Exposure to Artificial Light at Night and the Consequences for ...

Artificial lighting in general, and LEDs in particular, add to the list of numerous anthropogenic pressures that, decade after decade, are ... Articles JacquesEpelbaum InstitutNationaldelaSantéetdelaRechercheMédicale(INSERM),France ThomasDickmeis KarlsruheInstituteofTechnology(KIT),Germany RandyJ.Nelson WestVirginiaUniversity,UnitedStates Theeditorandreviewers'affiliationsarethelatestprovidedontheirLoopresearchprofilesandmaynotreflecttheirsituationatthetimeofreview. Abstract Introduction TheIntegrationoftheLightSignalinLivingOrganisms OrientationinSpace:Phototaxis,Phototropism OrientationinTime:TheCircadianClocks ImpactofAlanandLEDsonLivingOrganisms LEDsandEcosystems LEDsandOtherAnthropogenicFactors:SomeExamples Conclusion AuthorContributions ConflictofInterest Funding Acknowledgments Abbreviations Footnotes References SuggestaResearchTopic> DownloadArticle DownloadPDF ReadCube EPUB XML(NLM) Supplementary Material Exportcitation EndNote ReferenceManager SimpleTEXTfile BibTex totalviews ViewArticleImpact SuggestaResearchTopic> SHAREON OpenSupplementalData REVIEWarticle Front.Neurosci.,16November2020 |https://doi.org/10.3389/fnins.2020.602796 ExposuretoArtificialLightatNightandtheConsequencesforFlora,Fauna,andEcosystems JackFalcón1*,AliciaTorriglia2,DinaAttia3,FrançoiseViénot4,ClaudeGronfier5,FrancineBehar-Cohen2,ChristopheMartinsons6andDavidHicks7 1LaboratoireBiologiedesOrganismesetEcosystèmesAquatiques(BOREA),MNHN,CNRSFRE2030,SU,IRD207,UCN,UA,Paris,France 2CentredeRecherchedesCordeliers,INSERMU1138,OphtalmopoleHôpitalCochin,AssistancePublique-HôpitauxdeParis,UniversitédeParis-SU,Paris,France 3ANSES,FrenchAgencyforFood,EnvironmentalandOccupationalHealth&Safety,Maisons-Alfort,France 4MuséumNationald’HistoireNaturelle,Paris,France 5LyonNeuroscienceResearchCenter(CRNL),WakingTeam,InsermUMRS1028,CNRSUMR5292,UniversitéClaudeBernardLyon1,Lyon,France 6CentreScientifiqueetTechniqueduBâtiment,SaintMartind’Hères,France 7Inserm,CNRS,InstitutdesNeurosciencesCellulairesetIntégratives,UniversitédeStrasbourg,Strasbourg,France Thepresentreviewdrawstogetherwide-rangingstudiesperformedoverthelastdecadesthatcataloguetheeffectsofartificial-light-at-night(ALAN)uponlivingspeciesandtheirenvironment.Weprovideanoverviewofthetremendousvarietyoflight-detectionstrategieswhichhaveevolvedinlivingorganisms-unicellular,plantsandanimals,coveringchloroplasts(plants),andtheplethoraofocularandextra-ocularorgans(animals).Wedescribethevisualpigmentswhichpermitphoto-detection,payingattentiontotheirspectralcharacteristics,whichextendfromtheultravioletintoinfrared.Wediscusshoworganismsuselightinformationinawaycrucialfortheirdevelopment,growthandsurvival:phototropism,phototaxis,photoperiodism,andsynchronizationofcircadianclocks.Theseaspectsaretreatedindepth,astheirperturbationunderliesmuchofthedisruptiveeffectsofALAN.Thereviewgoesintodetailoncircadiannetworksinlivingorganisms,sincethesefundamentalfeaturesareofcriticalimportanceinregulatingtheinterfacebetweenenvironmentandbody.Especially,hormonalsynthesisandsecretionareoftenundercircadianandcircannualcontrol,henceperturbationoftheclockwillleadtohormonalimbalance.Thereviewaddresseshowtheubiquitousintroductionoflight-emittingdiodetechnologymayexacerbate,orinsomecasesreduce,thegeneralizedever-increasinglightpollution.NumerousexamplesaregivenofhowwidespreadexposuretoALANisperturbingmanyaspectsofplantandanimalbehaviourandsurvival:foraging,orientation,migration,seasonalreproduction,colonizationandmore.Weexaminethepotentialproblemsatthelevelofindividualspeciesandpopulationsandextendthedebatetotheconsequencesforecosystems.Westress,throughafewexamples,thesynergisticharmfuleffectsresultingfromtheimpactsofALANcombinedwithotheranthropogenicpressures,whichoftenimpacttheneuroendocrineloopsinvertebrates.Thearticleconcludesbydebatinghowtheseanthropogenicchangescouldbemitigatedbymorereasonableuseofavailabletechnology–forexamplebyrestrictingilluminationtomoreessentialareasandhours,directinglightingtoavoidwastefulradiationandselectingspectralemissions,toreduceimpactoncircadianclocks.WeendbydiscussinghowsocietyshouldtakeintoaccountthepotentiallymajorconsequencesthatALANhasonthenaturalworldandtherepercussionsforongoinghumanhealthandwelfare. Introduction Humanactivitiesarealmostexclusivelyassociatedwithbrightlylitenvironments.ThelastcenturyhasseenanunprecedentedincreaseintheuseofArtificialLightatNight(ALAN),withacurrentongoingglobalincreaserateofmorethan6%peryear(Hölkeretal.,2010).Thisisdramaticallyaffectinglandaswellasaquaticandopenseaareas.Mediterraneanandtemperatezones,mangrovesandforestregionsinproximitytoagriculturalareasareparticularlyaffected(Votsietal.,2017).Today,morethan80%oftheworldspopulationlivesundera“litsky”atnight(Falchietal.,2016),actuallyaffectingupto99%inEuropeandNorthAmericaandontheincreaseintheMiddleEast(Tamiretal.,2017)andAsia(Jiangetal.,2017).ALANactsbothdirectlyandindirectly(throughskyglow)uponorganisms.Theilluminanceatgroundlevelcanequalthatofthefullmoon(0.01<<1lx)(Bennieetal.,2015a,2016;Figure1)andcanevenbeamplifiedbythecloudceiling.ALANwasfirstintendedtodetectobstacles,increaseroadsafetyandsecurepotentiallydangerousareasatnight,buthasnowbeenextendedtoallaspectsofhumanactivities,includingindustrial,commercial,amenityspacesortouristpurposes.Illuminationlevelsoftenexceedrealneeds;insomeareastheaestheticaspects(lightingofmonuments)oradvertising(lightingofcommercialareas,shopwindows,streetsignsandilluminatedposters)havebeengivenprecedent.Itfollowsthatuntouchednaturalareas-essentialtothedevelopmentofwildlife-areconstantlydecreasing.Theconsequencesonbiotopesandlivingorganisms(includinghumans)aremultiple.Basicresponsesandfunctionsrelatedtoorientationinspace(phototaxis,phototropism)andtime(circadianrhythms)areaffectedbyALAN.Theseprocessesaretheresultofmillionsofyearsofevolution,whileALAN-inducedchangesareoperatingonatimescaleofonlyafewdecades.Thisisparticularlyevidentwhenitcomestotemporalevents,whichdependonthepredictablealternationoflight(L)anddarkness(D)duringthe24hLDcycle,dayafterdayandseasonafterseason.Fromtheveryearliesttimesoflifeonearth,organismsdevelopedtime-measurementsystems-circadianclocks-whichallowedthemtoforecastandanticipatethesenaturalchanges,essentialforaligningphysiologicalactivitywiththeappropriatetime.Asaresult,mostofthebasicfunctionsoflivingorganismsarecontrolledbytheseinternal,geneticallydetermined,clocks.Theseclocksdependabsolutelyonthe24hLDcycletoaccuratelysynchronizetheiractivitywithsolartime,andinturntheyorchestrateamyriadofdownstreambiochemical,physiologicalandbehaviouraleventssothattherightprocessoccursattherighttime.Thus,changingthenaturalLDcyclecannotbewithoutconsequencesforbiologicalorganisms.Inhumans,perturbationofthecircadiansystemresultsinmajorphysiologicalimpacts(Attiaetal.,2019),forexampleinalteredhormonalbalance,includingmelatoninsecretion.Melatoninisonekeycircadianclockoutputinvolvedinthesynchronizationofmanyrhythmicfunctions;inadditionitissuspectedtopossesspowerfulanti-oxidativeproperties(Reiteretal.,1997).Inhumans,acorrelationbetweenALANandtheappearanceofvariousdisorders(activity/sleeprhythms,mentalhealthdisorders,energymetabolism,weightgainandobesity,sensitivitytosomecancers[breast,prostate])hasbeendocumentedquiteextensively(Dominonietal.,2016;Attiaetal.,2019)butthelevelofproofremainslowbecauseinmostcasesthelightintensitiesusedarefarabovethelevelsencounteredinALAN. FIGURE1 Figure1.(A)Illuminancemeasuredinthehorizontalplanefromatypicalstreetlight(PhillipsCosmopolis,metalhalidelamp).Theilluminanceleveldecaysrapidlywithdistancetothelamp.