Microzooplankton play pivotal roles in marine ecosystems. They act as the main primary consumers of phytoplankton in most planktonic foodwebs, from ultra-oligotrophic areas to upwelling regions, where they outcompete copepods and other larger zooplankton. They are also important contributors to the diet of mesozooplankton (e.g., in oligotrophic ecosystems, the microzooplankton-associated carbon supply for copepods surpasses that of phytoplankton), and play key roles as nutrient recyclers and CO2 producers. These roles are not yet fully understood and seldom properly parameterized in ecological and predictive models. Therefore, in this session we welcome studies that contemplate the significance of microzooplankton functional diversity and varying impacts in plankton communities and ecosystems, both as grazers and as prey of larger organisms, as well as any new insights into their individual behaviour, physiology (including mixotrophy), and temporal and spatial variability in diverse ecosystems.