Predominance of heavily calcified coccolithophores at low CaCO3 saturation during winter in the Bay of Biscay

Coccolithophores are an important component of the Earth system, and, as calcifiers, their possible susceptibility to ocean acidification is of major concern. Laboratory studies at enhanced pCO2 levels have produced divergent results without overall consensus. However, it has been predicted from these studies that, although calcification may not be depressed in all species, acidification will produce "a transition in dominance from more to less heavily calcified coccolithophores" [Ridgwell A, et al., (2009) Biogeosciences 6:2611-2623]. A recent observational study [Beaufort L, et al., (2011) Nature 476:80-83] also suggested that coccolithophores are less calcified in more acidic conditions. We present the results of a large observational study of coccolithophore morphology in the Bay of Biscay. Samples were collected once a month for over a year, along a 1,000-km-long transect. Our data clearly show that there is a pronounced seasonality in the morphotypes of Emiliania huxleyi, the most abundant coccolithophore species. Whereas pH and CaCO3 saturation are lowest in winter, the E. huxleyi population shifts from <10% (summer) to >90% (winter) of the heavily calcified form. However, it is unlikely that the shifts in carbonate chemistry alone caused the morphotype shift. Our finding that the most heavily calcified morphotype dominates when conditions are most acidic is contrary to the earlier predictions and raises further questions about the fate of coccolithophores in a high-CO2 world.颗石藻（Coccolithophores）是地球系统的重要组成部分，作为钙化生物（calcifiers），它们对海洋酸化（ocean acidification）的潜在敏感性备受关注。针对升高的pCO₂水平开展的实验室研究得到了不一致的结果，尚未形成广泛共识。不过，基于此类研究已有预测指出：尽管并非所有物种的钙化作用（calcification）都会受到抑制，但海洋酸化将引发“优势类群从钙化程度更高的颗石藻向钙化程度更低的类群转变”[Ridgwell A 等，2009，《生物地球科学》（Biogeosciences），6:2611-2623]。近期一项观测研究[Beaufort L 等，2011，《自然》（Nature），476:80-83]也表明，酸性更强的环境中颗石藻的钙化程度更低。 本研究报道了在比斯开湾开展的大型颗石藻形态学观测研究结果：研究团队沿一条1000公里长的断面，历时一年以上每月采集一次样本。数据清晰显示，全球丰度最高的颗石藻物种赫氏颗石藻（Emiliania huxleyi）的形态型存在显著的季节周期性。尽管冬季海水pH与碳酸钙（CaCO₃）饱和浓度均处于全年最低水平，但赫氏颗石藻种群中高钙化形态的占比从夏季的不足10%上升至冬季的90%以上。但仅凭碳酸盐化学环境的变化，不足以解释该形态型的转变。本研究发现，在环境酸性最强的冬季，高钙化形态型反而占据主导地位，这与此前的预测相悖，也为高二氧化碳世界中颗石藻的生存前景提出了新的疑问。