Which organism produced the first significant oxygen in Earth's atmosphere?

• A. Cyanobacteria
• B. Algae
• C. Protozoa
• D. Fungi

Answer: (A)

Oxygen first entered Earth's atmosphere through oxygenic photosynthesis carried out by cyanobacteria, ancient photosynthetic bacteria that used water as an electron donor and released O2 as a byproduct. Their activity in the oceans slowly built up oxygen over billions of years, eventually driving the Great Oxygenation Event around 2.4 billion years ago and transforming the atmosphere to include abundant free oxygen. Algae later contributed further oxygen as eukaryotic photosynthesizers, but they did not initiate the early rise in atmospheric oxygen. Protozoa are mostly consumers rather than producers of oxygen, and fungi are not photosynthesizers at all. So cyanobacteria are the source of the first significant oxygen in Earth's atmosphere.