Phytoplankton are tiny organisms that form the base of food webs in lakes, rivers, and oceans, and sometimes cause harmful algal blooms. Understanding how phytoplankton respond to changing temperatures is crucial, but we currently lack the knowledge to predict their future state. Our project investigates a type of phytoplankton called cyanobacteria that thrive in hot springs. We will analyze their genetic adaptations and responses to temperature changes, both over short timescales and across long-term laboratory evolution. This will help us uncover how they survive extreme heat. We will then test how well our findings apply to cyanobacteria in freshwater sources across the U.S. This research will help predict which phytoplankton are most vulnerable to warming and explore ways to engineer heat-resilient cyanobacteria that produce supplements, biofuels, and other valuable products. We will also create educational programs to train future scientists in cutting-edge biological data analysis and engage the public in how microbiology can inform our understanding of life on earth. Phytoplankton responses to warming are mechanistically poorly understood, limiting our ability to predict their future fitness, forecast harmful algal blooms, or cultivate them effectively for bioproducts. This project aims to elucidate thermal adaptation mechanisms in cyanobacteria by integrating heat stress responses with eco-evolutionary processes. We will leverage thermophilic cyanobacteria that evo