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Abstract. Marine phytoplankton such as bloom-forming, calcite-producingcoccolithophores, are naturally exposed to solar ultraviolet radiation (UVR,280–400nm) in the ocean's upper mixed layers. Nevertheless, the effects ofincreasing carbon dioxide (CO₂)-induced ocean acidification and warming have rarelybeen investigated in the presence of UVR. We examined calcification andphotosynthetic carbon fixation performance in the most cosmopolitancoccolithophorid, Emiliania huxleyi, grown under high(1000µatm, HC; pH_T: 7.70) and low (400µatm,LC; pH_T: 8.02) CO₂ levels, at 15^∘C,20^∘C and 24^∘C with or without UVR. The HCtreatment did not affect photosynthetic carbon fixation at 15^∘C,but significantly enhanced it with increasing temperature. Exposure to UVRinhibited photosynthesis, with higher inhibition by UVA (320–395nm) thanUVB (295–320nm), except in the HC and 24^∘C-grown cells, in whichUVB caused more inhibition than UVA. A reduced thickness of the coccolith layerin the HC-grown cells appeared to be responsible for the UV-inducedinhibition, and an increased repair rate of UVA-derived damage in theHC–high-temperature grown cells could be responsible for lowered UVA-induced inhibition.While calcification was reduced with elevated CO₂ concentration,exposure to UVB or UVA affected the process differentially, with the formerinhibiting it and the latter enhancing it. UVA-induced stimulationmore »of calcification washigher in the HC-grown cells at 15 and 20^∘C, whereas at24^∘C observed enhancement was not significant. The calcificationto photosynthesis ratio (Cal∕Pho ratio) was lower in the HC treatment,and increasing temperature also lowered the value. However, at 20 and24^∘C, exposure to UVR significantly increased the Cal∕Phoratio, especially in HC-grown cells, by up to 100%. This implies thatUVR can counteract the negative effects of the “greenhouse” treatment onthe Cal∕Pho ratio; hence, UVR may be a key stressor when considering theimpacts of future greenhouse conditions on E. huxleyi.