Seawater carbonate chemistry and growth, photosynthetic and calcification rates of a coastal strain of Emiliania huxleyi

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.

沿海水域的碳酸盐化学特征相较于开阔海洋具有更强的变异性，无论是波动幅度还是时间尺度均是如此。然而，关于沿海浮游植物对pH/pCO2动态变化的响应机制，目前相关研究仍十分有限。为此，本研究探究了沿海分离株赫胥黎艾氏球石藻（Emiliania huxleyi，菌株PML B92/11）在波动与稳定pCO₂条件下的生理表现，具体处理包括：稳定环境pCO₂（400 μatm）、稳定高pCO₂（1200 μatm）以及日间波动高pCO₂（600–1800 μatm）。无论是稳定还是波动的高pCO₂条件，均会抑制该藻的钙化速率。然而，在日间波动高pCO₂条件下培养的细胞，其比生长速率更高，且钙化与光合作用的比值更低。与稳定高pCO₂处理相比，波动pCO₂条件可缓解高pCO₂对有效光化学量子产率及相对光合电子传递速率的负面影响。研究结果表明，在未来海洋酸化背景下，赫胥黎艾氏球石藻的生长或可从pH/pCO₂的昼夜波动中获益，但其钙化作用会因CO₂浓度升高及波动而降低。这提示我们，在研究与全球海洋变化相关的沿海碳循环时，需充分考虑pH动态波动的影响。