How did photosynthesis change the chemistry of the early oceans and atmosphere?

The evolution of photosynthesis by early organisms, such as cyanobacteria, fundamentally transformed Earth’s chemistry by shifting the atmosphere from a reducing state to an oxidizing one.12

Impact on the Oceans

In the early oceans, photosynthetic organisms produced oxygen as a metabolic by-product. Because this oxygen was harmful to primitive cells, it was rapidly consumed by reacting with dissolved ferrous iron released from volcanic activity. This process precipitated iron out of the water, leading to the formation of massive banded-iron formations on the ocean floor. This "oxygen sink" lasted for millions of years until the majority of dissolved iron was depleted, eventually allowing oxygen to accumulate in seawater and outgas into the atmosphere.134

Impact on the Atmosphere

As photosynthesis became the dominant source of oxygen, atmospheric chemistry changed significantly:
* Oxygen Rise: Oxygen levels increased from near zero to eventually reach modern values, enabling the development of complex aerobic organisms.12
* Carbon Dioxide Decline: Photosynthetic organisms consumed carbon dioxide, reducing its concentration in the atmosphere. This decrease weakened the greenhouse effect, which helped regulate Earth's temperature as the Sun's energy output increased over time.25