Abstract
Abstract. As major calcifiers in the open ocean, coccolithophores play a key role in the marine carbon cycle. Because they may be sensitive to changing CO2 and ocean acidification, there is significant interest in quantifying past and present variations in their cellular calcification by quantifying the thickness of the coccoliths or calcite plates that cover their cells. Polarized light microscopy has emerged as a key tool for quantifying the thickness of these calcite plates, but the reproducibility and accuracy of such determinations has been limited by the absence of suitable calibration materials in the thickness range of coccoliths (0–4 µm). Here, we describe the fabrication of a calcite wedge with a constant slope over this thickness range, and the independent determination of calcite thickness along the wedge profile. We show how the calcite wedge provides more robust calibrations in the 0 to 1.55 µm range than previous approaches using rhabdoliths. We show the particular advantages of the calcite wedge approach for developing equations to relate thickness to the interference colors that arise in calcite in the thickness range between 1.55 and 4 µm. The calcite wedge approach can be applied to develop equations relevant to the particular light spectra and intensity of any polarized light microscope system and could significantly improve inter-laboratory data comparability.
Highlights
The calcification by coccolithophorid algae represents a major source of CaCO3 production in the open oceans (Siesser, 1993)
To move beyond the responses of calcification in clonal cultures in short-duration experiments, recent efforts have focused on characterizing the variations in coccolithophorid calcification from measurements of the thickness of coccoliths, in populations from the water column and ocean sediments. Such measurements are effectively made using images of coccoliths taken by polarized light microscopy, for which the interference color varies with the thickness of the calcite in the coccolith (Beaufort, 2005)
Since the relationship between interference colors and thickness must be established for each particular microscope–camera configuration and validated routinely, we propose that a calcite wedge may serve as one such optimal calibration material
Summary
The calcification by coccolithophorid algae represents a major source of CaCO3 production in the open oceans (Siesser, 1993). To move beyond the responses of calcification in clonal cultures in short-duration experiments, recent efforts have focused on characterizing the variations in coccolithophorid calcification from measurements of the thickness of coccoliths, in populations from the water column and ocean sediments Such measurements are effectively made using images of coccoliths taken by polarized light microscopy, for which the interference color varies with the thickness of the calcite in the coccolith (Beaufort, 2005). In this work, following previous studies (Bollmann, 2014; Craig, 1961; Fuertes et al, 2014), we employ circular polarized light (Bass, 2009; Frohlich, 1986; Hecht, 2002; Higgins, 2010) so that our interference colors are independent of the calcite c-axis orientation and coccoliths are viewed without extinction patterns With this calcite wedge, the user can calibrate microscope-specific equations relating color to thickness. Our method with a calcite wedge permits calibration continuously over the thickness range from 0 to 4 μm, contributing to reproducible and accurate thickness measurements of a wide range of genera of modern and ancient coccoliths
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