231–254.[23]C.N.Mulligan,C.N.Yong,B.F.Gibbs,Heavy metal removalfrom sediments by biosurfactants,J.Hazard Mat.85(2001)111–125.[24]H.Jeong-Jin,S.Yang,C.Lee,Y.Choi,T.Kajuichi,Ultrafiltra-tion of divalent metal cations from aqueous solution using polycarboxylic acid type biosurfactant,J.Coll.Inter.Sci.202 (1998)63–73.[25]D.L.Gutnick,H.Bach,Engineering bacterial biopolymers for thebiosorption of heavy metals;new products and novel formula-tions,Appl.Microbiol.Biotechnol.54(2000)451–460.[26]H.C.Jyh,R.C.Dawn,L.W.Lion,M.L.Shuler,W.C.Ghiorse,Trace metal mobilization in soil by bacterial polymers,Environ.Health Perspect.103(Suppl.1)(1995)53–58.[27]J.H.Kyung,S.Tokunaga,T.Kajiuchi,Evaluation of remedi-ation process with plant derived biosurfactant for recovery of heavy metals from contaminated soils,Chemosphere49(2002) 379–384.[28]W.Kovalick,Perspectives on risks of soil pollution and experiencewith innovative remediation technologies strategies2000,Proc.World.Congr.Chem.Eng4th(1991)282–295.[29]I.Noriyuki,M.Sunairi,M.Urai, C.Itoh,H.Anzai,M.Nakajima,S.Harayama,Extracellular polysaccharides of Rhodo-coccus rhodochrous S-2stimulate the degradation of components in crude oil by indigenous marine bacteria,Appl.Environ.Microbiol.68(2002)2337–2343.[30]R.S.Todd,M.C.Andrea,R.M.Maier,A rhamnolipid biosurf-actant reduces cadmium toxicity during naphthalene biodegrada-tion,Appl.Environ.Microbiol.66(2000)4585–4588.[31]P.M.Maslin,R.M.Maier,Rhamnolipid enhanced mineralizationof phenanthrene in organic metal co-contaminated soils,Biore-med.J.4(2000)295–308.[32]C.N.Mulligan,C.N.Yong,B.F.Gibbs,On the use of biosurf-actants for the removal of heavy metals from oil contaminated soil,Environ.Prog.18(1999)31–35.[33]G.W.Gray,G.Wilkinson,The action of EDTA on Pseudomonasaeruginosa,J.Appl.Bacteriol.28(1965)153–164.[34]Z.Kamel,Toxicity of cadmium to two Streptomyces sp.Asaffected by clay minerals,Plant Soil93(1986)193–205.[35]J.V.Headly,J.Gandrass,J.Kuballa,K.M.Peru,Y.Gong,Ratesof sorption and partitioning of contaminants in river biofilms, Environ.Sci.Technol.32(1998)3968–3973.[36]A.W.Decho,Microbial biofilms in intertidal systems:an over-view,Cont.Shelf Res.20(2000)1257–1273.[37]H.C.Flemming,Sorption sites in biofilms,Water Sci.Technol.32(2000)27–33.[38]H.H.P.Fang,L.Xu,K.K.Chan,Effects of toxic chemicals onbiofilm and biocorrosion,Water Res.36(2002)4709–4716. [39]C.White,G.M.Gadd,Copper accumulation by sulfate-reducingbacterial biofilms,FEMS Microbiol.Lett.183(2000)313–318.[40]C.White,G.M.Gadd,Accumulation and effects of cadmium onsulfate-reducing bacterial biofilms,Microbiology144(1998) 1407–1415.296P.Singh,S.S.Cameotra/Biochemical and Biophysical Research Communications319(2004)291–297。
