外文翻譯-生物預(yù)處理秸稈轉(zhuǎn)化能源的初步研究

外文文獻(xiàn)原文：StatusofBiologicalPretreatmentofLignocellulosics:Potential,ProgressandChallengesShulinChen,XiaoyuZhang,DeepakSingh,HongboYu,XueweiYangDepartmentofBiologicalSystemsEngineering,WashingtonStateUniversity,Pullman,WA99164.SchoolofLifeScience&Technology,HuazhongUniversityofScience&Technology,Wuhan,Hubei,P.R.China,430074.AbstractThefeasibilityofproducingbiofuelsandbiochemicalsfromlignocellulosicbiomassviathebiochemicalplatformdependslargelyonadvancingtechnologiesobtainingsugarsfromthecelluloseandhemicellulosesoftheplantcellwalls.Thispaperprovidesanoverviewonthemeritandchallengesrelatedtodevelopingbiologicalpretreatmentprocessesasanewalternativetobreakthebarriersoftheplantcellwallstructureforsubsequentenzymaticextractionofsugarsfromcellulosepolymer.Althoughhavingattractedlittleattentionduetomanyinherentlimitations,biologicalpretreatmenthasgreatpotentialbecauseofthemultiplebenefits,includingbeingmoreenvironmentallybenign,lessenergyintensive,lessinhibitorproduction,andco-productpossibilities.Increasingknowledgeontermiteandwhite-rotfungiprovidesinsightsindevelopinganewgenerationofpretreatmenttechnologiestorealizethesebenefits.Thisreviewsummarizestheenzymesystemprimarilylignindegradingenzymes,describescurrentunderstandingofbiodegradationofplantcellwallsbymicroorganisms,comparesbiologicalversusthermochemicalpretreatmentprocesses.Thereviewalsosummarizestheprogressinbiopulping,suggestsafutureperspectivefordevelopingthebiologicalpretreatmentprocesses.IntroductionTheonlywaytoobtainrenewabletransportationfuelsandchemicalsisthroughtheuseofplantbiomassthatstorestheinterceptedsolarenergyviaphotosynthesisintheformsoforganiccarbon.Naturehasalsodevelopedvariouspathwaysforutilizingandrecyclingtheseplantmaterialswithminimuminputofadditionalenergy.Indoingso,naturehasbeenabletomaintainasustainable,yetbalancedecosystemformillionsofyears.Thesenaturallyoccurringbiologicalprocessesshouldbeadoptedinaddressingmajorbarriersofbiorefinering-utilizationoflignocellulosicsforbiofuelsandbiochemicalproductionastheyareoftenthemostenergyefficientyetcreateslittleimpacttotheenvironment.Therehasbeenincreasingurgencyfordevelopingbiomassbasedfuelsandchemicalsastheconcernsoverclimatechangeincreasesandfossilfuelresourcesdecline.Forexample,the36billiongallonsofrenewablefuelsproductionperyearby2022ofwhich21billiongallonsmustbeadvancedbiofuels[1].Thebaseforfuturebiofuelandbiochemicaldevelopmentisthesupplyofbiomassfeedstock.Amongalltypesofbiomass,lignocellulosics,thecellwallsoftrees,cropresidues,grasses[2]andalgae[3]ismostabundant.Lignocellulosicbiomassisthemostabundantrenewablebiologicalresourceonearth,withayearlyproductionof~200

