Oxygenic photosynthesis is made possible by the accumulation of oxygen in earth's atmosphere and oceans. the primary producers of oxygen from 2.4 billion to 650 million years ago were mostly

Cyanobacteria

Explanation:

Most Scientist believe that for half Earth's 4.6 billion years age there was no oxygen on the planet . The Cyano bacteria  or the blue green algae become the first microorganism to produce oxygen in the planet about 2.4 billion years from now. This was thought to happen somewhere between 2.5 and 2.3 billion years ago.

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