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

Abstract

Abstract. 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.

Reduced H+channel activity disrupts pH homeostasis and calcification in coccolithophores at low ocean pH

“…PIC production decreased more strongly than POC production in acidified conditions, leading to lower PIC:POC ratios. These trends are in close agreement with other laboratory studies examining the response of C. braarudii to changing carbonate chemistries 10,15,31,32,37 . Calculation of the H + load from values of PIC and POC production indicated that H + production by calcification exceeded H + consumption by photosynthesis under all scenarios, being highest at optimal PIC:POC ratios (Fig.…”

Biscutum constans coccolith size patterns across the mid Cretaceous in the western Tethys: Paleoecological implications

“…However, it has not been explained why sub-optimal temperature should disturb cytoskeleton functionality and therefore the intra-cellular production of coccoliths. Moreover, a recent work underlined the lack of a common response among different species towards different temperatures, indicating that coccolithophores do not always produced bigger or smaller coccoliths under sub-optimal temperature values (Faucher et al, 2020). Furthermore, for living coccolithophore, previous authors claimed that coccolith formation, is less adversely affected by nutrient deficiency than growth and cell division (Paasche, 1998): coccoliths continue to be produced under P or N depletion and, indeed, morphometrical studies did not reveal any drastic variation in size and morphology under nutrient limitation (Faucher et al, 2020).…”

“…Moreover, a recent work underlined the lack of a common response among different species towards different temperatures, indicating that coccolithophores do not always produced bigger or smaller coccoliths under sub-optimal temperature values (Faucher et al, 2020). Furthermore, for living coccolithophore, previous authors claimed that coccolith formation, is less adversely affected by nutrient deficiency than growth and cell division (Paasche, 1998): coccoliths continue to be produced under P or N depletion and, indeed, morphometrical studies did not reveal any drastic variation in size and morphology under nutrient limitation (Faucher et al, 2020). The lack of a direct link between size and temperature variations or nutrient content evidenced in this study for B. constans, and the lack of a common interpretation of temperature and nutrient content effects on B. constans size from other works on Cretaceous intervals, is consistent with data from experiments on living coccolithophores and evidence a strong uncertainty of correlating nutrient and temperature with size fluctuations.…”

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

“…Well, that is only my idea, and the authors do not have to change their conclusion if they can offer a better discussion than this version. 7) These two papers may help for a rich discussion, Sheward et al (2017) and Aloisi (2015), which are absent from their references list.…”