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Coccoliths are major contributors to the particulate inorganic carbon in the ocean that is a key part of the carbon cycle. The coccoliths are a few micrometres in length and weigh a few picogrammes. Their birefringence characteristics in polarized optical microscopy have been used to estimate their mass. This method is rapid and precise because camera sensors produce excellent measurements of light. However, the current method is limited because it requires a precise and replicable set-up and calibration of the light in the optical equipment. More precisely, the light intensity, the diaphragm opening, the position of the condenser and the exposure time of the camera have to be strictly identical during the calibration and the analysis of calcite crystal. Here we present a new method that is universal in the sense that the thickness estimations are independent from a calibration but result from a simple equation. It can be used with different cameras and microscope brands. Moreover, the light intensity used in the microscope does not have to be strictly and precisely controlled. This method permits the measurement of crystal thickness up to 1.7 µm. It is based on the use of one left circular polarizer and one right circular polarizer with a monochromatic light source using the following equation: d=λπΔnarctanILRILL, where d is the thickness, λ the wavelength of the light used, Δn the birefringence, and ILR and ILL the light intensity measured with a right and a left circular polarizer. Because of the alternative and rotational motion of the quarter-wave plate of the circular polarizer, we coined the name of this method “bidirectional circular polarization” (BCP).

The rise of calcareous nannoplankton in Mesozoic oceans has deeply impacted ocean chemistry and contributed to shaping modern oceans. Nevertheless, the calcareous nannoplankton colonization of past marine environments remains poorly understood. Based on an extensive compilation of published and unpublished data, we show that their accumulation rates in sediments increased from the Early Jurassic (∼200 Ma) to the Early Cretaceous (∼120 Ma), although these algae diversified up to the end of the Mesozoic (66 Ma). After the middle Eocene (∼45 Ma), a decoupling occurred between accumulation rates, diversity and coccolith size. The time series analyzed points toward a three-phase evolutionary dynamic. An invasion phase of the open-ocean realms was followed by a specialization phase occurring along with taxonomic diversification, ended by an establishment phase where a few small-sized species dominated. The current hegemony of calcareous nannoplankton in the world ocean results from a long-term and complex evolutionary history shaped by ecological interactions and abiotic forcing.

Coccolithophores are one of the most abundant phytoplankton and calcifying organisms, well-known to produce intricate calcareous exoskeletons made of coccoliths. Here the authors show, by using X-ray nanotomography, the dependence of the grid size on the calcite nucleation site number and on the mass of coccoliths.

Although the role of Earth's orbital variations in driving global climate cycles has long been recognized, their effect on evolution is hitherto unknown. The fossil remains of coccolithophores, a key calcifying phytoplankton group, enable a detailed assessment of the effect of cyclic orbital-scale climate changes on evolution because of their abundance in marine sediments and the preservation of their morphological adaptation to the changing environment 1,2 . Evolutionary genetic analyses have linked broad changes in Pleistocene fossil coccolith morphology to species radiation events 3 . Here, using high-resolution coccolith data, we show that during the last 2.8 million years the morphological evolution of coccolithophores was forced by Earth's orbital eccentricity with rhythms of around 100,000 years and 405,000 years-a distinct spectral signature to that of coeval global climate cycles 4 . Simulations with an Earth System Model 5 coupled with an ocean biogeochemical model 6 show a strong eccentricity modulation of the seasonal cycle, which we suggest directly affects the diversity of ecological niches that occur over the annual cycle in the tropical ocean. Reduced seasonality in surface ocean conditions favours species with mid-size coccoliths, increasing coccolith carbonate export and burial; whereas enhanced seasonality favours a larger range of coccolith sizes and reduced carbonate export. We posit that eccentricity pacing of phytoplankton evolution contributed to the strong 405,000-year cyclicity that is seen in global carbon cycle records., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Coccolithophores of the Noëlaerhabdaceae family are covered by imbricated coccoliths, each composed of multiple calcite crystals radially distributed around the periphery of a grid. The factors that determine coccolith size remain obscure. Here, we used synchrotron-based three-dimensional Coherent X-ray Diffraction Imaging to study coccoliths of 7 species of Gephyrocapsa, Emiliania and Reticulofenestra with a resolution close to 30 nm. Segmentation of 45 coccoliths revealed remarkable size, mass and segment number variations, even within single coccospheres. In particular, we observed that coccolith mass correlates with grid perimeter which scales linearly with crystal number. Our results indirectly support the idea that coccolith mass is determined in the coccolith vesicle by the size of the organic base plate scale (OBPS) around which R-unit nucleation occurs every 110-120 nm. The curvation of coccoliths allows inference of a positive correlation between cell nucleus, OBPS and coccolith sizes.

Heterococcoliths are micron-scale calcite platelets produced by coccolithophores. They have been the most abundant and continuous fossil record over the last 215 million years (Myr), offering great potential for geochemical studies, although the heterococcolith fossil record remains underutilised in this domain. We have mapped heterococcoliths' composition using X-ray fluorescence (XRF) with a 100-nm resolution beam to decipher element distributions in heterococcoliths and to investigate the potential development of geochemical proxies for palaeoceanography. The study presents two Middle Jurassic Watznaueria britannica heterococcoliths from Cabo Mondego, Portugal. XRF analysis was performed with a 17 keV incident energy beam at the European Synchrotron Radiation Facility ID22NI beamline to study elements from Sr down to S. Ca, Sr and Mn are distributed following the heterococcolith crystalline arrangement. Cl, Br and S display an homogeneous distribution, whereas K, Fe, Cu, Zn and Rb are concentrated at the edges and in the central area of the heterococcoliths. Distributions of K, Fe, Ti, Fe, Cu, Zn, Rb and to a lesser extent V and Cr are highly influenced by clay contamination and peripheral diagenetic overgrowth. Mn is related to diagenetic Mn-rich CaCO3 overgrowth on top of or between heterococcoliths shields. Cl and Br are likely to be present in heterococcoliths inside interstitial nano-domains. We assume that the cytoplasm [Cl(-) ] and [Br(-) ] are mediated and constant during heterococcolithogenesis. Assuming a linear correlation between cytoplasm [Cl(-) ] and sea water [Cl(-) ], heterococcolith Cl may have potential as a salinity proxy. As S is incorporated into heterococcoliths by sulphated polysaccharides, our study suggests a role for such polysaccharides in heterococcolithogenesis for at least 170 Myr. The low Sr/Ca in the W. britannica specimens studied here may either highlight an unusual cellular physiology of Mesozoic coccolithophores or result from low growth rates in oligotrophic environments., (© 2016 John Wiley & Sons Ltd.)