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CU-Boulder GEOL 5700 - The Deep, Hot Biosphere

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Proc.Natl.Acad.Sci.USAVol.89,pp.6045-6049,July1992MicrobiologyThedeep,hotbiosphere(geochemistry/planetology)THOMASGOLDCornellUniversity,Ithaca,NY14853ContributedbyThomasGold,March13,1992ABSTRACTTherearestrongindicationsthatmicrobiallifeiswidespreadatdepthinthecrustoftheEarth,justassuchlifehasbeenidentifiedinnumerousoceanvents.Thislife isnotdependentonsolarenergyandphotosynthesisforitsprimaryenergysupply,anditisessentiallyindependentofthesurfacecircumstances.Itsenergysupplycomesfromchemicalsources,duetofluidsthatmigrateupwardfromdeeperlevelsintheEarth.Inmassandvolumeitmaybecomparablewithallsurfacelife.Suchmicrobiallifemayaccountforthepresenceofbiologicalmoleculesinallcarbonaceousmaterialsintheoutercrust,andtheinferencethatthesematerialsmusthavederivedfrombiologicaldepositsaccumulatedatthesurfaceisthereforenotnecessarilyvalid.Subsurfacelifemaybewide-spreadamongtheplanetarybodiesofoursolarsystem,sincemanyofthemhaveequallysuitableconditionsbelow,whilehavingtotallyinhospitablesurfaces.Onemayevenspeculatethatsuchlifemaybewidelydisseminatedintheuniverse,sinceplanetarytypebodieswithsimilarsubsurfaceconditionsmaybecommonassolitaryobjectsinspace,aswellasinothersolar-typesystems.WearefamiliarwithtwodomainsoflifeontheEarth:thesurfaceofthelandandthebodyoftheoceans.Bothdomainssharethesameenergysource-namely,sunlight,usedintheprocessofphotosynthesisingreenplantsandmicroorga-nisms.InthisprocessthemoleculesofwaterandofCO2aredissociated,andtheproductsofthisthenprovidechemicalenergythatsupportsalltheotherformsoflife.Mostofthisenergyismadeavailablethroughtherecombinationofcarbonandhydrogencompoundsconcentratedintheplantswiththeoxygenthatbecamedistributedintotheatmosphereandoceansbythesamephotosyntheticprocess.TheendproductisagainlargelywaterandCO2,therebyclosingthecycle.Thiswasthegeneralconceptaboutlifeandthesourcesofitsenergyuntil-12yearsago,whenanotherdomainoflifewasdiscovered(1).Thisdomain,the"oceanvents",foundfirstinsomesmallregionsoftheoceanfloor,butnowfoundtobewidespread(2),provedtohaveanenergysupplyforitslifethatwastotallyindependentofsunlightandallsurfaceenergysources.Theretheenergyforlifewasderivedfromchemicalprocesses,combiningfluids-liquidsandgasses-thatcameupcontinuouslyfromcracksintheoceanfloorwithsubstancesavailableinthelocalrocksandintheoceanwater.SuchsourcesofchemicalenergystillexistontheEarth,becausethematerialsherehaveneverbeenabletoreachtheconditionofthelowestchemicalenergy.TheEarthwasformedbytheaccumulationofsolidmaterials,condensedinavarietyofcircumstancesfromagaseousnebulasurroundingthesun.Muchofthismaterialhadneverbeenhotafteritscondensation,anditcontainedsubstancesthatwouldbeliquidorgaseouswhenheated.IntheinterioroftheEarth,heatisliberatedbyradioactivity,bycompression,andbygravitationalsorting;andthiscausedpartialliquefactionandgasification.Asliquids,gases,andsolidsmakenewcontacts,chemicalprocessescantakeplacethatrepresent,ingeneral,anapproachtoalowerchemicalenergycondition.Someoftheenergysoliberatedwillincreasetheheatingofthelocality,andthisinturnwillliberatemorefluidsthereandsoacceleratetheprocessesthatreleasemoreheat.Hotregionswillbecomehotter,andchemicalactivitywillbefurtherstimulatedthere.Thismaycontributeto,oraccountfor,theactiveandhotregionsintheEarth'scrustthataresosharplydefined.Wheresuchliquidsorgasesstreamuptohigherlevelsintodifferentchemicalsurroundings,theywillcontinuetorepre-sentachemicaldisequilibriumandthereforeapotentialenergysource.Therewilloftenbecircumstanceswherechemicalreactionswithsurroundingmaterialsmightbepos-sibleandwouldreleaseenergy,butwherethetemperatureistoolowfortheactivationofthereactions.Thisisjustthecircumstancewherebiologycansuccessfullydrawonchem-icalenergy.Thelifeintheoceanventsisoneexampleofthis.Thereitisbacteriallifethatprovidesthefirststageintheprocessofdrawingonthisformofchemicalenergy;forexample,methaneandhydrogenareoxidizedtoCO2andwater,withoxygenavailablefromlocalsulfatesandmetaloxides.Hydrogensulfideisalsofrequentlypresentandleadstotheproductionofwaterandmetalsulfides;theremaybemanyotherreactionsofwhichwearenotyetaware.Ofalltheformsoflifethatwenowknow,bacteriaappeartorepresenttheonethatcanmostreadilyutilizeenergyfromagreatvarietyofchemicalsources.HowwidespreadislifebasedonsuchinternalenergysourcesoftheEarth?Aretheoceanventsthesolerepresen-tativesofthis,ordotheymerelyrepresenttheexamplesthatwerediscoveredfirst?Afterall,thediscoveryoftheseisrecent,andwemaywellexpectthatotherlocationsthatarehardertoinvestigatewouldhaveescapeddetectionsofar.Bacteriacanliveathighertemperaturesthananyotherknownorganisms;110'Chasbeenverified,andsomebiolo-gistsconsiderthattheuppertemperaturelimitmaybeashighas150'C(providingalwaysthatthepressureissufficienttoraisetheboilingpointofwaterabovethistemperature).TherecanbelittledoubtthatventingofliquidsandgasesfromareasoftheEarth'smantlebeneaththecrustisnotlimitedtoafewcracksintheoceanfloor.Indeedfossilized"dead"oceanventshavealreadybeendiscovered(3),show-ingthatthephenomenoniswidespreadandoccurredindifferentgeologicepochs.Asimilarsupplyoffluidsseemstobewidespreadalsoinlandareas,whereitismuchhardertoinvestigate,butithasbeennotedthatmanyareasofbase-mentrockscontainmethaneandotherhydrocarbons.Thishasbeenseeninnumerousminingandtunnelingoperationsforalongtime.Majorfaultlineshavebeennotedtobehighspotsofhydrocarbonseepage(4).Hydrocarbonshavealsobeenencounteredindeepdrillinginbasementrocks,asintheSovietsuperdeepwellintheKolapeninsulaandinthepilotholeoftheGermanContinentalDeepDrillingProject.Thelargequantitiesofmethanehydrates(methane/waterices)foundinmanyareasoftheoceanfloor,andthoughttocontain6045Thepublicationcostsofthisarticleweredefrayedinpartbypagechargepayment.Thisarticlemustthereforebeherebymarked"advertisement"inaccordancewith18U.S.C.§1734solelytoindicatethisfact.Proc.Natl.Acad.Sci.USA89(1992)moremethanethanallotherknownmethanedeposits(5,


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