T.W. Engelmann (1843–1909) performed an experiment where he split light into its spectral components using a prism and illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria (like Azotobacter). He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. This accumulation occurred because these are the regions of maximum oxygen evolution, corresponding to the absorption spectra of chlorophyll a and b .