Correct Answer - C
He split light into its spectral components (VIB-GYOR) by using a prism and then illuminated a green alga (Cladophora) placed in the suspension of aerobic bacteria. The bacteria were used to detect the sites of `O_(2)`, evolution.
Engelmann observed that bacteria accumulated mainly in the region of blue and red light of split spectrum. A first action spectrum of PHS was thus described.
.Algae often float on surface of water during day but sink down during night due to evolution and trapping of oxygen bubbles during the day in their PHS.
