Using the microorganism Volvox we recently observed quite interesting behaviors: at an individual level Volvox exhibits directional phototaxis towards the light source, where the phototaxis mechanism can be explained by the orchestral coordination of the many cilia on its surface. At the collective level where the density of Volvox in the fluid is high, we observed that a population of Volvox exhibits distinct collective dynamics compared to individual behavior in response to light stimuli, where hundreds of Volvox cells form a stable, rotating two-dimensional layer of active crystal (aggregate) suspended in a three- dimensional fluid chamber. In this project, we aim to establish a novel mechanistic framework that integrates experiments, theoretical analysis, and simulations to capture quantitatively the hydrodynamic interactions governing both individual and collective behaviors of microorganisms.