Is Carbon Limitation a Driver of Phytoplankton Ecology and Evolution?

Our understanding of how the oceans and marine life respond to large changes in global climate, such as increased atmospheric carbon dioxide (CO2) concentrations, largely comes from the study of deep ocean sediments. Over time, these sediments build up layer upon layer and preserve the fossil remains of the tiny single celled marine organisms - or plankton - that once lived near the surface of the ocean. By drilling into these sediments, and recovering cores of sedimentary material and the fossils they contain, we can reconstruct a history of changing ocean conditions and life over millions of years.

The proposed project will study one of these plankton groups - a group of algae called coccolithophores - that produce and surround their cells in plate-like scales made out of the mineral calcite (calcium carbonate). Coccolithophores can also photosynthesise, using dissolved carbon from sea water together with light energy to make the complex carbon molecules they require for energy and cell growth. Like all photosynthetic organisms - including plants and freshwater algae - the coccolithophores need carbon as a 'resource' which they use during photosynthesis. They also need forms of carbon to make their calcium carbonate scales. From laboratory work, it is expected that increasing the amount of dissolved carbon in sea water - for example by increasing atmospheric CO2 concentrations - reduces the limitations on how coccolithophore cells allocate carbon between photosynthesis and the production of calcium carbonate scales, allowing cells to grow larger and produce larger scales. Changes in cell size and in the size of these scales, although minute, little more than a hundredth of a millimetre in size, when multiplied by the many billions of cells living in the surface ocean can have a significant impact on how food, energy and carbon are cycled through the oceans.

In the modern oceans, it's still early to see the effects of changing carbon concentrations on this group of phytoplankton. However, data from ocean sediments preserves a record of the coccolithophores stretching back millions of years, to a time when atmospheric CO2 concentrations were at levels similar to those predicted in the coming century. This project - by measuring the chemistry of coccoliths produced by small and large cells through time - seeks to provide the most detailed record available to date of the degree to which carbon availability limited the growth and cell size of coccolithophore algae. This project seeks to understand if the availability of carbon is a major control on the ecology and evolution of the marine phytoplankton, and so make better precautionary predictions of the likely changes to marine phytoplankton ecosystems in the coming century.