(B)Comparisonofmeasuredilluminancefromnaturalsourcesoflighttoartificiallightsources–axisisonalogarithmicscale,andbarspresentapproximaterangesbasedonfieldmeasurements.FromBennieetal.(2016).Nospecialpermissionrequired. Here,weprovideanoverviewofthetremendousvarietyoflight-detectionstrategieswhichhaveevolvedinunicellularorganisms,plantsandanimals.Wefurthergiveacomprehensivedescriptionofthedifferentvisualpigmentswhichpermitphoto-detectioninalllivingorganismsfromultraviolettoinfrared.Thereviewthenmovesontodiscusshowlivingorganismsactuallyuselightinformationinameaningfulway,crucialfortheirdevelopment,growthandsurvival:phototropism,phototaxis,photoperiodism,andsynchronizationofcircadianclocks.Theseaspectsaretreatedindepth,astheirperturbationunderliesmuchofthepotentiallydisruptiveeffectsofALAN.Thereviewgoesintoconsiderabledetailoncircadiannetworksinlivingorganisms,sincethesefundamentalfeaturesexistinvirtuallyalllifeformsandareofcriticalimportanceinregulatingtheinterfacebetweenenvironmentandbody.ItisnecessarytounderstandthediverseprinciplesunderlyingtheirfunctioningacrossthedifferentphylainordertoappreciatewhyALANcanrepresentsuchadisruptiveinfluence.Althoughmuchofthedatareportedintheliteraturenecessarilycomesfromolderlightingtechnology,thereviewaddresseshowtheapproachingubiquitousintroductionoflight-emittingdiode(LED)technologymayexacerbate,orinsomecasesreduce,thegeneralizedever-increasinglightpollution.Afocusisputonthefundamentalroleofshortwavelengthemissions,sincethesearethemostrelevantwavelengthswhenconsideringsignallingthroughvertebratephotoreceptivetissuesandsynchronizationofcentralcircadianclocks.Neverthelessthepaperalsostressesthatduetothehugerangeoflightdetectionsystemsusedbylivingorganisms,otherwavelengthsmayalsobeproblematic.NumerousexamplesaregivenofhowwidespreadexposuretoALANisperturbingmanyaspectsofplantandanimalbehaviourandsurvival.Weexaminethepotentialproblemsatthelevelofindividualspeciesandpopulationsbeforeextendingthedebatetotheconsequencesforintegratedecosystems.ItalsoemphasizesadditiveharmfuleffectsresultingfromtheimpactsofALANtogetherwithotheranthropogenicpressures.ThearticleconcludesbydebatinghowtheseanthropogenicchangescouldbeeasilymitigatedbymorereasonableuseofavailabletechnologyandhowsocietyshouldtakeintoaccountthepotentiallymajorconsequencesthatALANhasonthenaturalworldandtherepercussionsforongoinghumanhealthandwelfare. TheIntegrationoftheLightSignalinLivingOrganisms Nothinginbiologymakessenseexceptinthelightofevolution(DobzhanskycitedinLamb,2013). Thecaptureoflightinformationgoesbacktoancestralcyanobacteria,thefirstknownrepresentativesoflifeonearth,whichappeared∼3.8billionyearsago.Itallowsorganismstoorientateinspace(phototropismforanimals,phototaxyforplants)andtime(synchronizationoftheendogenousclocksthatdrivethedaily,lunarandannualrhythmsofmetabolic,physiologicalandbehaviouralfunctions).Livingbeingshaveimplementedahugevarietyofsystemsandmechanismsinordertocapturelight,fromsimplephotoreceptiveorganellestohighlycomplexstructuressuchasthechloroplastofplantsandthecameraeyesofvertebrates,insectsandcephalopods. Inunicellularorganisms,photoreceptionismediatedbyaphotoreceptororganelleexistingaseitherasinglespot(cyanobacteria,euglena)oramoreelaboratedstructure(dinoflagellates),containingalltheelementsfoundinavertebrateeye,i.e.,pigment,acornea-shapedsurface,alensandalamellarstructure(Gehring,2005,2011,2014).Ithasbeenhypothesizedthattheseorganellesmightcorrespondtochloroplastsincorporatedbyhorizontaltransmission,buthavinglosttheirphotosyntheticactivity(Gehring,2012). Cyanophyceae,thecurrentrepresentativesoftheancestralcyanobacteriaare,liketheoriginalform,capableofcapturinglightandensuringphotosynthesis.Theyexistassinglecellunitsorassociatedinfilaments,andcanfixcarbondioxide[CO2]andreleaseoxygen[O2],buthavenochloroplast.PhototaxyandphotoperiodicsynchronizationofcircadianclockshavebeendemonstratedinCyanobacteria(Gehring,2012),asintheterrestrialCyanobacteriumLeptolyngbyasp.,whichshowstwomaximaofabsorption(λmax)at456and504nm.PopulationsofCyanobacteriaareincreasingworldwide,favouredbytrophicand/orecologicalimbalances(includingeutrophicationofwater),andposemajorphysical(invasion,obstructions)andtoxicological(productionofdangerousorevendeadlytoxins)problems(SvrcekandSmith,2004). TheChloroplastofPlants Theingestionofcyanobacteriabyprimitiveeukaryoticcells∼1.5/1.6billionyearsagoledtotheformationofchloroplasts(Figure2),foundinthecytoplasmofeukaryoticphotosyntheticcells(Kirchhoff,2019).IntheunicellularalgaoftheChlamydomonasgenus,thereisonechloroplastpercell,whilemulticellularplantspossessseveraltensofchloroplastsinonecell,withtheleavesshowingthehighestdensity.Thechloroplastallowsphotosynthesis,i.e.,itabsorbslightenergytofixinorganicCO2andproducesglucoseandO2(thehighestproductionofO2isfromalgaeandmarinephytoplankton,followedbyforests).Moreover,itisinvolved,byinteractingwithphotoreceptivemoleculesandcircadianclockgenes,intheresponsetolight(Jaubertetal.,2017). FIGURE2 Figure2.Thechloroplastofplantsandphotosyntheticalgaeabsorbsbasicelementsandusessunlighttoproducesugarandotherorganicmoleculestofulfiltheirneeds(Kirchhoff,2019)@JackFalcón. ThePhotoreceptiveCellsandOrgansofAnimals Therhabdomericandciliaryphotoreceptorsarethetwomaintypesofphotoreceptivecellsfoundintheanimalkingdom.Bothshowahighlysegmentedandpolarizedorganization,withaphotoreceptivepolemadeoffoldsorstacksofmembrane,acellbodyandanopposingpoleforneurotransmission(Figure3A).Evolutionofphotoreceptorcellsandorgansrunsinparallel,andstudieshaveshownthateyesandotherphotoreceptivestructureshaveamonophyleticoriginthatstartedwithasingleprototype(Fainetal.,2010;Gehring,2012;Lamb,2013;Gavelisetal.,2015).Evolutionledtotheappearanceofavarietyofcomplexoculartypes(Figure3B).Thus,whilethecamera-typeeyecontainingciliaryphotoreceptorscharacterizestheeyesofhumansandothervertebrates,camera-typeeyesarealsofoundinjellyfishandcephalopods,whichinsteadpossessrhabdomericphotoreceptorsasisthecaseinmostinvertebrates.However,coexistenceofrhabdomericandciliaryphotoreceptorsisnotuncommon,asobservedinthecephalochordateAmphioxus,thelivingproxyofallvertebrates(ZhangQ.L.etal.,2019).Theretinaofthehagfisheye,aswellasthepinealglandoffish,frogsandsauropsids,iscomposedmainlyofphotoreceptorcellsconnecteddirectlytoganglioncells.Thefirstareoftheciliarytypeandthesecondarederivedfromrhabdomericphotoreceptors,asshownatleastinthehagfish(Autrumetal.,2012;Lambetal.,2007;Lamb,2013).Theretinaofallothervertebrateshasbecomemorecomplex,withtheappearanceofbipolar,horizontalandamacrinecellsinanintermediateposition.Themostrecentdataindicatethatbipolarcellsarederivedfromciliarytypephotoreceptors,whiletheganglioncellsderivefromtherhabdomericline;amacrineandhorizontalcellswouldalsobelongtotherhabdomericline(Lamb,2013). FIGURE3 Figure3.(A)Rhabdomericmicrovilli-based(invertebrates)andcilia-based(vertebrates)photoreceptorsdisplayconservedcellpolarityandtopology.TheyevolvedmostprobablyfromacommonancestorinearlyBilateria.Thephotosensorypoleismadeofstacksofplasmamembraneseparatedfromthebaso-lateralmembranebyazonulaadherens.N,nucleus.(B)Themainopticaldesignsofeyes:(a)Thepinholeeye;light(yellowarrow)fallsdirectlyuponthephotoreceptors(brownlayer).(b)Theconcave-mirroreye;lightcrossestheretina,andisthenfocusedbackontotheretinauponreflectionfromahemisphericreflectivemirror(tapetum,greyzone).(c)Thecameratypeeye;lightisfocusedbythelenstoformanimageontheretina.