Drivers of extinction in the marine phytoplankton

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Marine phytoplankton form the basis of pelagic food webs and ecosystems. With modern anthropogenic environmental impacts - including eutrophication, climate change and ocean acidification - there is evidence that this group is already experiencing biogeographic range shifts and changes to community structure. The longer-term risk of species extinction within this group is, however, more poorly understood. Here, we seek to investigate the dynamics, causes and community restructuring during a major extinction of a closely-related group oceanic surface-dwelling alage, approximately 2 million years ago. The alage concerned produce calcium carbonate (calcite) plate-like scales, which are produced by and surround the cell during life, but on death accumulate in great numbers in deep ocean sediments. These fossil scales, called coccoliths, can be used to reconstruct the abundance and diversity of species that were living in the surface ocean across millions of years. The chemistry of coccoliths can also be used as an indicator of cell processes and the conditions in the surface ocean environment at the time the coccoliths were formed. Togther, we propose to use the variations in abundances and diversity of species, as well as chemical indicators of cell growth rates and environment, to determine the main drivers of species extinction in this case. In particular, was this mainly driven by biotic competition with other algal species and groups, or was it due to changing surface ocean environments, which drove the loss of the particular niche occupied by the extinct species. Understanding this process, will help constrain the dynamics of extinction in the plankton, and the mechanisms by which environmental stress is transferred into extinction risk.