Can morphological features of coccolithophores serve as a reliable proxy to reconstruct environmental conditions of the past?

Morphological changes in coccoliths, tiny calcite platelets covering the outer surface of coccolithophores, can be the result of physiological responses to environmental changes. Coccoliths recovered from sedimentary successions may therefore provide information on paleo-environmental conditions prevailing at the time when the coccolithophores were alive. To calibrate the biomineralization responses of ancient coccolithophore to climatic changes studies often compared the biological responses of living coccolithophore species with paleo-data from calcareous nannofossils. However, there is uncertainty whether the morphological responses of living coccolithophores are representative for those of the fossilized ancestors. To investigate this, we cultured four living coccolithophore species (Emiliania huxleyi, Gephyrocapsa oceanica, Coccolithus pelagicus subsp. braarudii, and Pleurochrysis carterae) that have been evolutionarily distinct for millions of years, exposed them to changing environmental conditions (i.e. changing light intensity, Mg / Ca ratio, nutrient availability, temperature and carbonate chemistry) and evaluated their responses in coccolith morphology (i.e. size, length, width, malformation). The motivation for this study was that if the species show the same morphological response to changes in any of the tested abiotic environmental factors, then there is a reason to assume that this response is conserved over geological timescales and that coccolith morphology can serve as a paleo-proxy for that specific factor. In contrast with this concept, we found that the four species responded differently to changing light intensity, Mg / Ca ratio, nutrient availability and temperature in terms of coccolith morphology. The lack of a common response reveals the difficulties in using coccolith morphology as a proxy for paleo-environmental conditions. However, a common response was observed under changing seawater carbonate chemistry (i.e. rising CO₂) which consistently induced malformations. This commonality provides some confidence that malformations found in the sedimentary record could be indicative for high CO₂ levels. [ABSTRACT FROM AUTHOR]