Question

With the origin of aerobic phototrophs the atmosphere of the earth became

• A. Oxidizing
• B. Reducing
• C. Devoid of nitrogen
• D. Devoid of carbon dioxide

Answer 1

The correct option is A Oxidizing

Oxygen has not always been as abundant as it is today. Most scientists believe that for half of Earth’s 4.6-billion-year history, the atmosphere contained almost no oxygen. Cyanobacteria or blue-green algae became the first microbes to produce oxygen by photosynthesis, perhaps as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago. Cyanobacteria are microbes that live primarily in seawater and were the first organisms on Earth to perform oxygenic photosynthesis. They produced organic carbon, the building blocks of life’s molecules, and release oxygen gas (O${}_2$). The decomposition of cyanobacteria removes oxygen from seawater, and in turn, from the atmosphere hence releasing oxygen.

So the correct option is 'oxidizing'.