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tons[4,5],only3%ofwhichisusedinnon-foodareas,suchasthepaperandpulpindustries[6].Currentcelluloseconsumptionisthreefoldhigherthansteelconsumptionandequalscerealconsumption[7].Astheuseofthesematerialsfortheproductionofbiofueldoesnotconstituteaconflictwiththeproducingfoodforhumanconsumption,lignocellulosicbiomasswillbethemajorfeedstockforthefuturebiorefineries.Theplantcellwalls(PCW)aretheprimarymaterialswhereenergyandorganiccarbonarestored.ThecompositionandstructureofPCWdeterminesthedesignofthedownstreamprocessesusingPCWasrawmaterialstoproducevarioustargetmolecules.Plantconsistsofanorderlyarrangementofcellswithwallscomposedofvaryingamountsofamixtureofcellulose(ca.40%),hemicellulose(ca.20-30%)andlignin(ca.20-30%)[8].CelluloseisalinearpolymerofD-glucoseunitslinkedbyβ-1,4-glycosidicbonds.Hemicelluloseisheteropolymercontainingmanysugarmonomers(xylose,mannose,galactose,arabinose,andrhamnose)formingrandomamorphousstructures.Ligninontheotherhand,isacrosslinkedmacromoleculeconsistingofprimarilythreemonolignolmonomers(p-coumeraylalcohol,coniferylalcohol,andsinapylalcohol)thataremethoxylatedtovariousdegree.Lignincelluloseisacompact,inpartcrystallinecomplex,andpolysaccharidecomponentswhichformmicrofibersaredenselypackedinlayersoflignin,protectingthemagainsttheactivityofhydrolyticenzymesandotherexternalfactors,servingasastabilizerofthecomplexstructure[9].Suchastructurecanbeillustratedusingwoodasanexample.Generally,thesupportingcellhasalumenafterthecellisdead,whichcanbemoreorlessemptyorfilledwithwater.Theinnerlayerisaheterogeneousmixtureofcomponentsofunknowncomposition.Thesecondarywalloutsidetheinnerlayercanbefurtherdividedintothreesublayers(S3,S2,S1)allconsistingofcellulosemicrofibrilsembeddedinanamorphousmixtureofdifferenthemicellulosesandlignin[10].TheconcentrationofthecelluloseishighestintheS2sublayerofthesecondarywallanddecreasetowardsthemiddlelamella.TheS3layer,whichisnearthelumenisrichinhemicelluloses.Ligninhasthehighestconcentrationinmiddlelamella.However,sincethesecondarywallismuchthicker,itcontainsmostofthelignin(60-80%).LignificationisinitiatedintheprimarywallsadjacenttothecornersofthecellundergoingcellulosedepositionintheS1layersnearthecambiumandthenproceedsintheintercellularlayersandprimarywalls[11].Intheprimarycellwall,crystallineandamorphouscellulosecoreissurroundedbyhemicellulosepolymer.Ingrass,themiddlelamellaandprimarywallcontainsmoreC-Ccouplingofmonolignolsintoahighlybranchedpolymer.Incontrast,theβ-O-4couplingofmonolignolsleadstoarelativelylinearpolymerinthesecondarycellwall[12].Thehighlybranchedligninsintheprimarycellwallsaremoreinhibitorytocellwalldegradationthanthelinearpolymerinthesecondarywalls[13].TheessenceofconvertinglignocellulosicstofuelsandchemicalsistoobtainthedesirableformoforganiccarbonmoleculesfromthePCWtobeusedeitherasprecursormoleculesorenergysourcesforthetargetedfuelproducts.Therearetypicallytwomajorplatformsforbiomassconversion.Thefirstoneisbiochemicalplatforminwhichvarioussugarmoleculesarefirstobtainedfromthebiomass.Thesugarsarethenusedbymicroorganismstobeconvertedsubsequentlytotargetfuelmoleculessuchasethanol.Whereastheotheristhermochemicalplatforminwhichthelignocellulosicsiseitherfirstbrokenintoamixtureofmoleculesorsimplemolecules.Someofthemolecules,uponseparationfromthemixture,canbeuseddirectlyasfuels;whereasthesimplemolecules,uponfurtherprocessing,canbesynthesizedasfuelmolecules.Forthepurposeofthispaper,ourdiscussionswillbelimitedtothebiochemicalplatform.