(dande)Thecompoundeyes;lightreachesthephotoreceptorsexclusivelyfromthesmallcorneallens(dtype)locateddirectlyabove,orfocusedthroughalargenumberofcornealfacetsandconestobedirectedtowardssinglerhabdoms(etype).RedrawnfromWarrant(2019). CompoundandCameraTypeEyes Adozendifferenteyestructureshavebeenidentifiedinanimals,whichdevelopedthroughdifferentevolutionarypathways(divergent,parallel,orconvergent)(Shubinetal.,2009).Somearejustscatteredphotoreceptors(aloneorafewtogether)allalongthebody,foundinsmallinvertebratesandinlarvaeofinsectsandworms.Theyaredesignatedasprimitiveeyesbecausetheyareassociatedwithapigmentedcellpositionedononeside,permittingtheperceptionoflightdirectionality.Thesestructuresaresimpledosimetersofthesurroundinglightintensityallowingnegativeorpositivephototaxy(escapeorattractivebehaviourrespectively).Intubularwormsthesegroupsofcellsformwellsorpiteyes;thepiteyeformsasmallhollowinwhichphotoreceptorcellsdisplaydifferentorientations,thusallowingspatialdetectionoflight(Figure3Ba).Fromthesepiteyesappearedthesphericalconcavemirroreyeswithapupil,butwithoutacrystallinelens,asseenborderingthemantleofthebivalves(clams,scallops)(Figure3Bb).Moreelaboratedcameraeyesarefoundinvertebrates,molluscs(squid,octopus),jellyfish,someannelids,arthropods(includingspiders),insectlarvaeandcopepods(Figure3Bc).Finally,thecompoundeye,themostwidespreadmodel,ischaracteristicofinsects(75%ofexistinganimalspecies),mostcrustaceans,myriapods,somebivalvesandpolychaetes(Figure3Bd,e).Compoundeyesareformedofidenticalunitscalledommatidia,whicheachcontainsaclusterofphotoreceptorcellssurroundedbysupportingcellsandpigmentedcells.Eachommatidiumpossessesacorneaandaconicallensthatfocuseslighttowardstherhabdomericphotoreceptors.Inthemajorityofdiurnalspecies,eachommatidiumisisolatedfromitsneighboursbyapigmentlayer,whichmakescommunicationbetweenthemimpossible(Figure3Bd).Innocturnalspeciestheabsenceofpigmentallowsthediffusionoflightfromoneommatidiumtoitscloseneighbours,conferringagainofsensitivity(Figure3Be). Theeyewithitsretinaisnottheonlystructurethatallowslightdetection,asbothinvertebratesandvertebratespossessadditionalextra-retinallightsensitivestructures. ExtraretinalPhotoreceptioninVertebrates Aquaticvertebrates,amphibiansandlizardspossessapinealcomplexformedbyapinealglandassociatedwitheitheraparapinealorganoraparietaleye(dependingonthespecies)(Collinetal.,1988;Falcón,1999;Figures4A-J).Theglandappearsasanevaginationoftheroofofthediencephalon,locatedatthesurfaceofthebrain.Inthemajorityofcases(particularlyinpoikilothermicspecies)theskulldirectlyabovethepinealglandisthinnerandtranslucentandtheskinislesspigmented(Figures4A-D).Inlargefish(e.g.,thetuna)wherethebrainislocateddeepinsidethehead,atranslucentcartilaginoustubedirectslightfromthesurfacetothepinealgland(personalobservations).Alltheseanatomicalcharacteristicsallowbetterlightpenetration.Inadditiontothepinealgland,frogsandlizardspossessaparietaleye(Figures4E-J)locatedbetweentheskullandtheskin,whichsendsanervethatcrossestheskulltoreachthebrain.Inaddition,theparietaleyeoflizardspossessesalens(Figure4J).Inbirds,snakesandmammalsthesespecializationshaveregressed:thepinealglandofadultmammalsisoftenlocatedmoredeeplyinthebrainandhaslostitsabilitytodetectlightdirectly,eventhoughtheystillexpresstheproteinsnecessaryforphototransduction(Figures4K,L).Furthermore,duringdevelopmentmammalianpinealocytesdisplaymorphologicalfeaturescharacteristicofciliaryphotoreceptorcellsbutwhichsubsequentlyregress(BlackshawandSnyder,1997). FIGURE4 Figure4.Extraretinalphotoreceptioninvertebrates.(A)DorsalviewoftheheadofthePolarCodBoreogadussaida;thepinealorgan(PO)islocatedinthesagittalaxisjustbehindtheeyesinanareawithunpigmentedmeninges(@JackFalcón).(B)DorsalviewofthebrainsoftheRedMulletMullussurmulletusshowingthelocationofthepinealorgan(thickarrow),locatedinbetweenthetwocerebralhemispheres(Ch);OT,optictectum;Cer,cerebellum;fromBaudelot(1883)(nopermissionrequired).(C)Schematicsagittalsectionsthroughtheepithalamusareaof,fromtoptobottom,lampreys,chondrichtyensandteleostfish;fromStudnicka(1905).Notethattheskullabovethepinealorganisthinner,asalsoseeninpanel(D)(nopermissionrequired).ThehistologicalsagittalsectionisfromtheSeaBreamSparusaurata;thepinealislocatedinakindoflargepitbelowtheskull(notethatthetegumentabovealsoappearsthinner)(giftfromProfessorJ.A.MuñozCueto,Cadiz,Spain).(E,F)HeaddorsalviewsshowingthespotpositionofthefrontalorganintheAmericanBullfrogRanacatesbeiana(E)andtheparietaleyeoftheZebra-tailedLizardCallisaurusdraconoides(F)(arrows)(@JackFalcón).(G,H)Schematicsagittalsectionsthroughtheepithalamusareasoffrogs(G)andlizards(H);thepinealorgansarelocatedbelowtheskull,whilethefrontal/parietaleyesarelocatedintheskinconnectedtothebrainbyastalk(Studnicka,1905)(nopermissionrequired).(I)DorsalfossilskulloftheancestralamphibianThoosuchusjakovlevishowingthelocationofthefrontalorganholejustequidistantfromtheeyes(withpermissionfromhttps://commons.wikimedia.org/wiki/File:Thoosuchus_jakovlevi.JPG).(J)ThepinealeyeofthetuataraSphenodonpunctatusresemblesasimplifiedretinawithaneyecupandalens-likestructure;sagittalsectionfromDendy(1911)(nopermissionrequired).(K)Intheavianbrainthepinealorganformaglandinbetweenthecerebralhemispheresandthecerebellum(giftfromProfessorJ.P.Collin).(L)Inhumanstheglandislocateddeepinthebrain(@JackFalcón). Thepinealepitheliumofnon-mammalianvertebratesdisplaysthecharacteristicsofasimplifiedretinaasitcontainscone-typephotoreceptorsconnectedtoganglioncells,thelattersendingtheiraxonstowardsspecificbraincentres.Itisofinteresttonotethatretinalandpinealbrainprojectionsoverlapinsomeareas,thusprovidingconvergentlightinformation(EkströmandMeissl,2003).Incontrasttotheretina,thepinealorganisonlyadosimeteroflightintensity,albeitofgreatsensitivity.Inadditiontothisnervousinformationpinealphotoreceptorsalsoproducethe“time-keepinghormone”melatonin(seeLocalizationoftheCircadianSystem–Vertebrates)(Falcón,1999).Inthecourseofevolutionsnakesandmammalshavelosttheparapinealandparietalorgans,aswellasthedirectphotosensitivityofthepinealgland,andtheynolongerproducenervousinformation(Collinetal.,1988).Inthesespecies,thepinealcells(pinealocytes),receivelightinformationviatheretinaandacomplexnervepathway;onlythenocturnalproductionofmelatoninpersists(Kleinetal.,1997).Birdsdisplayfeaturescharacteristicofbothearlyandlatevertebrates. Inadditiontotheseorganizedphotoreceptiveorgans,intracerebralphotoreceptors,theexistenceofwhichhadbeenpostulatedearlyinthelastcentury(VonFrisch,1911;BenoitandAssenmacher,1954),havebeenfoundinfish,lizardsandbirds(Hangetal.,2016;Haasetal.,2017)(seealsobelowFigure11).Theirroleremainsenigmatic;somemaycontributetotheannualcontrolofreproduction(BenoitandAssenmacher,1954). Finally,ectothermicvertebrates(fish,amphibians,andlizards)possessphotosensitivecellsonthesurfaceoftheirskin,whichparticipateinthecontrolofmigrationinlampreys(BinderandMcDonald,2008),theaggregation/dispersionofskinpigmentsinfishandfrogs(Moriyaetal.,1996;Chenetal.,2014),orbaskinginreptiles(TosiniandAvery,1996). Extra-RetinalPhotosensitivityinInvertebrates Inadditiontotheirrhabdomericeyes,insectspossessocelliandeyelets,whichmayhavevariousshapesandlocations(Figures5A-E).Theocelliofinsectsaresimplelenseyesconsistingofasingle,largeaperturelens,followedbyseveralhundredsofrhabdomericphotoreceptorswhichconvergeontoafewtensofinterneurons(Berryetal.,2011).Drosophilaeyeletscontain4to6rhabdomericphotoreceptorsandarederivedfromthelarvaevisualorgans(Helfrich-Försteretal.,2002).Compoundeyesandocellihaveacommonancestralorigin(Friedrich,2006),andtheseextra-retinalphotoreceptorsarelikelytobeinvolvedinbehaviourandsynchronizationofendogenousrhythms.Spidersdonothaveocelli,butmaypossessfrom1to4pairsofeyeswithdifferentfunctions(Figure5F) FIGURE5 Figure5.Extra-ocularlightperceptioninvariousinsectspecies(A-E)andeyesofaspider(F).ArrowspointtoocellarstructuresasfoundinNeteliasp.(A),Heptageniasp.