24. Emiliania huxleyi calcite mass variability during periods of atmospheric CO2 rising in the Mediterranean Sea

Michaël Grelaud (1) and Patrizia Ziveri (1)*

1 Institut de Ciència i Tecnologia Ambientals, Universitat Autònoma de Barcelona, Bellaterra, Barcelona, Spain

Background
Coccolithophores are marine phytoplanktonic organisms that play a significant role in both the marine food web and the carbon cycle. They are responsible for the photosynthetic fixation of inorganic carbon, regulating the particulate inorganic-organic carbon ratio and a large portion of the calcium carbonate (CaCO3) production. It is challenging to understand how the current rising of atmospheric CO2 concentration and the subsequent change in carbonate chemistry (i.e. ocean acidification) will impact the marine calcifying organisms. Here we present data of the most ubiquitous species (Emiliania huxleyi) from Mediterranean surface sediments and sedimentary cores that cover the industrial age, the Holocene, the last glacial-interglacial transition and the marine isotopic stage 5e.

Methods
Morphometric parameters (mass and size) of E. huxleyi coccolith were automatically measured, using the birefringence properties of calcite when viewed in cross-polarised light, in sediments of the Mediterranean Sea and covering key periods of rising atmospheric CO2. Moreover, the morphotypes distribution of E. huxleyi was investigated by scanning electron microscopy and compare to the averaged mass.

Findings
The calcite mass of E. huxleyi is anticorrelated with the atmospheric CO2 concentrations both during the warm MIS 5e and the last deglaciation, whilst there is no clear correlation between the atmospheric CO2 concentration and the E. huxleyi calcite mass in the most recent records (less than 400 years). Remarkably, the calcite mass variation of E. huxleyi observed over the last five centuries and in the surface sediment is of the same magnitude as for the last glacial-interglacial transition or the MIS 5e. The distribution of the morphotypes influences the averaged mass of E. huxleyi.

Conclusions
These results highlight the vulnerability and complexity of coccolithophore responses to the rapid rise of atmospheric CO2 and environmental changes since the beginning of the industrial age.