Fossil Plankton may be the key to understanding prehistoric climate change

Coccolithophores surround themselves with platelets of calcium carbonate, shown here greatly magnified. Calcium carbonate is transparent and so the platelets do not compromise the organism's ability to photosynthesize. Credit: Hermoso and McClelland. 


Ancient fossilised algae may be the gateway to understanding carbon dioxide and the role it played in transforming the prehistoric climate of the planet, a new study has found.

Alongside collaborators from PML and the University of Washington, St. Louis, the team from the University of Oxford’s, Earth Sciences Department, studied coccolithophores, a single-celled algae that lives on carbon dioxide, water and sunlight. Some plankton are known for making shells whose chemistry records the environment they live in. As a species, coccolithophores are distinct from other algae, as they produce their shells, and the chemistry contained within their shells, entirely within the cell. Their microscopic, mineralised plates (known as coccoliths) contain a chemical imprint of the cell, as well as the environment and leave a different and highly variable chemical signal in the sediments preserving them.

Scientists have long been able to measure these chemical signals, and limited evidence about how much environmental and physiological conditions impacted the chemical imprint of these ancient plankton, has previously made it very difficult for them to interpret the data. But the new findings, recently published in the journal Nature Communications, could change that.

Recreating the prehistoric environment in laboratory conditions, the team grew different species of this algae, each, with varying carbon levels. The mathematical model of the results they created allowed the team to show which factors controlled the chemical signals in the coccoliths.

By observing that the algae’s chemical signature was species-specific, and dependent on the rate at which they grow and calcify, they have found direct links between the chemistry of the coccoliths, the physiology of plankton, and their external environments. Studying these differences in more detail could shed light on prehistoric ocean conditions and support scientists to gain further understanding of how sensitive the Earth is - or was, to carbon dioxide and how much, and drastically the climate changed during this time.

Their findings not only support understanding of individual algal physiology and evolution, but also the geological fossil patterns and chemistry that have been observed throughout history.

Describing the study’s unique value, lead author and Professor of Biogeochemistry at Oxford, Rosalind Rickaby said: “Our model allows scientists to understand algal signals of the past, like never before. It unlocks the potential of fossilised coccolithophores to become a routine tool, used in studying ancient algal physiology, and also ultimately as a recorder of past CO2 levels. With further research it has the potential to tell us the atmospheric carbon dioxide levels of the Earth during the dinosaur era. Obviously, we know that during that time there was no ice on the planet, today we have ice at both poles and knowing the amount of carbon dioxide needed to transform the planet in this way, will help us to understand why.”

PML Marine Ecosystem Modeller and co-author on the study Jorn Bruggeman commented: "Our model provides unprecedented insight how marine calcifying algae use inorganic carbon, including CO2. This makes it possible to interpret the chemical composition of fossilized algae in terms of their physiology and environment at the time. It also sheds light on the physiological trade-offs experienced by the algae when acquiring carbon, which can helps us understand and model the role of these key species in the marine ecosystem in present and future."

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