Geology Department, University of Oviedo, Jesus Arias de Velasco S/N, 33005, Oviedo, Asturias, Spain.
Nature. 2013 Aug 29;500(7464):558-62. doi: 10.1038/nature12448.
Coccolithophores are marine algae that use carbon for calcification and photosynthesis. The long-term adaptation of these and other marine algae to decreasing carbon dioxide levels during the Cenozoic era has resulted in modern algae capable of actively enhancing carbon dioxide at the site of photosynthesis. This enhancement occurs through the transport of dissolved bicarbonate (HCO3(-)) and with the help of enzymes whose expression can be modulated by variable aqueous carbon dioxide concentration, [CO2], in laboratory cultures. Coccolithophores preserve the geological history of this adaptation because the stable carbon and oxygen isotopic compositions of their calcite plates (coccoliths), which are preserved in the fossil record, are sensitive to active carbon uptake and transport by the cell. Here we use a model of cellular carbon fluxes and show that at low [CO2] the increased demand for HCO3(-) at the site of photosynthesis results in a diminished allocation of HCO3(-) to calcification, which is most pronounced in larger cells. This results in a large divergence between the carbon isotopic compositions of small versus large coccoliths only at low [CO2]. Our evaluation of the oxygen and carbon isotope record of size-separated fossil coccoliths reveals that this isotopic divergence first arose during the late Miocene to the earliest Pliocene epoch (about 7-5 million years ago). We interpret this to be a threshold response of the cells' carbon acquisition strategies to decreasing [CO2]. The documented coccolithophore response is synchronous with a global shift in terrestrial vegetation distribution between 8 and 5 Myr ago, which has been interpreted by some studies as a floral response to decreasing partial pressures of carbon dioxide () in the atmosphere. We infer a global decrease in carbon dioxide levels for this time interval that has not yet been identified in the sparse proxy record but is synchronous with global cooling and progressive glaciations.
颗石藻是一种利用碳进行钙化和光合作用的海洋藻类。这些藻类和其他海洋藻类在新生代期间对二氧化碳水平下降的长期适应，导致现代藻类能够在光合作用的部位主动增强二氧化碳。这种增强是通过溶解的碳酸氢根（HCO3(-)）的运输以及在实验室培养中，通过可变的水相二氧化碳浓度[CO2]调节其表达的酶来实现的。颗石藻保存了这种适应的地质历史，因为其方解石板（颗石）的稳定碳和氧同位素组成对细胞的主动碳吸收和运输敏感，而这些方解石板在化石记录中得以保存。在这里，我们使用细胞碳通量模型表明，在低[CO2]下，光合作用部位对 HCO3(-)的需求增加，导致 HCO3(-)向钙化的分配减少，在较大的细胞中最为明显。这导致只有在低[CO2]下，小颗石与大颗石的碳同位素组成之间出现很大的差异。我们对大小分离的化石颗石的氧和碳同位素记录的评估表明，这种同位素差异首先出现在中新世晚期到上新世早期（约 700 万至 500 万年前）。我们将其解释为细胞碳获取策略对二氧化碳下降的阈值响应。有记录的颗石藻反应与 800 万至 500 万年前全球陆地植被分布的全球性转变同步，一些研究将这种转变解释为对大气中二氧化碳分压下降的植物反应。我们推断，在这个时间间隔内，全球二氧化碳水平下降，但在稀疏的代理记录中尚未确定，与全球变冷和渐进式冰川作用同步。
