This action funds an NSF Postdoctoral Research Fellowship in Biology for FY 2025. The fellowship supports research and training of the fellow that will contribute to biology in innovative ways. Nitrogen is an important nutrient for photosynthetic plants and algae, but there is often not enough of it in natural and agricultural settings. Only a few species of bacteria can take abundant nitrogen from the air and turn it into a form that can be used by biological life. This process is called nitrogen fixation. Because of this, most plants and algae get fixed nitrogen from their surroundings, while some form partnerships with nitrogen-fixing bacteria. This research will study a rare case in which a nitrogen-fixing cyanobacterium has become a permanent part of an alga. It now exists as a special compartment called a “diazoplast.” Having a nitrogen-fixing compartment inside an alga creates unique challenges. The process of nitrogen fixation uses a lot of energy and cannot happen when oxygen is present. Because of this, it must take place in a low-oxygen space. This research will help us understand the mechanisms that are necessary for the evolution of a nitrogen-fixing compartment inside a photosynthetic cell. The diazoplast relies on metabolites from the host to fuel nitrogen fixation. The mechanisms responsible for metabolite import into the diazoplast and protection of nitrogenase, the enzyme responsible for nitrogen fixation, from O2 remain unknown. The fellow will identify