Question

Assertion :Atmosphere III (oxidizing) came into existence about 3.6 billion years ago. Reason: Fossils, stromatolites suggest the dominance of oxygenic cyanobacteria from that age.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
• C. Assertion is correct but Reason is incorrect.
• D. Both Assertion and Reason are incorrect.

Answer 1

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

The early second atmosphere was reducing atmosphere due to absence of molecular oxygen and presence of powerful reducing agents. It was rich in water vapour, carbon monoxide, carbon dioxide, nitrogen and hydrogen. As we know that first oxygenic photoautotrophs (Cyanobacteria) appeared around 3.4 billion years ago that use light as source of energy and carbon dioxide as chief carbon source. These photoautotrophs use water as electron donor during photophosphorylation that results in production of oxygen, hence the name oxygenic bacteria which in turn added oxygen to otherwise primitive reducing atmosphere and made it oxygenic (the atmosphere III). Assertion is correct. Stromatolites are the calcium/silicon layers formed by dead remains of photosynthetic algae. The earliest accepted stromatolites, as found in the oldest rock of Archaean eon, are 3.4 billion years old. Thus, it proves that the first oxygenic phototrophic prokaryotes appeared in archaeozoic era around 3.4 billion years ago. Reason is correct and is the correct explanation of Assertion.