(B),grasshopperLocustamigratoria(C),Eristalinussepulchralis(D),Vespacabro(E),andPhilodromusdispar(F).Photocredits:P.Falatico(A,B,D,E;@http://aramel.free.fr/),JFalcón(C),D.Vaudoré(F;@https://www.galerie-insecte.org/galerie/ref-183890.htm).Nospecialpermissionsrequired. PhotopigmentsandVisualPerception Phytochromes Phytochromesarefoundinplants,fungi,bacteriaandcyanobacteria,unicellularalgaeanddiatoms.Theyarecovalentlyassociatedwithaphytochromobilinaschromophoreinplantsandcyanobacteria,andbiliverdininotherbacteriaandfungi(Bhooetal.,2001;Glukhovaetal.,2014;Huche-Thelieretal.,2016).Inplants,severalformsofphytochromesmaybepresentsimultaneously(fiveinArabidopsisthaliana,threeinsorghum,blackcottonwoodandrice,andtwoinpea)(Demotes-Mainardetal.,2016).Theydisplaymaximalsensitivityintheredrangeofwavelengths,althoughresponsetootherwavelengthsisalsoobservedbutwithmuchlowersensitivity(Figure6A).Phytochromesexistsintwostates:theinactivestatehasasensitivitymaximuminthered(58080%ofdenovotranscriptswererhythmic(possiblyundercircadiancontrolbutalsopossiblyevokedbythelight-darkcycleorthesleep-wakecycle)(Mureetal.,2018). FIGURE10 Figure10.Simplifiedschematicrepresentationofthecircadianclockin(A)mammals,(B)insects,(C)Cyanobacteria,(D)fungi,and(E)plants.FordetailsseeSainiR.etal.(2019).Abbreviations:CCA1,circadianclockassociated1;CCG,clockcontrolledgenes;Clk,clock;CRY,cryptochrome;CYC,cycle;ELF,earlyflowering;FRH,FRQ-interactingRNA,helicase;FRQ,frequency;GI,gigantea;LHY,lateelongatedhypocotyl;LUX,luxarrhythmo;PER,period;Rev-Erbβ(orphannuclearreceptorfamily1);PRR,pseudo-responseregulator;RORα,retinoicacidreceptor(RAR)-relatedorphanreceptors;TIM,timeless;TOC1,timingofcabexpression1;VVD,vivid;WC,whitecollar;WCC,whitecollarcomplex.ModifiedfromSainiR.etal.(2019)Nospecialpermissionrequired. TABLE1 Table1.Someexamplesofdemonstratedimpactsoftheclocksonorganisms. ItisbelievedthatcircadianclocksappearedveryearlyinevolutionasanadaptivefunctionlinkedtoDNAreplication.BylimitingDNAreplicationtothenightphase,UV-induceddamagetoDNAcouldbeblocked(PegoraroandTauber,2011).Overgeologicaltimeselectivepressureturnedthissimplepassiveprocessintoanactiveone,allowinganticipationofpredictablechanges.Amongthemyriaddailyandannualfunctionsdisplayingclock-controlledrhythmicityaretherest/activitycycle,foodintake,flowering,verticalandhorizontalmigration,growth,reproduction,andmanymore(Table1).Inadditiontotheirubiquitouscharacterandthepersistenceofrhythmicactivityunderconstantlight(LL)ordarkness(DD)(free-running),othercharacteristicsofacircadianclockinclude(1)geneticdetermination(i.e.,eachspecieshasitsproperperiodcloseto24h,butinter-individualvariationsareobservablewithinthesamespecies),(2)synchronizationbyotherfactors(e.g.,rainfalls,mooncycles,foodintake,tides)inadditiontotheLDcycle;(3)temperaturecompensation,i.e.,theclock’speriodisnotaffectedbytemperature;(4)lengtheningorshorteningoftheperiodwithlightintensityunderconstantlight(LL);(5)inductionofphaseadvancesorphasedelaysbylightsequencesappliedatdifferenttimesunderDD;(6)resynchronizationbyanenvironmentalstimulusonceconstantconditionshaveended.Virtuallyallcellspossessinternalclockmachinery. Itisworthmentioningthatinadditiontothecircadianclocksmanyorganismshavedevelopedcircannualtimemeasuringsystems.Asisthecaseforthecircadianclocks,circannualclocksareancestral,ubiquitous,autonomous,entrainedbyphotoperiodandtemperaturecompensated(Lincoln,2019).Thelocationandmechanismsofthecircannualclocks,stillpoorlyunderstood,arediscussedelsewhere(Numataetal.,2015;WestandWood,2018;WoodandLoudon,2018;Murphy,2019). LocalizationoftheCircadianSystem Plants Thereisevidencethatmultipleanddistinctcircadianclocksarepresentindifferenttissuesofplants.Thefirstexamplewasobtainedfrombeanplants,inwhichstomatalopening,photosynthesis,andleafletmovementrhythmsdisplayeddifferentperiodsunderfree-runningconditions.Inaddition,itseemsthatinsomecellsthe24hLDcycleisthedominantsynchronizingfactor,whileinothersitisthe24htemperaturecycle.Thequestionhasarisenastowhetherthereisacentralpacemakerorahierarchicalcouplingbetweendifferentclocksinplantsasisthecaseinanimals,andhowthesedifferentclockactivitiessynchronizewitheachother.Ithasbeenhypothesizedthattheoscillationsinsugarconcentrationsand/ormicroRNA(miRNA)mightplaythisrole(Endo,2016). Moreisknownininvertebratesandvertebrates,whereallcellspossessmolecularclockmachinery,forminganetworkofmoreorlesspotentandhierarchicallyorganizedunits(Falcónetal.,2007b;Dibneretal.,2010;Vatineetal.,2011;ItoandTomioka,2016).Thehierarchicalordervariesaccordingtotheclassandspeciesconsidered. Vertebrates Infishandlizards,thecircadiansystemismadeofanetworkofindependentandinterconnectedlight-sensitiveoscillatoryunitslocatedintheretina,thepinealglandandprobablyalsointhebrain(Tosinietal.,2001;Falcónetal.,2007b).Studiesinthezebrafishindicatedthatvirtuallyallcellsfromanytissuearelightsensitivecircadianoscillators(SteindalandWhitmore,2019),butthegreatvarietyoffishspeciesprecludesmakinganygeneralization.Inanycase,thepinealglandappearstoactasapotentmasteroscillator,dependingonthespecies(Underwood,1989;Whitmoreetal.,1998;Figure11).Thephotoreceptorcellsintheretinaandpinealglandactuallyconstitutefullcircadiansystemsbythemselves,astheypossessthelighttransductionmachinerythatprovidesinputtotheclock,aswellasthemachinerythatproducestheoutputsignalofthisclock,i.e.,melatonin(PickardandTang,1994;Bollietetal.,1997;Gothilfetal.,1999).Amajordifferencebetweentheretinaandpinealglandliesinthefactthatretinalmelatoninisgenerallyusedandmetabolizedlocally(Figure11).Inthepinealgland,melatoninistypicallyproducedinhigheramountsatnightthanduringtheday,andisimmediatelyreleasedintothebloodorcerebrospinalfluid.Thedurationofthisnocturnalsignalreflectsthedurationofthenight,whileitsamplitudevarieswithtemperatureinaspecies-specificmanner(Underwood,1989;Falcón,1999).Thus,dailyandannualvariationsinthemelatoninsecretionprofileprovideareliableindicationofdailyandcalendartime,whichisusedasatime-keepingsignaltosynchronizephysiologicalandbehaviouralprocesseswithdailyandannualvariationsinphotoperiodandtemperature(seesection“ClockOutputsandPhotoperiodism”). FIGURE11 Figure11.Schematicrepresentationofthephotoneuroendocrineorganizationinthenon-mammalianbrain.Thedrawingpicturesafrontalsectionofthebraindiencephalicarea.Lightinformationiscapturedbythelateraleyesandthepinealorgan.Photosensitiveunits,expressingdifferenttypesofopsins,havealsobeenidentifiedalongthe3rdventricle(3rdV;yellowandgreencircles).Majorcircadianclockmachineriesarepresentinthepinealandretinalphotoreceptorsaswellasinthebasaldiencephalon(preopticarea[POA]andsuprachiasmaticnuclei[SCN])oflizardsandbirds.Thepinealglandoffishandlizardsalsointegratestemperatureinformationfromtheexternalenvironment.Theconcomitantactionoflight,temperatureandotherinternalfactors,shapestherhythmicnervous(blue)andhormonal(red;melatonin)outputs(seetextfordetails),providingatemporalmessagetransmittedtotheneuroendocrineaxisanddownstreamtargets(peripheralendocrineorgans).Melatoninactsthroughspecificreceptors(stars)distributedindifferenttissuesandorgans.Whilethemainretinaloutputsubservesvisualfunction,afewotherfibresalsoterminateindifferentpartsofthebasaldiencephalon,wheresomeconvergewithfibresoriginatingfromthepinealgland.Someofthetargetedareasalsoexpressmelatoninreceptors.Thisdoubleortripleinputcontributestosynchronizingtheneuronalactivityofthebasaldiencephalon.InsauropsidsthePOAandSCNneuronsalsorelayretinalinformationtothepinealgland.TheentireneuroendocrineaxisistargetedbyALANtogetherwithmultipleotherdisruptorsincludingtemperaturerisesandpollutants[e.g.,endocrinedisruptors]actingdirectlyorindirectlyatdifferentlevelsoftheloop. Thestrengthandreliabilityofthemelatonintime-keepingsignalisreflectedinitsconservationthroughoutvertebrateevolution.Howeverthemodalityofmelatoninproductionhasbeenprofoundlymodifiedfromfishtomammalsasaresultofdramaticstructuralandfunctionalmodificationsofthewholecircadiannetwork.Inmammals,thecircadiancomponentsarelocatedindistinctspecializedareas.A“masterclock”islocatedinthesuprachiasmaticnuclei(SCN;∼5,000to30,000cells)ofthehypothalamus,whichinteractswithanetworkofperipheraloscillators(HarderandOster,2020).PhotoperiodicinputtotheSCNcomesfromtheretinaviatheretino-hypothalamictract:whilelightinformationencodedbytheretinaismostlydirectedtothevisualcortexthroughganglioncells(RGC),asmallnumberofthese-themelanopsin-containingorintrinsicallyphotosensitive(ip)RGC(seesection“TypeIIorAnimalRhodopsins”)-sendinformationtotheSCN(aswellasnumerousotherbrainnuclei)(Do,2019).OnedownstreameffectoroftheSCNisthepinealgland,withitsrhythmicmelatoninproduction;buttheglandhaslostallintrinsicphotoreceptiveandcircadianproperties(Collinetal.,1988;Kleinetal.,1997).RhythmicinformationfromtheSCNistransmittedtothepinealglandviaapoly-synapticneuralpathway(Kleinetal.,1997;Falcónetal.,2007b).ThefewstudiesperformedinSauropsida(birdsandreptiles)indicatethatmelatoninsecretionbythepinealglandiscontrolledbybothdirectandindirectphotosensitivity(Cassone,2014). Invertebrates Insectsincludemorethan1millionspecies,displayingahugediversityinallaspectsoforganizationandlifestyle,andthereismuchvariationintheanatomicalorganizationofthecircadiannetworkintheinsectbrain(Blochetal.,2013).Despitethisdiversity,therearestrikingsimilaritiesintheprincipalorganizationofcircadianclocks.InthefruitflyDrosophilamelanogasterthenetworkconsistsofafewhundredneurons(Hermann-LuiblandHelfrich-Foerster,2015).Amasterclockislocatedinscatterednucleilocatedintheopticlobesandbrain,composinganeuronalnetwork(TomiokaandMatsumoto,2010;Hermannetal.,2013;Hermann-LuiblandHelfrich-Foerster,2015).Theseneuronsutilizemainlyneuropeptidesassignallingmolecules,includingpigment-dispersingfactor(PDF),whichappearstobewell-conservedinputativemasterclockneuronsofallinsectsstudiedsofar(includingapterygotes,orthopteroids,coleoptera,hymenoptera,lepidopteraanddipteraTomiokaandMatsumoto,2010).InD.melanogaster,PDFisconsideredasthemainoutputfactorofclocks,actingasaneuromodulatorandsynchronizingsignalbetweenthedifferentcentralclockneuronclusters(Helfrich-Forsteretal.,2011;Hermannetal.,2013).Inadditiontothesecentralclocks,thereisevidenceindicatingthatmanyotherorgansortissues,eithernervous(eyeandeyestalk,antenna)orperipheral(gustatorysystem,Malpighiantubules,prothoracicgland,epidermissecretingendocuticle,testisandgerminalvesicle),expresscircadianclockproperties(Tomiokaetal.,2012).Photoperiodicinformationcapturedbytheocular,andinsomeinstancestheocelliphotoreceptors,entrainsthecentraloscillators,whichinturndeliverinformationtoslaveperipheraloscillators.Incricketsandcockroachesthispathwayisessential(TomiokaandMatsumoto,2010;Tomiokaetal.,2012).Inotherspecies(e.g.,Drosophila)thecentralbrainandsomeoftheperipheraloscillatorsarefullyintegratedcircadiansystemsastheyareabletocapturelightandthussynchronizetheirclocksandoutputfunctionsinvitro(Tomiokaetal.,2012),inamannersimilartothatdescribedforthezebrafish(Whitmoreetal.,1998).Intheeye,theRh1andRh6rhodopsinsareimplicatedinentrainmenttoredlight(D.melanogaster),whileinthebrainandperipheraloscillatorsitislikelytobetheUVA/bluepigmentCry1(drosophilaD.melanogasterandMonarchbutterflyDanausplexippus)(seesection“Phytochromes”)(TomiokaandMatsumoto,2010).Itisnoteworthythatthecentralbraincircadiansystemishighlyplasticasphotoperiodicchangeshavebeenreportedinfibredistributionornumberofclockneurons(Shiga,2013). TheMolecularMechanismsofCircadianClocks Thepurposehereistohighlighttheuniversalityoftheunderlyingprincipleaswellasthewiderangeofsituationsencounteredregardingthequalitativeaspectsofclockentrainmentbylight(Bhadraetal.,2017;SainiR.etal.,2019). Irrespectiveoftheorganismstudied,themolecularclockmechanismconsistsofoneormoretranscription/translationnegativefeedbackloopsofvaryingcomplexity(Figure10).Becausethefunctioningoftheclockinvolvessimilaroperatingmechanismswithdifferentmolecularactors,itisthoughtthatclockshaveappearedindependentlyseveraltimesduringevolution(PegoraroandTauber,2011).Thenumberoftheseactorsvariesfromafew(fungi,greenalgae)tomany(plants,animals)(SainiR.etal.,2019).Themolecularmechanismsofthecircadianclocks,havebeendescribedindetailinCyanobacteria,fungi(Neurosporacrassa),plants(Arabidopsisthalliana),greenalgae(Chlamydomonasreinhardtii,Ostreococcustauri),insects(Drosophilamelanogaster)andseveralrepresentativesofvertebratesincludinghuman(TomiokaandMatsumoto,2010,2015;UkaiandUeda,2010;Nakamichi,2011;PeschelandHelfrich-Forster,2011;Vatineetal.,2011;Hurleyetal.,2015;ItoandTomioka,2016;KoritalaandLee,2017;GilandPark,2019).Strongconservationoftheoperatingmodesisobservedbetweeninsectsandmammals,includingatthelevelofthemolecularactors(TomiokaandMatsumoto,2015;Figure10).Itisworthmentioningthatpost-transcriptionalregulationandproteinmodification,suchasphosphorylationandoxidation,havebeenhypothesizedasalternativeswaystobuildingatickingclock(Milliusetal.,2019). LightInputtotheClock Lightisthemaininputtotheclocks.Theeffectsonthecircadiantimingsystemsdependontheintensity,duration,spectrumandpatternofthelightstimulus;forareviewinhumansseePrayagetal.(2019).Intheanimalsinvestigatedthusfar,shortandmiddlewavelengthsarestronglyinvolvedinsynchronizationandentrainment.Invertebrates,theeffectivewavelengthsarecomprisedbetween420and500nm,thehighestefficiencybeingobtainedbetween450and480nm(Ramosetal.,2014;Prayagetal.,2019).Inmammals,thiscorrespondstothespectralresponseofmelanopsinfromtheipRGCoftheretina(see“TypeIIoranimalrhodopsins”).However,itisnotexcludedthatthemechanismsoflight-inducedclockentrainmentaremorecomplexthanbelieved.Indeed,ithasbeenobservedthatcolouropponentmechanismscaninducephaseadvancesorphasedelaysinthecircadianrhythm,dependingonlightintensityandspectralcomposition,inthepinealorganoffish,frogsandlizards(Spitschanetal.,2017).Opposingeffectsofwavelengthsoncircadianphaseshiftshavebeenshowninthecave-dwellingbatHipposiderosspeoris(bluevs.green)andwildrabbitOryctolaguscuniculus(bluevs.yellow).ItisnoteworthythatasubsetofipRGC,sensitivetoUVisalsoindirectlysensitive(viaconeperception)toyellowwavelengthsinthemouseMusmusculus. IninsectssuchasD.melanogasterandotherflies,Cry1isinvolvedbothinlightcapture(seesection“Cryptochromes”)andmolecularfunctionoftheclock(Figure10;Saunders,2012).Cry1issensitivetobluelight(λmax470).Inaddition,Rh1andRh6areimplicatedinentrainmenttoredlight,andRh1,Rh5,andRh6togreenandyellowlight(TomiokaandMatsumoto,2010). Inplants,avarietyofsituationsisobservedregardingthewavelengthsthatentraintheclocks.Interrestrialhigherplants,e.g.,A.thaliana,phytochromes(seesection“Phytochromes”)mediatetheeffectsofredandinfraredwavelengths(λ:700-750nm),whileCry1andCry2mediatetheeffectsofbluelight(Figure10;Chenetal.,2004;McClung,2006).InmicroalgaesuchasC.reinhardtiitheclockisresetbyawiderangeofwavelengths:violet,blue/greenandred(Niwaetal.,2013;Ryoetal.,2016).Finally,infungithelightentrainmentoftheclockismediatedbytheWC1bluephotoreceptorspecies(Bhadraetal.,2017). ClockOutputsandPhotoperiodism Clockscontrolawiderangeofperipheraloscillatorsandrelateddownstreamprocesses,manyofthemvital,tokeepinphasethemyriadrhythmiceventsthattakeplaceoverthecourseofadayorayear.Wepresentbelowashortoverview(summarizedinTable1),withthehelpofafewexamplestakenfromunicellularorganisms,fungi,plantsandanimals. UnicellularAlgae,Plants,andFungi