Physiological and biochemical responses of Emiliania huxleyi to ocean acidification and warming are modulated by UV radiation

Abstract. Marine phytoplankton such as bloom-forming, calcite-producing coccolithophores, are naturally exposed to solar ultraviolet radiation (UVR, 280–400 nm) in the ocean's upper mixed layers. Nevertheless, the effects of increasing carbon dioxide ( CO 2 )-induced ocean acidification and warming have rarely been investigated in the presence of UVR. We examined calcification and photosynthetic carbon fixation performance in the most cosmopolitan coccolithophorid, Emiliania huxleyi , grown under high (1000  µ atm, HC; pH T : 7.70) and low (400  µ atm, LC; pH T : 8.02) CO 2 levels, at 15  ∘ C, 20  ∘ C and 24  ∘ C with or without UVR. The HC treatment did not affect photosynthetic carbon fixation at 15  ∘ C, but significantly enhanced it with increasing temperature. Exposure to UVR inhibited photosynthesis, with higher inhibition by UVA (320–395 nm) than UVB (295–320 nm), except in the HC and 24  ∘ C-grown cells, in which UVB caused more inhibition than UVA. A reduced thickness of the coccolith layer in the HC-grown cells appeared to be responsible for the UV-induced inhibition, and an increased repair rate of UVA-derived damage in the HC–high-temperature grown cells could be responsible for lowered UVA-induced inhibition. While calcification was reduced with elevated CO 2 concentration, exposure to UVB or UVA affected the process differentially, with the former inhibiting it and the latter enhancing it. UVA-induced stimulation of calcification was higher in the HC-grown cells at 15 and 20  ∘ C, whereas at 24  ∘ C observed enhancement was not significant. The calcification to photosynthesis ratio (Cal  ∕  Pho ratio) was lower in the HC treatment, and increasing temperature also lowered the value. However, at 20 and 24  ∘ C, exposure to UVR significantly increased the Cal  ∕  Pho ratio, especially in HC-grown cells, by up to 100 %. This implies that UVR can counteract the negative effects of the “greenhouse” treatment on the Cal  ∕  Pho ratio; hence, UVR may be a key stressor when considering the impacts of future greenhouse conditions on E. huxleyi .