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This lesson focuses on how oxygen levels in the atmosphere increased from virtually zero to the current level of approximately 21%, as required by AQA GCSE Chemistry specification (5.9.1). The appearance and increase of oxygen is one of the most important events in the history of life on Earth, and it was driven primarily by photosynthesis. You need to understand the organisms involved, the chemical reaction of photosynthesis, and the consequences of rising oxygen levels.
For the first 1–2 billion years of Earth's history, there was essentially no free oxygen in the atmosphere. The early atmosphere was dominated by carbon dioxide and water vapour, with no organisms capable of producing oxygen. Any small amounts of oxygen produced by chemical reactions (such as the breakdown of water vapour by ultraviolet radiation — a process called photolysis) were immediately consumed by reactions with reactive metals and other elements on the surface.
Exam Tip: Free oxygen means oxygen gas (O₂) that is not combined with other elements. The early atmosphere contained oxygen atoms bonded in molecules like CO₂ and H₂O, but there was no free O₂ gas.
The increase in atmospheric oxygen was driven almost entirely by photosynthesis — the process by which organisms use light energy to convert carbon dioxide and water into glucose and oxygen.
The first organisms to carry out photosynthesis were cyanobacteria (sometimes called blue-green algae, although they are actually bacteria, not algae). Cyanobacteria appeared approximately 2.7–3.4 billion years ago. They were the first organisms to use water as a source of hydrogen atoms in photosynthesis, releasing oxygen as a waste product.
Evidence for early cyanobacteria comes from fossilised structures called stromatolites — layered dome-shaped structures formed by mats of cyanobacteria in shallow water. Living stromatolites can still be found today in places such as Shark Bay, Western Australia.
The word equation for photosynthesis is:
carbon dioxide + water → glucose + oxygen
The balanced symbol equation is:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This reaction requires light energy, which is absorbed by the green pigment chlorophyll in chloroplasts.
Photosynthesis had two simultaneous effects on the atmosphere:
Over billions of years, the cumulative effect of photosynthesis by cyanobacteria, algae, and eventually land plants transformed the atmosphere from one with no free oxygen to one containing approximately 21% oxygen.
Exam Tip: Always state both effects of photosynthesis on the atmosphere: it decreased CO₂ AND increased O₂. Stating only one effect will cost you marks in a question about atmospheric change.
The period when oxygen first began to accumulate significantly in the atmosphere is known as the Great Oxidation Event (GOE), which occurred approximately 2.4–2.0 billion years ago. Before this, the oxygen produced by cyanobacteria reacted with dissolved iron in the oceans and with minerals on land, and did not accumulate in the atmosphere.
flowchart TD
A["Cyanobacteria begin photosynthesis<br/>~3.4 billion years ago"] --> B["Oxygen reacts with dissolved<br/>iron in oceans"]
B --> C["Banded iron formations<br/>deposited on ocean floor"]
C --> D["Dissolved iron used up<br/>~2.4 billion years ago"]
D --> E["Free oxygen begins to<br/>accumulate in atmosphere"]
E --> F["Great Oxidation Event<br/>~2.4–2.0 billion years ago"]
F --> G["Oxygen levels gradually rise<br/>to modern levels (~21%)"]
style A fill:#27ae60,color:#fff
style D fill:#e67e22,color:#fff
style F fill:#c0392b,color:#fff
style G fill:#2980b9,color:#fff
The main evidence comes from banded iron formations (BIFs) — layers of iron-rich rock found in ancient sediments dating from about 3.5 to 1.8 billion years ago. These formed when oxygen produced by cyanobacteria reacted with dissolved iron (Fe²⁺) in the oceans:
4Fe²⁺ + O₂ + 8OH⁻ → 2Fe₂O₃ + 4H₂O
The iron oxide (Fe₂O₃) precipitated out of the water and settled on the ocean floor. Once all the dissolved iron had been oxidised, free oxygen could begin to accumulate in the atmosphere and dissolved in the ocean water.
After cyanobacteria established oxygen production, more complex photosynthetic organisms evolved:
| Organism | Approximate Appearance | Contribution to Oxygen |
|---|---|---|
| Cyanobacteria | 3.4 billion years ago | First oxygen producers; began the oxygenation of the atmosphere |
| Eukaryotic algae | 1.5 billion years ago | More efficient photosynthesis; diversified in the oceans |
| Land plants | 470 million years ago | Colonised the land; greatly increased photosynthesis rate |
| Forests | 360 million years ago | Vast forests during the Carboniferous period produced enormous amounts of oxygen and locked carbon into coal deposits |
The Carboniferous period (approximately 360–300 million years ago) was particularly important. Vast swamp forests covered much of the land, and the rate of photosynthesis was extremely high. Dead plant material accumulated faster than it could decompose (partly because organisms that could break down lignin had not yet evolved), and this material was buried and eventually formed coal deposits. This process both increased atmospheric oxygen and removed carbon dioxide.
During this period, oxygen levels may have reached 30–35% — higher than today — allowing giant insects such as dragonflies with 70 cm wingspans to thrive.
Exam Tip: The Carboniferous period is a useful example to mention in extended-answer questions. It demonstrates how biological activity (photosynthesis and burial of organic matter) can dramatically affect atmospheric composition.
The increase in atmospheric oxygen had profound consequences:
| Consequence | Explanation |
|---|---|
| Aerobic respiration became possible | Organisms could use oxygen to release energy from glucose more efficiently than anaerobic respiration. This allowed the evolution of larger, more complex organisms. |
| Ozone layer formed | Oxygen (O₂) in the upper atmosphere was converted to ozone (O₃) by ultraviolet radiation. The ozone layer blocks harmful UV radiation, making it possible for life to colonise the land. |
| Anaerobic organisms declined | Many organisms that thrived in the oxygen-free early atmosphere were poisoned by oxygen. They were forced into oxygen-free environments (such as deep ocean sediments, swamps, and the guts of animals). |
| Oxidation of surface minerals | Iron and other reactive metals on the surface were oxidised, forming rust-coloured rocks. |
The ozone layer was a crucial consequence of rising oxygen levels:
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