“钙板金藻”（又名球石藻，在海洋浮游生物中广泛分布）在藻类中是独特的，因为它们将碳既用于钙化、又用于光合作用。在这项研究中，Clara Bolton和Heather Stoll采用一个细胞碳流量模型发现，当二氧化碳浓度低时，这些生物将会把碳优先分配给光合作用而不是钙化，特别是在较大的细胞中。这一点反映在小球石和大球石的同位素特征之间的一个差别上，该差别在二氧化碳浓度高时会减小。这一模式可以在化石记录中看到。作者在距今大约600万年前的小球石和大球石之间发现了一个同位素分化，将此解读为细胞的碳获取对于当时二氧化碳浓度的普遍下降的一种临界反应。（生物谷 Bioon.com）
生物谷推荐的英文摘要
Nature doi:10.1038/nature12448
Late Miocene threshold response of marine algae to carbon dioxide limitation
Clara T. Bolton & Heather M. Stoll
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 era1 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 cultures2, 3. 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 atmosphere4, 5, 6. We infer a global decrease in carbon dioxide levels for this time interval that has not yet been identified in the sparse proxy record7 but is synchronous with global cooling and progressive glaciations8, 9.
