Processes that either promote growth or cause mortality drive the abundance of all organisms. For microbes that have a lifespan measured in hours to days, small changes in growth and mortality rates can have significant impacts on their population dynamics. While factors that influence phytoplankton growth are often examined, it has been difficult to parameterize phytoplankton population dynamics due to incomplete estimates of phytoplankton mortality. Through my research, I aim to close this gap, by revealing a fundamental mechanistic understanding of the processes that mediate phytoplankton mortality in the ocean. This talk will give an update on Harvey Lab research on the different mechanisms of mortality in phytoplankton, how mortality may be partitioned between important death processes, and how these different modes of mortality influence biogeochemical fluxes.