Answer:
action spectrum of photosynthesis
Explanation:
The action spectrum of photosynthesis represents the regions of white light that are utilized during the process of photosynthesis by green plants.
According to studies, the action spectrum of photosynthesis corresponds to the blue and red regions of white light.
Hence, the two distinct regions of bacterial growth along the length of the filamentous algae observed by Engelmann represent the blue and red region of the white light.
As photosynthesis was taking place across the two regions, oxygen was being produced. Oxygen is necessary for the growth of aerobic bacteria and as such, they were able to grow across the regions where oxygen was being produced.