16 points how did cyanobacteria affect earth’s early atmosphere? a. they replaced oxygen with carbon dioxide in the atmosphere. b. they eliminated the harmful ultraviolet rays of the sun. c. they replaced carbon dioxide with oxygen in the atmosphere. d. they absorbed ozone molecules from the atmosphere.

The correct answer is-

C.They replaced carbon dioxide with oxygen in the atmosphere.

Cyanobacteria ( also known as blue green algae) is a large phylum of bacteria that is capable of synthesizing its own food using carbon dioxide and water in the presence of sunlight ( photosynthesis).

They contain a green pigment called chlorophyll that absorbs energy from sunlight and helps in the formation of food ( glucose). They release oxygen as a byproduct in the atmosphere.

As these organisms utilize carbon dioxide ( that is present in the atmosphere) to give oxygen in return, therefore, they affected early atmosphere of Earth by replacing carbon dioxide with oxygen.

Thus, option C) is the right answer.

The correct answer is option C, They replaced carbon dioxide with oxygen in the atmosphere.

Reason -

Cyanobacteria are aquatic organisms that are capable to phosynthesise and produce their own food.  They are unicellular bacteria found , some 3.5 billion years ago. They produced the oxygen during the Archaean and Proterozoic Eras however before that the atmosphere has an unstable chemical composition which do not support life. They also led to the origin of plants which further added to the production of oxygen in the atmosphere.