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  1. 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; pHT: 7.70) and low (400µatm,LC; pHT: 8.02) CO2 levels, at 15C,20C and 24C with or without UVR. The HCtreatment did not affect photosynthetic carbon fixation at 15C,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 24C-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 20C, whereas at24C observed enhancement was not significant. The calcificationto photosynthesis ratio (CalPho ratio) was lower in the HC treatment,and increasing temperature also lowered the value. However, at 20 and24C, exposure to UVR significantly increased the CalPhoratio, especially in HC-grown cells, by up to 100%. This implies thatUVR can counteract the negative effects of the “greenhouse” treatment onthe CalPho ratio; hence, UVR may be a key stressor when considering theimpacts of future greenhouse conditions on E. huxleyi.

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