From Engelmann's experiment
Engelmann used this device to illuminate a strand of Cladophora (not Spirogyra) with light from the visible spectrum, exposing different sections to different wavelengths (or colors of light). He added the oxygen seeking bacteria B. termo to this setup and noted where they accumulated (Note: Four years later, Hauser concluded that B. termo had been mislabeled and was not one, but three species of bacteria of the genus Proteus ). Their clumping allowed him to see which regions had the highest concentration of oxygen. He concluded that the most photosynthetically active regions will have the highest concentrations of bacteria. The bacteria accumulated in the regions of red and violet light, showing that these wavelengths of light generated the most photosynthetic activity.
It can be concluded that the photosynthetically active regions of light are present at the blue-violet region and the red region
What I am unable to understand is , if photosystems I and II are activated at light wavelengths of 680nm and 700nm (primarily at the red region of the spectrum) then what type of photosynthesis is happening at the blue region due to which there is oxygen evolution?
The antenna molecules (light harvesting complex) which absorb majorly at this wavelength are transferring energy to the photocentre due to which there is excitation of the photocentre and thus OEC gets activated.
Even if this were true, then should'nt blue light be as effective as red light in helping in generating the quantum yield?