Mostbiologicalprocessesfocusontheuseofthesugarsastheenergyandcarbonsourcethroughfermentationtoderivedifferentproducts.Thepredominantsugarunitisglucosesthatareboundedcovalentlyintheformofcellulosepolymerwithintheplantcellwallstructure.ObtainingthesugarsfromthePCWisoneofthemajorbottlenecksinbiorefinering.Thesugarunitslockedinthecelluloseandhemicellulosepolymersarethesoleenergyandcarbonsourcesusedinfermentationforbiofuelandbiochemicalproduction.Thebreakdownofcellulosepolymerisofternaccomplishedbiochemicallybycellulolyticenzymescalledcellulases[14].Lignocellulose,anextremelycomplexandwidelyvaryingnanoscalecompositeofcellulose,iswelldesignedtoresistattack.Thecomplexofligninandhemicellulosesstructureandthehydrophobicityofthecellwallpreventenzymefromaccessingthecellulosepolymer.Thus,thestructureoftheligninandhemicellulosesneedstobedegradedormodifiedtofreespaceforcellulasetoaccess.Thisisoftenaccomplishedthroughapretreatmentprocess.Thecurrentpretreatmenttechnologiesformakinganeasyaccessforthecellulaseenzymetocatalyzecellulosedegradationtosugarsarephysiochemicalinnature.Thepurposeoftheseprocessesistodegradethehemicellulosesorligninstructure.Hydrothermalprocessesincludesteamexplosion[15],carbondioxideexplosion[16],orhotwatertreatment[17].Likewise,chemicalprocessesincludedilute-acidtreatment[18],alkalitreatment[19],organosolvprocessusingorganicsolvents[20],ammoniafiberexplosion(‘‘AFEX”)[21],ammoniarecyclepercolation[22],andozonolysis[23].However,allthesepretreatmentmethodssufferinherentseriousdrawbacks.Thechemicalandphysicalmeansareoftenlimitedbythelackofselectivity.Althougheffective,theseprocessestendtodamagethebasicunitsofthesebiopolymersbyreactionstoformnewandoftenunwantedcompounds,suchasthereleaseoftoxicandhazardouspollutants[24,25].Currently,thereisnoanysinglepretreatmenttechnologythatisperfectlyacceptableintheconversionofbiomassintobiofuel.Thereisincreasingneedsfornewpretreatmentapproaches.Asweareenteringaneweraofindustrialbiotechnology,syntheticbiology,metabolicengineering,andsystembiologyprovidenewtoolstoengineermicrobestoproducedadvancedbiofuelssuchashydrocarbons.Therealadvancementinbiofuelproductionusingthesetoolswillbelimitedifthesupplyofsugarstothesemicroorganismsremainsamajorbarrier.Consideringthefactthatnaturehasbeencreatedthroughevolutionbiologicalcatalystsasamixtureofenzymaticcomplexthatarecapableofunlockthecomplexstructureofligninmoleculesbyselectivelycleavingthechemicalbondsbetweenthebasicunits,weshouldseekexploitingthesimilarprocessesforbiologicallypretreatingthePCWwithlignindegradingenzymesandhemicellulosestoachievetheultimategoalofconsolidatingpretreatmentandsaccharification.Theseenzymeswouldbeappliedbeforeoraftertraditionalpretreatmenttominimizeand,eventually,replacethermochemicalprocesses,thuslesseningtheeffectsofoverallpretreatmentseverityatthemacromolecularlevelandsimplifyingprocessing.Intheinterestsofsustainabilityandenergyefficiency,biologicalprocessissuperiortothephysiochemicalonesastheyoccursundernaturalenvironmentanddonotproducedisruptionsthatarenottolerablebytheenvironment.Basedonthebeliefthatsuchabiologicalpretreatmentwillattractmoreattentioninthefuture,thisreviewaimsatprovidingacomprehensivesurveyonthistopic.Itisxpectedthatthispaperwillpresenttherationaleofbiologicalpretreatmentprocess,overviewthestateoftheartoftheknowledge,andidentifytheinformationandtechnologygaps.Finally,thereviewofferssomespeculationsandsuggestssomedirectionsforfutureresearchanddevelopment.Itneedstobepointedoutthatanidealpretreatmentprocessrequiresthedeconstructionofbothligninandhemicellulose.Thisreviewwill,however,focusprimarilyonlignindegradation.Thisarticlestartswithligninolyticenzymesystems,followedbyhowdifferentmicroorganismsdegradatedlignin.Comparisonsofbiologicalpretreatmentandthermochemicalpretreatmentarethenprovided,followedbybiopulpingasapplicationexamples,andconcludedwithfutureperspectiveandchallenges.1.LigninolyticenzymesystemLignincanbedegradatedbyenzymesproducedbyvariousorganismsamongwhichwhiterotfungushasbeenfoundthemosteffective.Ligninbiodegradationbywhiterotfungiinvolvesvariousenzymes,andthemostsignificantthreearelaccases(benzenediol:oxygenoxidoreductase,EC),ligninperoxidases(LiPs,EC4),andmanganeseperoxidases(MnPs,EC3)[26,27].LiPs,MnPs,andlaccasearephenoloxidaseswhichcatalyzesimilarreactions[10].Theyoxidizephenoliccompoundstocreatephenoxyradicals.Non-phenoliccompounds,ontheotherhand,areoxidizedtothecorrespondingcationradicals[10].Obviously,notallthewhiterotfungicanproduceallthethreeligninolyticenzymes,dependingonthespeciesofwhiterotfungiandthetypeofsubstrates[28,29].ThewhiterotfungusP.chrysosporiumevidentlycandegradeligninwithoutproducinglaccases[30]andproduceslaccasesinthepresenceofcellulose[1].Pycnoporuscinnabarinusisreportedtoproducelaccasesbutnotperoxidases[2].PleurotusdryinusproducesMnPsatthepresenceoflignocellulosicsubstratebutnoenzymeproductioninsyntheticmedium[3].2.ProcessesofbiologicaldeconstructionofplantcellwallsTherearemanywaysofplantdecayinnaturebydifferentorganismsinadditiontofungi.AlthoughmanyofthemechanismsofPCWdegradationbythesesystemsarestillunknown,thereisnodoubtthatfurtherunderstandingoftheseprocesseswillprovidecriticalinsightintodevelopingnewgenerationofpretreatmentprocesses.3.Comparisonofbiologicalpretreatmentwithtypicalthermochemicalprocesses3.1.EffectivenessMainpurposeofthepretreatmentforlignocellulosesistodismantlethematrixstructureofligninandhemicellulosestomodifytheporesinthematerialtoallowcellulolyticenzymestopenetratethebarrierinthePCWtodegradecellulosepolymer[15].Thus,thepretreatmentshouldbeeffectivetoavoiddegradationorlossofcarbohydrate,andavoidformationofinhibitoryby-productsforthesubsequenthydrolysisandfermentationanditmustbecost-effective[11].3.2.EnergyconsumptionComparingtothebiologicalpretreatment,thermochemicalmethodstoconvertthelignocellulosicbiomassutilizeslargeamountofenergyintheformofheatandchemicals.Forexample,alkalineprocessessufferfromsilicascalinginchemicalrecoverybecausemanyagriculturalfeedstocks,suchasriceandwheatstraw,haveveryhighsilicacontent.Thescalingproblemprohibitstherecoveryofalkalinechemicalsfrompretreatmentliquor[10].Similarly,theuseofdiluteacidpretreatmentisnotentirelysatisfactoryforwoodybiomass,inparticularsoftwoods.Therequirementofsizereductionpriortothepretreatment[10-12]makesthediluteacidprocesslesssuitableforpretreatingfeedstockswithstrongphysicalintegrity,suchaswoodybiomass,bamboo,andgiantreed,becauseofthehigh-energyconsumptioninsizereduction.Theuseofconcentratedacidhasbeenusuallybasedonthesolubilizationofplantpolysachharidesin72%(w/v)H2SO4or41%(w/v)HClatlowtemperatures,followedbydilutiontoa3-6%acidconcentrationandheatingat100-120℃for30-360min[13].Although,closetothetheoreticalsugarproductioncanbeachievedthroughthisprocess,itinvolveshighcapitalinvestment,acidconsumptionandacidrecoverycosts[14].3.3.Inhibitorsasby-productsAmajordisadvantageofthermochemicalpretreatmentprocessesistheproductionofby-productsthatofteninhibitdownstreamprocesses.Duringthermochemicalandhydrothermalprocesses,someoftheglucosereleasedfromcelluloseisdegradedto5-hydroxymethylfurfural(HMF),levulinicacid,andformicacid.Likewise,thepentosefromhemicelluloseisconvertedtofurfuralandformicacid.Duringsteamexplosion,ligninisprimarilydegradedthroughthehomolyticcleavageofβ-O-4etherandotheracid-labilelinkages,producingaseriesofcinnamylalcoholsderivativesandcondensationby-products[14].Releaseoftheselowmolecularmasscompoundsgraduallyincreasestowardshigherpretreatmentseverities[15].Theseproducts,alongwiththedegradedligninproductsandreleasedorganicacidsactasinhibitorsofenzymaticsaccharificationandethanolfermentation[16,17](Figure1).Thedetoxificationstep,ifrequiredafterthethermochemicalprocessisverycostlyandoftenineffective[12].Inaddition,thechemicalprocessesalonehaveseriousdisadvantagesintermsoftherequirementforthespecializedcorrosionresistantequipment,extensivewashing,andproperdisposalofthewastes.Biologicalpretreatmentusingwhiterotfungi,ontheotherhand,canavoidtheproductionoftheseunwantedby-products.3.4ReactionrateAlthough,thebiologicalpretreatmentprocessisasafe,lowenergyrequiringprocessforlignocellulosicmaterialdisintegration,thetypicaldegradationprocessusingfungioccursinalongerincubationtime.Someofthebiologicalpretreatmenttimeofincubationandreleasedsugarpercentagehasbeentabulatedbelow.Hatakkaetal.[19]studiedthebiologicalpretreatmentofwheatstrawby19fungalstrains.Incubatedatthe37℃inthesolidstatefermentation,Pleurotusostreatuswasabletomake35%ofthestrawintoreducingsugarsin35daysofincubation(Table1).Forthesamesubstrate,Phanerochaetesordidshowedthesimilarproductionratein28days[18].Totalsugarreleasedwashigher;38.9%fromricehullsbyP.ostreatusin60days.Pycnoporuscinnabarinustook28daystorelease35%totalreducingsugarsfromthewheatstraw[15].Likewise,Tanaguchietal.[16]demonstratedthatthereleaseof83%celluloseand52%hemicellulosewasobtainedfromricestrawafter60days.Table1:Biologicaldegradationofvarioussubstratesbydifferentfungalstrainsandcontributioninthesugarreleaseaswellasligninloss.SubstrateMicroorganismTotalsugarreleasedLigninlossTime(days)ReferenceWheatStrawPleurotusostreatus35%cNM35[19]SapwoodofpinelogsPhanerochaetegigantae71%hNM32[17]Aspen(Populustremuloides)chipsPleurotus(Fr.)P.Karst.SpeciesNM38%f56[18]RicestrawPleurotusostreatus83%a52%b25%e41%f60[16]JapaneseredcedarPycnopruscinnabarinus35%cNM28[17]WheatstrawPhanerochaetesordida35%cNM28[15]SoftwoodPinusdensifloraStereumhirsutum21.0%ab10.7%e14.5%f56[19]RicehullsPleurotusostreatus38.9%60[17]CocoapodhuskPleurotusostreatusNM17%g35[10]a:cellulose;b:hemicellulose;ab:totalsugarincreasedthanuntreatedcontrol;c:totalreducingsugars;d:degradationimprovement;e:totalweightloss;f:Klasonligninloss;g:totalligninloss;h:pitchcontentreduction;NM:Notmentioned.4.Near-termexamplesandfutureperspectiveAnexampleofnear-termapplicationofbiologicalpretreatmentisbiopulping.Byusingfungitoalterthelignininthecellwallsofthewood,it"softens"thewoodchips.Therefore,throughthebiopulpingprocess,energyconsumptiontoconvertthewoodintopapercouldbelessduetothepreferentialremovalofligninbythefungus[15].Therefore,biopulpingisfocusedonlignolyticenzymestosubstitutethechlorinatedagentsinthepaperpulpbleaching[16].Inadditiontothis,biopulpingdoesnotpollutetheairsincethereisnoneedofchemicals.Biopulpingwasenvisionedasamethodforsavingenergyandmakingastrongerpaperproduct[15].Inbiopulping,woodchipsareoftensupplementedwithnutrientslikecornsteepliquor.Thesupplementednutrientswillratherhelptoincreasethefungalbiomassatthesurfaceofthewoodchip,whichinturnenablesanevencolonizationofthedeeperareaofthechip[17].5.ConcludingremarksCementinghemicelluloseandcellulosetogetherinthelignocellulosiccellwalls,ligninprovidesplantstrengthandmakesplantshardformicrobestoattack.Nonetheless,naturehasdevelopedecologicallysustainableprocessesforextractingsugarsfromplantcellwallsandforrecyclingthelignocellulosicbiomass.Thecurrenttechnologiesdevelopedartificiallyarenotassustainableasthenaturalprocessastechnologiesrequireenergyandchemicalsandproduceinhibitors.Anidealpretreatmentprocesswouldallowsequentialdegradationofthelignininthecellwalltocreatethespacefortheaccessofcellulaseandhemicellulasetotheirsubstrate.Duringthisprocess,thebreakdownoftheligninandhemicelluloseschainisspeciallydesignedanddiligentlycontrolledtoavoidtheproductionofinhibitorsfromthederivatives.Additionally,thedegradationprocesswillbeaccomplishedundermildconditionwithminimumlossofhemicellulosesandcellulose,noheat,pressureorextremepHisrequired.Clearly,theprocessofbreakdownandrecyclingofthelignocellulosicmaterialsinnatureisclosetosuchanidealpretreatmentprocess.Althoughwithdrawbackbeingslowinreactionandpossiblelossofafractionofthesugars,thenaturalbiologicalprocesssuggestsapossibilityofamuchmoreenvironmentallyfriendlyandenergynon-intensiveprocess.UponbetterunderstandingthemechanismsofPCWdegradationinthesenaturalprocesses,theycanbesignificantlyimprovedbyemployingmodernbiotechnologyandengineeringtools.Newtechnologiesmayemergeasthenextgenerationofpretreatmentprocessesofferingmorecosteffectivepretreatmentoption.Theultimatetargetfortheresearchdevelopmentistoseekthemostsustainableapproachatacceptablecost.Thenear-termdevelopmentidentifiedasnewgenerationofpretreatmenttechnologiesmaybethecombinationofbiologicaltreatmentwithmildthermochemicaltreatment,providinghighefficiencyinthedelignificationprocesscosteffectivelywithoutproducingtoxicchemicalstotheenvironment.Inaddition,attentionshouldbepaidtotheuseoflignocellulosicbiomassfornotonlytheextractionofsugarycomponentsbutalsoforrecoveryofthefuelcompoundsandchemicalsfromlignin.Thefungalsystemasdemonstratedwithbiopulpingcanservedasastartingpointwhilethenewinsightsfromtermitesystemofferpromisingalternatives.Thedevelopmentoffurtherunderstandingofthesesystemsandtheavailabilityofincreasinglypowerfulbiotechnologyandengineeringtools,incombinationwithhumancreativities,willundoubtedlyspeeduptheprogresstowardanidealpretreatmentprocess.References:H?gerdalBH,HimmelME,SomervilleC,WymanC:WelcometoBiotechnologyforBiofuels.Biotechnol.Biofuel.1(1),1-4(2008).RubinEM:Genomicsofcellulosicbiofuels.Nature454(4),841-845(2008).SheehanJ,DunahayT,BenemannJ,RoesslerP:AlookbackattheU.S.DepartmentofEnergy’saquaticspeciesprogram:biodieselfromalgae.NRELTP-580-241901-296(1998).ReddyN,YangY:Biofibersfromagriculturalbyproductsforindustrialapplications.TrendsBiotechnol.23:22–27(2005).ZhangYHP,LyndLR:Towardanaggregatedunderstandingofenzymatichydrolysisofcellulose:noncomplexedcellulasesystems.Biotechnol.Bioeng.88,797–824(2004).KammB,KammM:Principlesofbiorefineries.Appl.Microbiol.Biotechnol.64,137–145(2004).DasH,SinghSK:Usefulbyproductsfromcellulosicwastesofagricultureandfoodindustry—acriticalappraisal.Crit.Rev.FoodSci.Nutr.44,77–89(2004).SjostromE:Plantchemistry:Fundamentalsandapplications,2nded.AcademicPress,NewYork/London,(1993).FengelD:Ultrastructuralorganizationofthecellwallcomponents.J.Polym.Sci.36,383-392(1971).KuhadRC,SinghA,ErikssonKL:Microorganismsandenzymesinvolvedinthedegradationofplantfibercellwalls.Adv.Biochem.Eng.Biotechnol.57,45-125(1997).WardropAB,BlandDE:Theprocessoflignificationinplantyplants.In:Biochemistryofplant.KratziK,BillekG(Ed.),NewYork,92-116(1959).JurasekL:Experimentingwithvirtuallignins.In:Ligninand