| Summary: | Anthropogenic CO2 emissions into the atmosphere are taken up by the ocean, leading to a decrease in pH and CaCO3 saturation state in a process called ocean acidification. Ocean acidification is thought to affect several marine calcifying groups, including the coccolithophores. Field studies on potential changes in coccolithophore calcification due to ocean acidification often rely on the coccolith as a proxy for calcification. Such research, however, is limited by researchers applying different approaches for estimating coccolith mass with little knowledge on how the different approaches relate to each other. Furthermore, several studies rely on flawed methodology to estimate coccolith mass from its interference colours produced under polarized light. In this thesis, a published method for estimating coccolith mass under circular polarized light (the CPR-method) was revised. These revisions addressed several important limitations with the CPR-method by rendering a revised and accurate Michel-Lévy chart, developing a colour management routine applicable to oil objectives, and using polymer films of known and low (~30nm) retardation for calibration to reduce polarization aberrations. After these limitations were addressed, coccoliths could be measured with unprecedented accuracy (±0:007 μm for thickness and ±~15-20% for mass). The CPR-method was then applied to plankton samples collected near the Canary Islands in January and September 1997. Relative abundance, coccolith thickness, and coccolith mass of different morphotypes of the coccolithophore species Emiliania huxleyi were evaluated, and the study revealed that "overcalcified" Type A coccoliths in September were lighter and thinner than "normally calcified" Type A coccoliths in January, and the thickness difference between Type A and Group B coccoliths was furthermore highly variable between samples. This result has important implications for several studies which have used E. huxleyi morphotypes as a proxy for coccolithophore calcification. The CPR-method was also applied to E. huxleyi and Gephyrocapsa oceanica strains cultured under different salinity conditions to evaluate the relationship between coccolith length and thickness. The study found that coccolith length and thickness were not related in E. huxleyi, and caution is therefore needed before using the ks model to estimate coccolith mass from length. Ph.D. 2021-10-25 00:00:00
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