Articles | Volume 14, issue 6
Biogeosciences, 14, 1493–1509, 2017
Biogeosciences, 14, 1493–1509, 2017

Research article 24 Mar 2017

Research article | 24 Mar 2017

Physiology regulates the relationship between coccosphere geometry and growth phase in coccolithophores

Rosie M. Sheward1,2, Alex J. Poulton3, Samantha J. Gibbs1, Chris J. Daniels3, and Paul R. Bown4 Rosie M. Sheward et al.
  • 1Ocean and Earth Science, University of Southampton, National Oceanography Centre, Southampton, SO14 3ZH, UK
  • 2Institute of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany
  • 3Ocean Biogeochemistry and Ecosystems, National Oceanography Centre, Southampton, SO14 3ZH, UK
  • 4Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK

Abstract. Coccolithophores are an abundant phytoplankton group that exhibit remarkable diversity in their biology, ecology and calcitic exoskeletons (coccospheres). Their extensive fossil record is a testament to their important biogeochemical role and is a valuable archive of biotic responses to environmental change stretching back over 200 million years. However, to realise the full potential of this archive for (palaeo-)biology and biogeochemistry requires an understanding of the physiological processes that underpin coccosphere architecture. Using culturing experiments on four modern coccolithophore species (Calcidiscus leptoporus, Calcidiscus quadriperforatus, Helicosphaera carteri and Coccolithus braarudii) from three long-lived families, we investigate how coccosphere architecture responds to shifts from exponential (rapid cell division) to stationary (slowed cell division) growth phases as cell physiology reacts to nutrient depletion. These experiments reveal statistical differences in coccosphere size and the number of coccoliths per cell between these two growth phases, specifically that cells in exponential-phase growth are typically smaller with fewer coccoliths, whereas cells experiencing growth-limiting nutrient depletion have larger coccosphere sizes and greater numbers of coccoliths per cell. Although the exact numbers are species-specific, these growth-phase shifts in coccosphere geometry demonstrate that the core physiological responses of cells to nutrient depletion result in increased coccosphere sizes and coccoliths per cell across four different coccolithophore families (Calcidiscaceae, Coccolithaceae, Isochrysidaceae and Helicosphaeraceae), a representative diversity of this phytoplankton group. Building on this, the direct comparison of coccosphere geometries in modern and fossil coccolithophores enables a proxy for growth phase to be developed that can be used to investigate growth responses to environmental change throughout their long evolutionary history. Our data also show that changes in growth rate and coccoliths per cell associated with growth-phase shifts can substantially alter cellular calcite production. Coccosphere geometry is therefore a valuable tool for accessing growth information in the fossil record, providing unprecedented insights into the response of species to environmental change and the potential biogeochemical consequences.

Short summary
Our culture experiments on modern Coccolithophores find that physiology regulates shifts in the geometry of their carbonate shells (coccospheres) between growth phases. This provides a tool to access growth information in modern and past populations. Directly comparing modern species with fossil coccospheres derives a new proxy for investigating the physiology that underpins phytoplankton responses to environmental change through geological time.
Final-revised paper