Late Miocene threshold response of marine algae to carbon dioxide limitation

New measurements of stable isotope vital effects in fossil coccoliths show a step increase in reliance of coccolithophore photosynthesis on active transport of dissolved bicarbonate in the late Miocene epoch, suggesting both a low threshold for adaptation of coccolithophores to carbon dioxide and a decrease in global carbon dioxide levels at that time. Phytoplankton cell size tracks atmospheric CO.sub.2 Coccolithophores, widely distributed in the marine plankton, are unique among algae in that they use carbon for both calcification and photosynthesis. In this study Clara Bolton and Heather Stoll use a model of cellular carbon fluxes to show that when carbon dioxide concentrations are low, these organisms will allocate carbon preferentially to photosynthesis rather than to calcification, particularly in larger cells. This is reflected in a difference between the isotopic signature of small and large coccoliths that diminishes at high levels of carbon dioxide. This pattern can be seen in the fossil record; the authors identify an isotopic divergence between small and large coccoliths at around 6 million years ago, and interpret this as a threshold response of the cells' carbon acquisition to a global decrease in carbon dioxide concentrations at the time. 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.sup.1 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 (HCO.sub.3.sup.-) and with the help of enzymes whose expression can be modulated by variable aqueous carbon dioxide concentration, [CO.sub.2], in laboratory cultures.sup.2,3. 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 [CO.sub.2] the increased demand for HCO.sub.3.sup.- at the site of photosynthesis results in a diminished allocation of HCO.sub.3.sup.- 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 [CO.sub.2]. 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 [CO.sub.2]. 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 ([Formula omitted]) in the atmosphere.sup.4,5,6. We infer a global decrease in carbon dioxide levels for this time interval that has not yet been identified in the sparse [Formula omitted] proxy record.sup.7 but is synchronous with global cooling and progressive glaciations.sup.8,9.
Author(s): Clara T. Bolton [sup.1] , Heather M. Stoll [sup.1] Author Affiliations: (1) Geology Department, University of Oviedo, Jesus Arias de Velasco S/N, 33005, Oviedo, Asturias, Spain, Main Coccolithophores are [...]