General responses of Thoracosphaera heimii grown under a range of pCO2

Ocean acidification is considered a major threat to marine ecosystems and may particularly affect calcifying organisms such as corals, foraminifera and coccolithophores. Here we investigate the impact of elevated pCO2 and lowered pH on growth and calcification in the common calcareous dinoflagellate Thoracosphaera heimii. We observe a substantial reduction in growth rate, calcification and cyst stability of T. heimii under elevated pCO2. Furthermore, transcriptomic analyses reveal CO2 sensitive regulation of many genes, particularly those being associated to inorganic carbon acquisition and calcification. Stable carbon isotope fractionation for organic carbon production increased with increasing pCO2 whereas it decreased for calcification, which suggests interdependence between both processes. We also found a strong effect of pCO2 on the stable oxygen isotopic composition of calcite, in line with earlier observations concerning another T. heimii strain. The observed changes in stable oxygen and carbon isotope composition of T. heimii cysts may provide an ideal tool for reconstructing past seawater carbonate chemistry, and ultimately past pCO2. Although the function of calcification in T. heimii remains unresolved, this trait likely plays an important role in the ecological and evolutionary success of this species. Acting on calcification as well as growth, ocean acidification may therefore impose a great threat for T. heimii.海洋酸化被视为海洋生态系统面临的主要威胁之一，尤其可能对钙化生物（calcifying organisms）造成显著影响，例如珊瑚、有孔虫（foraminifera）以及颗石藻（coccolithophores）。本研究针对常见的钙质性甲藻Thoracosphaera heimii，探究了升高的二氧化碳分压（pCO2）与降低的pH值对其生长及钙化作用的影响。研究发现，在升高的pCO2条件下，T. heimii的生长速率、钙化作用强度与孢囊稳定性均出现显著下降。此外，转录组分析（transcriptomic analyses）显示，大量基因的表达受到pCO2的敏感调控，其中尤以与无机碳摄取及钙化过程相关的基因为甚。有机碳合成过程中的稳定碳同位素分馏（stable carbon isotope fractionation）随pCO2升高而增大，而钙化过程的稳定碳同位素分馏则随pCO2升高而降低，这表明两类过程之间存在相互依存关系。本研究还发现，pCO2对方解石（calcite）的稳定氧同位素组成具有显著影响，这与此前针对另一株T. heimii的观测结果一致。本研究观测到的T. heimii孢囊的稳定氧、碳同位素组成变化，或可成为重建过去海水碳酸盐化学环境乃至古pCO2水平的理想手段。尽管T. heimii的钙化功能仍未明确，但该性状可能对该物种的生态与演化成功具有重要意义。因此，海洋酸化同时作用于钙化过程与生长过程，或会对T. heimii构成严重威胁。