The Evolution of Modern Eukaryotic Phytoplankton

The community structure and ecological function of contemporary marine ecosys-tems are critically dependent on eukaryotic phytoplankton. Although numerically inferior to cyanobacteria, these organisms are responsible for the majority of the flux of organic matter to higher trophic levels and the ocean interior. Photosynthetic eukaryotes evolved more than 1.5 billion years ago in the Proterozoic oceans. However, it was not until the Mesozoic Era (251 to 65 million years ago) that the three principal phytoplankton clades that would come to dominate the modern seas rose to ecological prominence. In contrast to their pioneering predecessors, the dinoflagellates, coccolithophores, and diatoms all contain plastids derived from an ancestral red alga by secondary symbiosis. Here we examine the geological, geo-chemical, and biological processes that contributed to the rise of these three, distantly related, phytoplankton groups. Coined in 1897, the term “phytoplank-ton ” describes a diverse, polyphylet-ic group of mostly single-celled photosynthetic organisms that drift with the currents in marine and fresh waters (1