| Summary: | The export of the calcitic platelets (coccoliths) of coccolithophores to the sea floor acts as a long-term sink of carbon. In order to understand the effects of current ocean acidification on coccolithophore calcification, it is important to estimate the amount of calcite that is bound in single coccoliths, which – due to their minuteness (1–10 μm) – cannot be individually weighed. To estimate single coccolith weights, this study takes advantage of the birefringence method (Beaufort, 2005), which links the interference color of calcite under polarized light with calcite thickness. Two similar techniques, one using crossed-polarized light (XPL) and one using circular polarized light (CPL), were tested to compare coccolith weight estimates of cultured single clones of Gephyrocapsa oceanica, which – in a previous study (Rickaby et al., 2010) – were grown in increasing concentrations of dissolved inorganic carbon (DIC) but constant pH-levels (~8.1) to reflect long-term buffering of the ocean. In contrast to expectations, results of this study revealed constant coccolith weight estimates throughout different levels of DIC, indicating that the calcification of G. oceanica is unaffected by DIC concentrations when pH levels are kept at ~8.1. At the same time, different coccolith weights were estimated between the two techniques, caused by a variable greylevel-to-thickness transfer function which was based on width measurements of conical stems (rhabdoliths) of the coccolithophore Rhabdosphaera. In the calibration, several rhabdolith specimens displayed fluctuating thickness/greylevel measurements due to natural variation of their shapes. Although relative coccolith weight comparisons were possible between the samples, the comparison of both techniques emphasizes the importance of establishing a more reliable calibration method in order to estimate absolute coccolith weights.
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