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

• Main Authors: S. Tong, D. A. Hutchins, K. Gao
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-01-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/561/2019/bg-16-561-2019.pdf

<p>Marine phytoplankton such as bloom-forming, calcite-producing coccolithophores, are naturally exposed to solar ultraviolet radiation (UVR, 280–400&thinsp;nm) in the ocean's upper mixed layers. Nevertheless, the effects of increasing carbon dioxide (<span class="inline-formula">CO₂</span>)-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, <i>Emiliania huxleyi</i>, grown under high (1000&thinsp;<span class="inline-formula">µ</span>atm, HC; pH<span class="inline-formula">_T</span>: 7.70) and low (400&thinsp;<span class="inline-formula">µ</span>atm, LC; pH<span class="inline-formula">_T</span>: 8.02) <span class="inline-formula">CO₂</span> levels, at 15&thinsp;<span class="inline-formula">^∘</span>C, 20&thinsp;<span class="inline-formula">^∘</span>C and 24&thinsp;<span class="inline-formula">^∘</span>C with or without UVR. The HC treatment did not affect photosynthetic carbon fixation at 15&thinsp;<span class="inline-formula">^∘</span>C, but significantly enhanced it with increasing temperature. Exposure to UVR inhibited photosynthesis, with higher inhibition by UVA (320–395&thinsp;nm) than UVB (295–320&thinsp;nm), except in the HC and 24&thinsp;<span class="inline-formula">^∘</span>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 <span class="inline-formula">CO₂</span> 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&thinsp;<span class="inline-formula">^∘</span>C, whereas at 24&thinsp;<span class="inline-formula">^∘</span>C observed enhancement was not significant. The calcification to photosynthesis ratio (Cal&thinsp;<span class="inline-formula">∕</span>&thinsp;Pho ratio) was lower in the HC treatment, and increasing temperature also lowered the value. However, at 20 and 24&thinsp;<span class="inline-formula">^∘</span>C, exposure to UVR significantly increased the Cal&thinsp;<span class="inline-formula">∕</span>&thinsp;Pho ratio, especially in HC-grown cells, by up to 100&thinsp;%. This implies that UVR can counteract the negative effects of the “greenhouse” treatment on the Cal&thinsp;<span class="inline-formula">∕</span>&thinsp;Pho ratio; hence, UVR may be a key stressor when considering the impacts of future greenhouse conditions on <i>E. huxleyi</i>.</p>