What did Engelmann observe when he split light into its spectral components and then illuminated Cladophora, placed in a suspension of aerobic bacteria?

T.W Engelmann split light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of O₂ evolution. He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. A first action spectrum of
photosynthesis was thus described. It resembles roughly the absorption spectra of chlorophyll a and b.