Blue green algae (BGA) blooms are becoming alarmingly frequent and conspicuous across the world, posing significant economic and health risks. The ability to fix atmospheric nitrogen (N) is a key trait conferring a competitive advantage to BGAs. N fixation is orchestrated by the enzyme nitrogenase, which is 59% iron (Fe) by mass. However little is known about the role of Fe in BGA growth. We will explore the role of Fe in determining BGA growth using experimental manipulations of Fe supply.