THE LIVING AND THE DEAD: FROM CELLULAR ARCHITECTURE TO GROWTH RATES, ENVIRONMENTAL CHANGE AND ECOLOGY IN MODERN AND FOSSIL COCCOLITHOPHORES

Coccolithophores are a phytoplankton group that form a composite exoskeleton cell covering of multiple intricate calcite plates (coccoliths). The fossil record of coccoliths is temporally and spatially extensive, but without intact cell coverings (coccospheres) we lose valuable information on the original size of the cells, which is a key functional trait for physiology, growth and ecology. Several geological successions have recently yielded high numbers of intact, exquisitely preserved fossil coccospheres of Paleogene age. These fossil ‘cells’ uniquely record organism size, coccolith length and the number of coccoliths that cover each cell. Using a novel combination of culture experiments, field populations and fossil ‘cells’, we have recently found that the drivers of cell size and number of coccoliths per cell are intimately linked with cellular growth and physiology, which has proved invaluable in developing a growth proxy for fossil coccolithophores.

Herein, we present new data from culture experiments on four important modern coccolithophore species with long evolutionary records – Calcidiscus leptoporus, C. quadriperforatus, Coccolithus braarudii and Helicosphaera carteri. These experiments reveal that, across several taxonomic orders of coccolithophores, relationships between coccosphere size and number of coccoliths per cell are strongly regulated by growth phase, and specifically whether a population is able to maintain exponential growth. Modern populations dividing exponentially have, on average, smaller cells with fewer coccoliths per cell compared to populations no longer dividing exponentially, and these are features we can also identify in the fossil remains of their ancestors. Species exhibiting smaller cells with fewer coccoliths within fossil communities are therefore interpreted as likely to be experiencing intervals of more optimal growth conditions than during periods where within-species coccosphere size and number of coccoliths per cell is increased. Integrating modern experimental results with fossil data therefore enables us to explore the impact of past environmental changes on coccolithophore growth and is a valuable new tool for further investigating the ecological preferences of fossil species through time.