Neurosporacrassawasthefirstfungiinwhichendogenouscircadiancontrolofitssexualandasexualdailyrhythmsofreproductionwasdemonstrated(Zámborszkyetal.,2014;Hurleyetal.,2015).Theasexualcycleconsistsintheproductionofconidiaduringthesubjectivenight,andsimilarrhythmsinconidiosporeformationhavenowbeenreportedinMyxomycetes,ZygomycetesandAscomycetes(CorreaandBell-Pedersen,2002).InN.crassaandothermultinucleatedfungi(PhysarumpolycephalumandAspergillusnidulansone),LDcyclesalsosynchronizethetimingofmitoticcycles(Edmunds,1988;Hongetal.,2014).TheinvolvementofthecircadianclockhasbeendemonstratedinNeurospora,inwhich15-20%ofthegenesareclock-controlled(Zámborszkyetal.,2014)(Table1). VirtuallyallfunctionsofunicellularalgaearerhythmicandsynchronizedbytheLDcycle,includingmetabolism,enzymaticactivities,photosynthesis,celldivisioncycle,mobility,morphologyandchromosometopology,andeventhesusceptibilitytodrugtreatmentsorinfectionbyviruses(Table1;Edmunds,1984).Theoutputsaregeneratedby24hLDrhythmsingenetranscription/translation(Welkieetal.,2019). Similarly,inmoredistantlyrelatedplantssuchasA.thaliana,therhythmscontrolledbythecircadianclockareplethoric,includinggeneexpression,Ca2+fluxes,chloroplastmovements,stomataopening,flowering,cotyledonandleafmovements,metabolicandhormonalactivities,ordefenceagainstpathogens(Baraketal.,2000;Table1).InalargescalestudycomparingninerepresentativesofArchaeplastida,includingunicellularalgae(Cyanophoraparadoxa,Porphyridiumpurpureum,ChlamidomonasReinhardtii),pluricellularalgae(Klebsormidiumnitens),mosses(Physcomitrellapatens),earlyvascularplants(Selaginellamoellendorffii),andlatevascularplants(Piceaabies,Oryzasativa,A.thaliana),itwasfoundthattheyhadsimilardiurnaltranscriptionalprograms,despitelargephylogeneticdistancesanddramaticdifferencesinmorphologyandlifestyle(Ferrarietal.,2019;Table1). Animals Vertebrates Thecircadianclocksofvertebratescontributetocontrollingamyriadofrhythmicmetabolic,physiologicalandbehaviouralfunctions(BoissinandCanguilhem,1998;Table1).Onemainoutputsignalfromthecircadiansystemofvertebratesismelatonin,thehormonesecretedprincipallyatnightbythepinealgland(“Vertebrates”andFigure11;Collinetal.,1988;EkströmandMeissl,2003;Falcónetal.,2007a). Atthemolecularlevel,theclocksgovernrhythmicvariationsinplasmalevelsofions,carbohydratesandlipids,andofbrainandplasmasteroids,andmonoamines(serotonin,dopamine)(Delahuntyetal.,1980;Olceseetal.,1981;Takahashi,1996;Tongetal.,2013;MendozaandChallet,2014;Hernandez-Perezetal.,2015;Vancuraetal.,2016;Songetal.,2017);furthermore,italsoregulatestheexpressionofgenesoractivitiesofenzymesinvolvedinthesechanges(Falcón,1999).Atthephysiologicallevel,theneuroendocrinesystem,fromthehypothalamustothepituitaryglandandperipheralorgans,displaysdailyandannualfluctuations,whichcontributestocontrollingawiderangeoffunctionsascriticalasgrowth,reproduction,stressresponse,foodintake,immunityorosmoregulation(Falcónetal.,2010;TonsfeldtandChappell,2012;WoodandLoudon,2014;Challet,2015;Kimetal.,2015;Leliavskietal.,2015;Figure11).Thecardiovascularsystem(bloodpressureandheartrate)andneuronalelectricalactivity(electroretinogramandelectroencephalogram)donotescapetheruleastheyalsofluctuaterhythmically(BoissinandCanguilhem,1998;PetersandCassone,2005;CameronandLucas,2009;Talathietal.,2009;WoodandLoudon,2014;Petsakouetal.,2015;Caveyetal.,2016;Pauletal.,2016;Figure11andTable1).Finally,inmanytissues,clocksalsocontrolthecelldivisioncycle(BoissinandCanguilhem,1998;SteindalandWhitmore,2019),aswellassomeadaptivecellularmovementsincludingretino-motormovements(therespectiveelongationandretractionofconesandrodsobservedinfishandamphibiansretinasattheL-to-DandD-to-Ltransitions)(Kwanetal.,1996;Songetal.,2017).Accordingly,dozensofbehaviouralactivitiesdisplaydailyandannualrhythms,includinglocomotoractivityandsleep,schoolingbehaviour(fish),pigmentationorfurrenewal,vertical(fish)andhorizontal(allvertebrates)migration,behaviouralthermoregulation(fish),vocalization(fish,birds),foodintake,matingandreproduction,etc…(Zachmannetal.,1992;Lincolnetal.,2006;Cancho-Candelaetal.,2007;Kantermannetal.,2007;FosterandRoenneberg,2008;Kulczykowskaetal.,2010;Cassone,2014;RufandGeiser,2015;Table1). Invertebrates Thedataoninvertebratesarenotasabundantasforvertebrates,andrelatemostlytoinsects,althoughmoreandmorestudiesrefertomarineinvertebrates.Allindicatethattheclocksmediatetheeffectsofphotoperiodandtemperatureonamyriadofrhythmicdailyandseasonalevents(Helfrich-Forsteretal.,2011;Arboledaetal.,2019).Themostobviousrelatetofeeding(e.g.,foraginginbees,andmoths,bugsandmosquitoesbites),reproduction(e.g.,courtshipbehaviour,matingandreproduction),andgrowth(larvalandadultdevelopment,diapause,longevity)(Helfrich-Forsteretal.,2011;Blochetal.,2013;RougvieandO’Connor,2013;Table1). TheneuromodulatorPDF,importantfortransmittingclockinformationtodownstreameffectors,alsoactsasacirculatinghormone(Blochetal.,2013).Thereisanatomicalandphysiologicalevidencethattheinvertebratecircadiansysteminfluencescirculatinglevelsofendocrinesignals,includingjuvenilehormone(JH),ecdysteroids,and“pheromonebiosynthesisactivatingneuropeptide.”JHplayskeyrolesinregulatingthereproductivephysiologyandbehaviourininsectsaswellasincontrollingtheage-relateddivisionoflabourinsocialinsects.ThelevelsoftranscriptsofJHbiosyntheticenzymesinthecorporaallatadisplaystrongdailyrhythmsinthebee,mosquitoandfruitfly.Inthehaemolymph,thecirculatinglevelsofJH,JH-bindingproteinandJH-degradingenzymesalsodisplaystrongcircadiandependentvariations(Blochetal.,2013).ItisbelievedthattheJHoscillationsmediatethecircadianrhythmsinthelevelsofneurotransmitters(pheromonebiosynthesisactivatingneuropeptide),andhormones(octopamine;serotonin;dopamine)thoughttobeimportantforlocomotoractivityorreproduction(includingtheproductionofpheromones,courtship,mating,andgameteproduction)(KoutroumpaandJacquin-Joly,2014).Similarly,itissuspectedthatPDFcontrolstherhythmicproductionoftheprothoracicotrophichormoneinvolvedintheregulationofecdysteroids,whichcontrolmoulting(Table1). Finally,theelectricalactivityofinvertebrates’eyes(electroretinogram)andoftheentirevisualsystemdisplaycircadianfluctuations(HernandezandFuentes-Pardo,2001).InthePrayingMantis,Hierodulapatellifera,rhythmsareassociatedwithcyclicchangesinthecolouroftheeyes,neuralcontrolofeyemovement,andgrosslocomotoractivity(Schirmeretal.,2014). ImpactofAlanandLEDsonLivingOrganisms “Natureisperfect.Ikeepadiary.Iwriteonwhichdayofthemonththeflowersbloomandonwhichdayofthemonththeinsectsbegintosing.Yearafteryear,thesedateshardlyvary.Theyareveryregular,thisisoneofthelawsofnature.Whatgoeswiththelawsisnature.Natureisinaccordancewiththelaws.That’swhyIbelievepeopleshouldlivebyimitatingnature…Naturedoesthetruthinsilence.” MasterEkiyoMiyazaki(1902–2008). TheGeneralizationofLEDIllumination