Answer:
They perform photosynthesis, releasing oxygen into the atmosphere as a by-product.
Step-by-step explanation:
Around the time that cyanobacteria appear around 3.2 billion years ago, the oxygen levels rise sharply.
Cyanobacteria are photosynthetic organisms. They possess the photosynthetic pigment chlorophyll, meaning they can trap light energy and use it to power chemical reactions to synthesise glucose. They convert carbon dioxide and water into sugar, releasing oxygen into the atmosphere as a byproduct. This causes oxygen levels to rise.