Marine phytoplankton are tiny microorganisms that photosynthesize, producing half of the oxygen we breathe and serve as the base of the ocean food web. Different types of phytoplankton live across diverse regions throughout the upper sunlit portion of the ocean. We use molecular tools in this study to identify the many kinds of phytoplankton from seawater samples collected across the Pacific, Atlantic, and Indian Oceans. We then grouped them by their ecosystem roles, such as how big they are or what nutrients they use. In parallel, we compared these real-world patterns with computer models that try to predict where the different phytoplankton should live. We found that the model does a good job matching observations for some major groups, but it also misses others that are crucial for the ocean and climate. Our findings help improve these global ecosystem models, which are often used to understand ocean dynamics and predict future changes. This research helps bring us a step closer to understanding how life in the ocean is organized and how it might change in the future.