Initiallymotivatedbythedesiretoprovidemoreenergy-efficientlightsourcesforpubliclighting(NairandDhoble,2015),theuseofLEDnowconcernsawiderangeoftechnological,socio-economicandcommercialapplications.Avarietyofsourcescontributesdirectlyorindirectly(glowing)tooutdoorsLEDlighting:officesandhomes,streetlighting(Figure1),vehicles,trafficsigns,commercialadvertising,tourism(architecturalandlandscapingenhancement),industry(factories,greenhouses),orrecreational(outdoorandindoorsports)areas.Aquaticenvironmentsarealsoaffected(shorelinesandcoastlinesinurbanandsuburbanareas,offshoreplatforms,commercialroutesorfishingareas,especiallynightfishing).FromsuchconsiderationsitcanbearguedthatinvestigationsontheeffectsofoutdoorsLEDarecloselyassociatedtothoseofALAN,asituationclearlyunfavourabletothepreservationofthenightsky. Artificiallightingingeneral,andLEDsinparticular,addtothelistofnumerousanthropogenicpressuresthat,decadeafterdecade,arechanginganequilibriumthathasresultedfrommillionsofyearsofevolution,affectingthetreeoflife,ofwhichmanisonlyonebranchamongthousandsofothers.Inthevastmajorityofcases,studiesinvestigatingtheimpactsofagivenfactorconsidermainlytheeffectsonhumanhealth,whileimpactsontheanimalandplantkingdomsareconsideredmainlywithinthecontextofimprovingproductivityinordertosatisfygrowinghumanneedsoflivestockandderivedproducts.ThisegocentricviewiscurrentlydirectingmostoftheresearchonLED;furthermore,themajorityofstudiesareconductedinacontrolledenvironment,whiletheimpactonnon-domesticatedspeciesandecosystemsarerarelytakenintoaccount. Wehavegivenaboveanoverviewoftheincrediblywiderangeofstrategiesthathavebeendevelopedbyunicellularandmulticellularorganisms(i)tocaptureandtransducelightinformationintomessagesconveyedtoappropriatetargets,(ii)toorientateinspaceandtimeandultimately(iii)toaccomplishtheiressentialbiologicalneeds.Thedevelopmentofinternalclocksreflectsadaptationtothehighlypredictableandreliablevariationsofthephoticenvironmentallowinganticipationandharmonizationofthemyriadofbiologicalfunctionstothedailyandannualchangesofphotoperiod.Itisthereforenotsurprisingthatdisturbancesofthisphoticenvironment,whetherinquality,quantityorduration,havemoreorlessmarkedimpactsonlivingorganisms.Belowwereview,throughafewrepresentativeexamples,howhumanactivitiesandALAN,aloneorincombinationwithotheranthropogenicfactors,alterindividuals,speciesandcommunities. EconomicalPurposes CultivationofMicroorganismsandPlants ManystudieshighlighttheinterestofLEDsforthegreenhousecultivationofplants(Yehetal.,2014;NairandDhoble,2015;Singhetal.,2015;Duecketal.,2016;Urrestarazuetal.,2016;Rehmanetal.,2017),fungi(Wuetal.,2013;Kimetal.,2014),andunicellularmicroalgae(Schulzeetal.,2014)ofagronomic,ornamentalormedicinalinterest.Onemajorfocusresidesinthepossibilitytochooseaparticularwavelength(ofnarrowspectralrange)oracombinationofwavelengths,targetingspecificaspectsofplantphysiologyingreenhouseenvironments(Rehmanetal.,2017).Inplants,daylength,lightintensity,andlightqualityaffectmorphology,growthanddevelopment.Theeffectsoflight(whetherbyLEDorothersources)onfungiandplantsdependontherangeoffrequenciestheydetect.Table2summarizestheeffectsofdifferentfrequenciesonthemetabolismandphysiologyofplants.Forexample,farblueandUVlightsareusefulforeliminatingbacterialandviralinfections(Yehetal.,2014;KumarandEngle,2016;Kimetal.,2017),whileanadequatecombinationofblueandred/infraredwavelengthsprovidesoptimaleffectsintermsofmetabolism(e.g.,photosynthesis,lipidsynthesis,energyproduction),germination,celldivision,budding,growth,flowering,nutritionalvalueandtaste,orproductionofcompoundswithhighaddedvalue(ergosterol,carotene).Littleinformationisavailableontheimpactofgreenlights. TABLE2 Table2.Effectsofwavelengthsonplants(fromXuetal.,2016). However,severalfactorsneedcarefulattention: (1)Theeffectsofawavelengthorcocktailofwavelengthsdependonthespeciesand,withinthesamespecies,onsexandstageofdevelopment;theyalsodependonintensity,positioning,periodicityorfrequencyofexposure(Duecketal.,2016;HernandezandKubota,2016).Forexample,cyanobacteriagrowpreferentiallyundergreen,yellowandredlight,whereasmicroalgaepreferentiallygrowunderblue(420white>red)(Geffenetal.,2015).Inhibitionoffoodintakehasalsobeenobservedregardlessoflightwavelength(VanLangeveldeetal.,2017).Incontrast,foragingactivitywasincreasedinspiders(Eriophorabiapicata)(Willmottetal.,2018).ReproductivesuccessandgrowthofmothsandspidersarealsocompromisedbyALAN:sexualactivityoffemalesandattractionofmalestofemalesweredisruptedbyLEDlightingofdifferentwavelengths(red>white>green)inOperophterabrumata(Geffenetal.,2015).InE.biapicata,a20lxwhiteLEDatnightacceleratedmaturationbutreducedthenumberandsizeofjuveniles(Willmottetal.,2018).InthemosquitoCulexpipiensf.molestus(familiarinurbanareas),ALAN(cool-whiteLED,100-300lx)appliedduringthefirst3hofthenightphaseresultedinfemalesproducingfewerandsmallereggs(Honnenetal.,2019);inaddition,malesandfemaleswerelessactiveduringtheALANphasebutfemalesbecamemoreactivethereafter.Thesex-dependentdifferenceswerealsoseeninclockgenesbecausethesameALANconditionsinducedupregulationofCycleinfemalesanddownregulationofClockinmales,withconsequencesonthemedianrelativeexpressionofclockgenesandactivitycycles(Honnenetal.,2019). InfirefliesALANhasbeenratedasthesecondmostseriousthreatafterhabitatloss,showingadverseeffectsonpopulations(Lewisetal.,2020).ALANinterfereswiththeproductionandperceptionofcourtshipmessages,glowing(e.g.,Lampyrisnoctiluca)orflashdialogues(Pteroptyxmaipo,Photurispyralis).Ultimately,sucheffectsimpingeuponreproductionofthespecies(BirdandParker,2014;Owensetal.,2020). IncoastalareasofChilethesandybeachisopodTylosspinulosusisactiveatnight.ALAN(120lx;whiteLED)disruptedisopodlocomotoractivityandcircadianrhythms,resultinginadramaticavoidanceoflitareasatnight(Duarteetal.,2019). Fish Reproduction Nightlightingaffectsreproductionoffishinseveralways,andinacomplexmanner(Figure11).WhiteLEDsoflowintensityinhibitedgonadotrophinexpression(FSH,folliculo-stimulatinghormone;LH,luteinizinghormone)infemalePerchPercafluviatilis,whereasmonochromaticwavelengths(blue,green,orred)hadnoeffect(Brüningetal.,2016).InthesamestudyALANofdifferentintensities(0.1to100lx)inhibitedsecretionofthetime-keepinghormonemelatoninregardlessoftheLEDwavelengthused(Brüningetal.,2016).UndersimilarconditionsmelatoninlevelswerealsoaffectedinRoachRutilus,whereasnoeffectwasseenongonadotrophinexpression(Brüningetal.,2018a).However,infieldexperimentsusingHPSlamps,abundanceofsexsteroids(17β-estradiol;11-ketotestosterone)andFSHandLHmRNAwasreducedinbothP.fluviatilisandR.rutilus,whilemelatoninlevelswerenotsignificantlyaffected(Brüningetal.,2018b).Indwarffish,ChrysipteraparasemaandC.cyanea,nocturnalexposuretomonochromatic,butnotwhite,LEDspromotedgonadalmaturation(Shinetal.,2013;Yehetal.,2014),themosteffectivewavelengthsbeinggreenandblueinC.parasema,andredinC.cyanea.OestradiolproductionwasalsostimulatedinC.parasema(Shinetal.,2013),andgonadotrophinswerestimulatedingoldfish,Carassiusauratus,whendaytimeilluminationwasreplacedbymonochromaticLEDs;greenlight,whichalsoincreasedtheexpressionofVAL-opsin,wasthemostpotent(Songetal.,2015).WhiteLEDlightatnight(∼23lxilluminance)totallyinhibitedhatchingintheClownfishAmphiprionocellaris,althoughnoimpactwasfoundonthefrequencyofspawningorfertilizationsuccess(Fobertetal.,2019).TheauthorsspeculatedthatfishwithsimilarspawningstrategiesmightrespondsimilarlytoALAN. Altogether,itisapparentthatALANcaninterferewithcomponentsofthereproductiveaxisinfish(Figure11).TheseconclusionsaresupportedbylongtermlaboratoryexperimentsinzebrafishD.rerio.After1yearunderLL(fluorescentbulbs,300lx)themolecularclockwasdisruptedintheovary,oestrogenlevelswereincreased(∼50%)whileprogesteronelevelsweredecreased(∼25%),andplasma,retinaandbrainmelatoninrhythmswereabolished(Khanetal.,2018).Moreimportantlyperhaps,therewasmolecularandhistologicalevidenceoftumorigenesisintheovariesoftheALANgroup.ALANalsoaffectedthewholetranscriptome,includinggenesinvolvedintumorigenesisandotherphysiologicaldisorders(Khanetal.,2018). Behaviour Behaviourisalsoaffectedincoastalandfreshwaterfish(Figure12A).IntwolakesofOntario(Canada),locomotoractivity,andthusenergyexpenditure,ofBlackBassMicropterusdolomieu,whichnestsandprotectsitsoffspring,wasabnormallyhighinthepresenceofcontinuousorintermittentnightlighting(WhiteLEDs,40lxatthewatersurface)mimickingtrafficlights(Fosteretal.,2016).Intermittentlightingwasthemostaggressive.Theeffectswereobservedbothduringdayandnightphasesandrenderedoffspringsurvivalmorerandom.Parentalcareoccursin60%offreshwaterfishfamilies;ALANcouldthushavenegativeconsequencesonmanyspeciesthatbuildnestsinlakeandriverlittoralzones.AnescapebehaviourhasalsobeenreportedintheLargemouthBass,Micropterussalmoides,inresponsetoLEDlights(green,yellow,orange,andred)pulsesappliedduringthedaytime(Sullivanetal.,2016).ThismayberelatedtotheobservationthatstreetlightingactedasalightbarrierinAtlanticsalmon,Salmosalar,fry(Rileyetal.,2013)(andsection“TheMigratingAtlanticSalmon-ACaseStudy”).Lightdisruptedthedailyrhythminfrydispersionanddelayeddownstreammigration.Thesechangesinmigratorybehaviourmayimpactonfishfitnessandincreasepredationrisk. FIGURE12 Figure12.