Dying is something phytoplankton do a great deal of. Phytoplankton -- bacteria and eukaryotes that photosynthesize, such as coccolithophores and diatoms -- fix as much carbon every year as all the plants on all the continents. Yet at any one time they account for just 1% of Earth's biomass. This means their rate of turnover is huge; on average, the world's phytoplankton population is replaced once a week. 
Image: Trichodesmium (sea 'sawdust'), a subtropical bloom-forming cyanobacteria on which many phytoplankton feed. It makes several key enzymes that are also found in the cell-death cascade of animals. And it is one of the most important nitrogen fixing organisms in the oceans. 1. From The Origins of Death - Apoptosis in cyanobacteria by Nick Lane, Nature, 28 May 2008
2. Spatial coupling of nitrogen inputs and losses in the ocean. Deutsch, C. et al. Nature, 11 January 2007. A bigger nitrogen fix. Gruber, N. (2005). Nature, 11 August 2005.