Cyanobacteria performed a chemical revolution. Before them, Earth's atmosphere was dominated by CO₂ — a greenhouse gas. Through photosynthesis, they used light energy to split water molecules and combine CO₂ with hydrogen to make glucose:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
The oxygen released was initially toxic to most life forms that had evolved in an anaerobic environment — this period is called the "Great Oxidation Event" (~2.4 billion years ago). Over millions of years, oxygen accumulated in the atmosphere. Two key things happened:
- CO₂ levels fell dramatically as it was consumed by photosynthesis and dissolved in oceans
- O₂ levels rose from near-zero to ~21% as photosynthesis outpaced consumption by chemical reactions and early aerobic organisms
Today, photosynthesis and respiration are broadly in balance — every molecule of O₂ consumed by respiration is replaced by photosynthesis. Human burning of fossil fuels is adding CO₂ that was locked underground for millions of years, disrupting this balance.
This how it works covers How Photosynthesis Changed the Atmosphere within Composition of Atmosphere for GCSE Chemistry. Revise Composition of Atmosphere in Atmosphere for GCSE Chemistry with 20 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 5 of 13 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Composition of Atmosphere
What is the approximate percentage of nitrogen in the current atmosphere?
Describe how Earth's early atmosphere was formed.