(A)ObservedabundancesinfishpopulationsfromtheharbourofSydney(Australia)undera12L/12Dcycleplottedin15minbins.UnderthenaturalLDcyclethenumberoffishishigherduringnight(blackline)thanduringday(blueline);theyweresedentaryatnightwithlowpredationactivity(P↓),whiledisplayingapredatorybehaviourduringday(P↑).ALAN(40-50lx,warmLEDlight),transformedthenocturnalpatternintoadiurnalone.ModifiedandadaptedfromBoltonetal.(2017).(B)Orientationresponseof4speciesofseaturtleshatchlingstocolouredlightsources.OliveRidleySeaTurtleLepidochelysolivacea,GreenSeaTurtleCheloniamydas,HawksbillSeaTurtleEretmochelysimbricata,wereattractedwhenilluminatedwithUV-Atoyellowwavelengths.TheLoggerheadSeaTurtleCarettadifferedinthatUV-Atogreenlightswereattractive,butyellowwavelengthswererepulsive,aneffectreversedbyredillumination.Fordetailssee(WitheringtonandMartin,2003)fromwhichthefigurewasmodifiedandadapted. Altogether,theavailablestudies,althoughscarce,suggestthatALANis“anunpredictablethreatforlightsensitivespecies,communities,andconsequentlybiodiversity”(Brüningetal.,2018a,b),adangerpotentiatedbytheobservationsthatresponsesdependonthespeciesandtheirlifestrategies(Fobertetal.,2019).Frogs OnlyafewstudiesexploredthephysiologicalconsequencesofALANonamphibians,allindicatingitislikelytohavenegativeeffectsonpopulations.Thus,whiteLEDlighting(equivalenttothatproducedbystreetlighting)affectsthenocturnaldistributionaswellaschoiceofpreferredsubstrateoftheunisexualBlue-SpottedSalamander(Ambystomalateraljeffersonianum),buthadnosucheffectonthefrogRanasylvaticus(Feukaetal.,2017).Theauthorsconcludedthatthesechoicesarelikelytoaffectthesurvivalofbothspeciesassalamandersmustchooseasubstrateoflowernutritionalqualitywhilefrogsbecomemoreexposedtonocturnalpredators.Infieldexperiments,nocturnalLEDlight(Blue/greenspectrumandintensitiesconsistentwiththosefoundunderstreetlight)reducedlarvaemetamorphosisdurationandjuvenilegrowthintheAmericantoadAnaxyrusamericanus(DananayandBenard,2018).InadditionALANalsoaffectedperiphytonbiomass,asmentionedbefore(section“Breeding”).InthePennsylvanianwoodfrogLithobatessylvaticustadpoles,ALAN(indoorswhiteLED)didnotchangemetamorphosisdurationbutreducedhatchingsuccess(Mayetal.,2019).Furthermore,whileA.americanuslarvaekeptahighrateofactivityunderilluminatednight(comparingtodaytime),L.sylvaticustadpolesmovedless,andaftermetamorphosisindividualsexposedtoALANweremoresusceptibletoNaClchallengeandtrematodes.Reducedactivityandalteredmetabolismwerealsoreportedinmalecommontoads,Bufoexposedfor20daystoALAN(whiteLED;0.1,5,or20lxilluminance)(Touzotetal.,2019).AstheeffectswereobservedattheonsetofthebreedingperiodtheauthorssuggestedthatALANcouldbeaseriousthreatformanynocturnalamphibianspecies. Reptiles Althoughscarce,studiesonreptilesindicateALANisamajorthreat.Manystudiesfocussedonseaturtlesofcoastalareasallaroundtheworld;theimpactofALANonnestingandhatchlingshasbeendocumentedsincetheearly80’s(WitheringtonandMartin,2003).Seaturtlenestingandhatchingoccuratnight,generallyeggsfromonenesthatchtogether,thoughsometimesamaingroupofhatchlingsmaybeprecededorfollowedbysmallergroups(WitheringtonandMartin,2003;Robertsonetal.,2016).CoastallightatnightcausesspawningatseaandabandonmentofnestsoralterationofthechoiceofnestingsiteinseveralspeciesoftheCaribbeanislands(GreenTurtle,Cheloniamydas;HawksbillTurtle,Eretmochelysimbricate;LeatherbackTurtle,Dermochelyscoriacea).Modellingstudiespredictlightpollutionwillsubstantiallyacceleratetheextinctionofthesespecies(Breietal.,2016).SimilardatawereobtainedalongAustralianshores:C.mydashatchlingsweredisorientedinthepresenceofshorelights,andthosethatreachedandenteredtheseareturnedtoshoreifreachinganarealitbyshore-basedartificiallights(Truscottetal.,2017).Low-pressuresodium-vapor(LPS)yellowlightswerebelievedtoprovideamore“turtle-friendly”environmentinLoggerheadsandGreenTurtles,asUV-blueandgreenwavelengthswerethemostattractivetohatchlings,whiletheredoneswerenot(WitheringtonandMartin,2003;Figure12B).However,morerecentinvestigationsindicatedLEDsemittinginthered(narrowband,600-670nm,λmax640nm)andyellow(wideband,600-750nm,λmax620nm)inducedtotaldisorientationofLoggerheadhatchlingsintheirracetowardstheseaatequalintensities(Robertsonetal.,2016).Themaximumeffectdependedonthenumberoflightingspotswithambercolouredemissionsbeingthemostpotentintheabsenceofmoonlight.Accordingtotheauthors,coastallightingisadramaticthreattothespecies. Littleinformationisavailableconcerningterrestrialreptiles,althoughalonglistofspecies,likelytobeaffectedbyALANinurbanandsuburbanlocations,hasbeendocumented(Perryetal.,2008).RecentobservationsonthenocturnalbehaviourandactivitypatternsoftwospeciesofdiurnalanolefromAntigua(WestIndies;Leach’sAnoleAnolisleachiiandWatts’sAnole,A.wattsi),describeanincreasedactivityunderALAN,albeitrestrictedtomalesandprimarilyrelatedtotheincreaseinthenumberofarthropodsattractedbylight(Maureretal.,2019). Birds ThereisabundanceofdataontheimpactofALANonbirdswithdozensofpublicationsoverthelastfiveyears(Dominonietal.,2013a,2016;Zhaoetal.,2014;Ronconietal.,2015;deJongetal.,2016b;Krügeretal.,2017;Raap,2018;Jiangetal.,2020).Overall,ALANdisruptsthecircadiansysteminbothsedentaryandmigratorybirds,affectingphototaxisandalteringendogenousdailyandannualrhythms.Theseeffectsareobservedbothinlandandabovetheseawherelightsemittedbydrillingandextractionplatforms,aswellasvessels,havesignificanteffects.Birdsareattractedbylightandbecomedisoriented.Collisionswithsolidstructures(orcontactwithflamesfromchimneys)havedramaticeffects,causingthedeathofhundredsoreventhousandsofindividuals(Ronconietal.,2015;Krügeretal.,2017;Rodriguezetal.,2017).Theseeffectsmayvarydependingonthequalityandintensityofthelightsource,LPSandLEDbeinglessharmfulthanmetalhalidelamps(Ronconietal.,2015).Inaddition,whencollisionisavoided,themigratorybirdsmayendupturningincirclesaroundtheplatforms,negativelyimpactingthetrajectoryandmigrationtime,energyexpenditureandultimatelysurvival.Inadditiontocollision,ALANaffectsthestopoverhabitatusebyinlandmigratingbirds,whichavoidbrightareas(McLarenetal.,2018). Artificial-light-at-nightalsoinducesindirecteffectsthroughthedisorganizationofthebirds’circadiansystem.InastudycomparingruralandurbantreesparrowsPassermontanusofMizoram(India)differenceswerefoundinthephaseand/oramplitudeofclockgenemRNAabundanceintheretina,pinealglandandhypothalamus(RenthleiandTrivedi,2019).Downstreamccggenes(includingmelatoninreceptors)alsodifferedintheirrhythmicexpressionandabundancebetweenruralandurbanbirds.Inaddition,therhythminmelatoninproductionitselfwasalsodifferent.ThemismatchesbetweentherhythmsofdifferentcomponentsofP.montanuscircadiansystemandeffectorsseeninurbanbirdsarelikelytohaveconsequencesoncircadiancontrolledprocesses.Indeed,indoorsexperimentswithP.montanusoftheBeijingarea(China)haveshownthatALANaltersthewholeneuroendocrinereproductiveaxis(ZhangX.J.etal.,2019);mRNAabundancecorrespondingtoFSH,THS(thyroidstimulatinghormone)andDio2(deiodinaseII)wereupregulatedwithlowilluminancelevels(85lx;coldwhite)anddownregulatedwithhighilluminancelevels(150and300lx)ofALAN.TheriseandamountofplasmaLHandoestradiolwereearlierandhigherinthe85lxgroup,andlaterandlowerintheothergroups,indicatingreproductiontimingandefficiencywerealtered. Light-emittingdiodescoveringawidespectrum(450