Question

On the earth surface, reducing atmosphere changed into an oxidizing atmosphere with the release of oxygen took place at this moment due to the

• A. Evolution of oxygenic photoautotrophs
• B. Evolution of anoxygenic photoautotrophs
• C. Evolution of chemo photoautotrophs
• D. Evolution of anaerobic heterotrophs

Answer 1

Answer: (A)

Evolution of oxygenic photoautotrophs

The conditions on the primitive earth were high reducing due to the absence of oxygen. Some 2.7 billion years ago, oxygenic photoautotrophs evolved. They were oxygenic because they released oxygen as the by-product and photoautotroph because they used sunlight as the source of energy. Due to the release of oxygen, the reducing atmosphere changes into the oxidizing atmosphere.

A. Evolution of oxygenic photoautotrophs leads to the shift from reducing atmosphere to the oxidizing atmosphere because they used water as the electron donor in the photosynthesis and released oxygen as the by-product.

B. Anoxygenic photoautotrophs used sunlight as an energy source but did not release the oxygen as the by-product because they did not use water as an electron donor. Sulfur was used for this purpose.

C. Chemophotoautotrophs also did not use the water as an electron donor so they did not release the oxygen. They used hydrogen gas or hydrogen sulfide.

D. Anaerobic heterotrophs did not release oxygen. They used the already available organic carbon for the energy requirements.

Hence, the correct answer is 'Evolution of oxygenic photoautotrophs'