Question

What did Engelmann observe from his prism experiments?
A. Bacteria could not detect the sites of O$_2$ evolution.
B. Bacteria releases excess carbon dioxide in red and blue light
C. Bacteria accumulated due to increase in temperature caused by increase in O$_2$ concentration
D. Bacteria are accumulated towards red and blue light.

Answer 1

Hint: Engelmann assumed that the bacteria was moving in response to oxygen generated through photosynthesis. He performed 3 important experiments which involves the process which is known as photosynthesis.

Complete answer:
Engelmann in 1881, observed the movement of bacteria towards the chloroplast in a strand of algae which is known as “Spirogyra”'. This is considered to be the first observation that is documented and called positive Aerotaxis in bacteria.
Further in the year 1882 he performed a spectrum experiment with the help of the device which was designed by Carl Zeiss. The device was a modifying microscope which consisted of a prism which produced a small or minute spectrum on the microscopic site. This device can measure different wavelengths of light which makes it a micro spectroscope.
Engelmann with the help of this device used a strand of Cladophora with the light from visible spectrum which is exposing to different wavelengths of light than he added oxygen seeking bacteria and he noted that they got accumulated. Now their accumulation helped him to see in which region there was a higher concentration of oxygen than he concluded that photosynthetically active regions will have the highest concentration of bacteria and most of the bacteria accumulated towards the red and blue light.

Thus, the correct option D i.e. Bacteria are accumulated towards red and blue light.

Note: Engelmann after one year later observed that the purple bacteria utilize ultraviolet light in the same way. However, his experiment had some drawbacks also because he failed to account for the fact that his son does not emit all the visible wavelengths of light at the same intensity.