Chlorophyll b affects the structure of photosystem I in a type of cyanobacteria. By introducing a specific enzyme into the bacteria, researchers were able to produce chlorophyll b and see how it influenced the photosystem. They found that as the amount of chlorophyll b increased, certain components of the photosystem changed. Specifically, the presence of chlorophyll b led to a decrease in red chlorophyll and a decrease in the formation of photosystem I trimers. This suggests that chlorophyll b plays a role in shaping the structure of photosystem I in this type of cyanobacteria.