While seawater acidification induced by elevated CO 2 is known to impact coccolithophores, the effects in combination with decreased salinity caused by sea ice melting and/or hydrological events have not been documented. Here we show the combined effects of seawater acidification and reduced salinity on growth, photosynthesis and calcification of Emiliania huxleyi grown at 2 CO 2 concentrations (low CO 2 LC:400 µatm; high CO 2 HC:1000 µatm) and 3 levels of salinity (25, 30, and 35). A decrease of salinity from 35 to 25 increased growth rate, cell size and photosynthetic performance under both LC and HC. Calcification rates were relatively insensitive to salinity though they were higher in the LC-grown compared to the HCgrown cells at 25 salinity, with insignificant differences under 30 and 35. Since salinity and OA treatments did not show interactive effects on calcification, changes in calcification:photosynthesis ratios are attributed to the elevated photosynthetic rates at lower salinities, with higher ratios of calcification to photosynthesis in the cells grown under 35 compared with those grown at 25. In contrast, photosynthetic carbon fixation increased almost linearly with decreasing salinity, regardless of the pCO 2 treatments. When subjected to short-term exposure to high light, the low-salinitygrown cells showed the highest photochemical effective quantum yield with the highest repair rate, though the HC treatment enhanced the PSII damage rate. Our results suggest that, irrespective of pCO 2 , at low salinity Emiliania huxleyi up-regulates its photosynthetic performance which, despite a relatively insensitive calcification response, may help it better adapt to future ocean global environmental changes, including ocean acidification, especially in the coastal areas of high latitudes.

The carbonate chemistry in coastal waters is more variable compared with that of open oceans, both in magnitude and time scale of its fluctuations. However, knowledge of the responses of coastal phytoplankton to dynamic changes in pH/pCO2 has been scarcely documented. Hence, we investigated the physiological performance of a coastal isolate of the coccolithophore Emiliania huxleyi (PML B92/11) under fluctuating and stable pCO2 regimes (steady ambient pCO2, 400 μatm; steady elevated pCO2, 1200 μatm; diurnally fluctuating elevated pCO2, 600–1800 μatm). Elevated pCO2 inhibited the calcification rate in both the steady and fluctuating regimes. However, higher specific growth rates and lower ratios of calcification to photosynthesis were detected in the cells grown under diurnally fluctuating elevated pCO2 conditions. The fluctuating pCO2 regime alleviated the negative effects of elevated pCO2 on effective photochemical quantum yield and relative photosynthetic electron transport rate compared with the steady elevated pCO2 treatment. Our results suggest that growth of E. huxleyi could benefit from diel fluctuations of pH/pCO2 under future-projected ocean acidification, but its calcification was reduced by the fluctuation and the increased concentration of CO2, reflecting a necessity to consider the influences of dynamic pH fluctuations on coastal carbon cycles associated with ocean global changes.

<p><strong>Abstract.</strong> While seawater acidification induced by elevated CO₂ is known to impact coccolithophores, the effects in combination with decreased salinity caused by sea ice melting and/or hydrological events have not been documented. Here we show the combined effects of seawater acidification and reduced salinity on growth, photosynthesis and calcification of <i>Emiliania huxleyi</i> grown at 2 CO₂ concentrations (low CO₂ LC: 400&amp;thinsp;μatm; high CO₂ HC: 1000&amp;thinsp;μatm) and 3 levels of salinity (25, 30 and 35&amp;thinsp;‰). A decrease of salinity from 35 to 25‰ increased growth rate, cell size and effective photochemical efficiency under both LC or HC. Calcification rates were relatively insensitive to combined effects of salinity and OA treatment but were highest under 3&amp;thinsp;5‰ and HC conditions, with higher ratios of calcification to photosynthesis (C&amp;thinsp;:&amp;thinsp;P) in the cells grown under 35&amp;thinsp;‰ compared with those grown at 25&amp;thinsp;‰. In addition, elevated dissolved inorganic carbon (DIC) concentration at the salinity of 35&amp;thinsp;‰ stimulated its calcification. In contrast, photosynthetic carbon fixation increased almost linearly with decreasing salinity, regardless of the <i>p</i>CO₂ treatments. When subjected to short-term exposure to high light, the low-salinity-grown cells showed the highest photochemical effective quantum yield with the highest repair rate, though HC treatment enhanced PSII damage rate. Our results suggest <i>Emiliania huxleyi</i> can tolerate low salinity plus acidification conditions by up-regulating its photosynthetic performance together with a relatively insensitive calcification response, which may help it better adapt to future ocean global environmental changes, especially in the coastal areas of high latitudes.</p>

We set up another two treatments (25‰ LCadd and 25‰ HCadd) to test if the strain of PML B92/11 of E. huxleyi had the ability to adjust the inorganic carbon allocation between photosynthesis and calcification under different carbonate chemistry conditions in which the DIC concentration was different (Herfort et al., 2002; Bach et al., 2013).After diluting the prepared artificial seawater (salinity = 35‰) with Milli -Q water to obtain 25‰ seawater, we added NaHCO3 to reach the same amount as that of artificial seawater (salinity = 35‰) before sterilization, nutrient addition and aeration (ambient or elevated CO2 levels). The procedures for determination of photosynthetic and calcification rates and chlorophyll a fluorescence under high light were the same as described in the main text. After the exponentially growing cells were acclimated to 25‰ LCadd or 25‰ HCadd treatment for about 14 generations, we took samples for the measurements.Under LC, with increased DIC concentration, the calcification to photosynthesis ratio (C:P) decreased by 61% (one-way ANOVA, p = 0.047) compared with the cells grown at the salinity 25‰ without NaHCO3 addition. In contrast, the C:P ratios of cells grown under the 25‰ HCadd treatment was 100% higher (one-way ANOVA, p =