The ability of photosynthetic organisms to harness solar energy and transform carbon dioxide into fuel, biomass, commodity chemicals, and/or produce hydrogen makes them attractive for integration into industrial processes. However, several obstacles remain in using photosynthetic organisms for these applications, including sensitivity of enzymes that produce hydrogen to molecular oxygen and an uneven distribution of light in bioreactors. Towards optimizing the use of photosynthetic organisms in these endeavors, previous studies have focused on understanding the photosynthetic and hydrogen-producing machinery of photosynthetic organisms to build artificial systems, engineering photosynthetic microorganisms to have improved metabolic yields, or redesigning bioreactors to improve light harvesting. Our laboratory instead, as outlined in this proposal, focuses on understanding the already existing mechanisms that photosynthetic organisms use to live in nearly every habitat on Earth. By studying the existing ways that organisms adapt to the light and oxygen available in different environments, we want to uncover ways to overcome inefficiencies in employing photosynthetic organisms for various applications.