What factors control coccolithophore growth rates?

Coccolithophores are a unique group of microscopic marine algae that produce small scales of CaCO3 (coccoliths), which form an outer shell (coccosphere) around the cell. Coccolithophores have important roles in the marine carbon cycle as they convert CO2 into both organic matter (via photosynthesis) and CaCO3 (via calcification), and coccoliths are effective agents at transferring CO2 as CaCO3 (calcite) from the surface to deep ocean. Coccoliths are also heavier than organic matter and add weight to sinking material so that it also reaches the deep sea. Although coccolithophores have a rich fossil record they face a bleak and uncertain future - increased atmospheric CO2 will cause ocean acidification and a chemical environment unfavourable to calcifying organisms, while global warming will change how the oceans are mixed and the availability of energy (sunlight) and nutrients (nitrogen, phosphorus) needed for growth. However, it is not fully understood how ocean chemistry or the availability of light and nutrients effects coccolithophore growth in the present-day ocean, and until this gap is addressed it is difficult to fully appreciate or predict how coccolithophores may react to climate change. Due to the inclusion of both calcite and organic matter within coccolithophore cells, both need to be considered when examining coccolithophore growth: understanding how coccolithophores balance cellular levels and rates of production of these two materials is key to understanding their growth and role in the marine carbon cycle. The aim of this project is to address these deficiencies by studying both natural coccolithophore communities in the ocean and individual species grown in the laboratory. In order to examine cellular levels of calcite and organic matter, photosynthesis and calcification, and growth rates it will be necessary to look closely at the structure of the coccosphere, coccolith and inner organic cell. The combination of knowledge from oceanic coccolithophore communities and those grown under more controlled conditions in the laboratory will create an overview of the principals governing coccolithophore growth. The end goal of this project will be to use this overview to generate a mathematical model that describes coccolithophore growth in relation to the coccosphere/cell structure and the availability of light and nutrients. Such a model can then be used to address global questions about coccolithophore ecology, their role in the marine carbon cycle and their future in a changing climate.