Cyanobacteria changed the atmosphere on early Earth because they led to increased a. reduced, soluble forms of iron. b. nitrogen levels. c. water levels in the oceans. d. atmospheric O2 levels.

The correct answer is d.  atmospheric O2 levels.

Explanation:

Cyanobacteria were considered to be the first aerobic organism on earth. Before the appearance of cyanobacteria, the earth atmosphere was reducing. Cyanobacteria were able to use light energy and perform photosynthesis.

As oxygen was the byproduct of photosynthesis it is released in the atmosphere and that increased the level of oxygen in world. Due to this oxygenic environment is created on the earth. Chloroplast in plants which helps in photosynthesis shows similarities with cyanobacteria.

So cyanobacteria changed the atmosphere on earth because they increased the atmospheric O2 level.

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