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 ofphotosynthesis was thus described. It resembles roughly the absorption spectra of chlorophyll a and b.