Oceanacidificationmaybenefitalgaethatareabletocapitalizeonincreasedcarbonavailabilityforphotosynthesis,butitisexpectedtohaveadverseeffectsoncalcifiedalgaethroughdissolution.Shiftsindominancebetweenprimaryproducerswillhaveknock-oneffectsonmarineecosystemsandwilllikelyvaryregionally,dependingonfactorssuchasirradiance(lightvs.shade)andnutrientlevels(oligotrophicvs.eutrophic).Thusexperimentsareneededtoevaluateinteractiveeffectsofcombinedstressorsinthefield.Inthisstudy,weinvestigatedthephysiologicalresponsesofmacroalgaenearaCO2seepinoligotrophicwatersoffVulcano(Italy).Thealgaewereincubatedinsituat0.2mdepthusingacombinationofthreemeanCO2levels(500,700–800and1200μatmCO2),twolightlevels(100and70%ofsurfaceirradiance)andtwonutrientlevelsofN,P,andK(enrichedvs.non-enrichedtreatments)inthenon-calcifiedmacroalgaCystoseiracompressa(Phaeophyceae,Fucales)andcalcifiedPadinapavonica(Phaeophyceae,Dictyotales).AsuiteofbiochemicalassaysandinvivochlorophyllafluorescenceparametersshowedthatelevatedCO2levelsbenefittedbothofthesealgae,althoughtheirresponsesvarieddependingonlightandnutrientavailability.InC.compressa,elevatedCO2treatmentsresultedinhighercarboncontentandantioxidantactivityinshadedconditionsbothwithandwithoutnutrientenrichment—theyhadmoreChla,phenolsandfucoxanthinwithnutrientenrichmentandhigherquantumyield(Fv/Fm)andphotosyntheticefficiency(αETR)withoutnutrientenrichment.InP.pavonica,elevatedCO2treatmentshadhighercarboncontent,Fv/Fm,αETR,andChlaregardlessofnutrientlevels—theyhadhigherconcentrationsofphenoliccompoundsinnutrientenriched,fully-litconditionsandmoreantioxidantsinshaded,nutrientenrichedconditions.Nitrogencontentincreasedsignificantlyinfertilizedtreatments,confirmingthatthesealgaewerenutrientlimitedinthisoligotrophicpartoftheMediterranean.Ourfindingsstrengthenevidencethatbrownalgaecanbeexpectedtoproliferateastheoceansacidifywherephysicochemicalconditions,suchasnutrientlevelsandlight,permit