Photosynthetic bacteria, particularly cyanobacteria, played a crucial role in changing Earth's atmosphere through the process of photosynthesis. During photosynthesis, these bacteria convert carbon dioxide and water into glucose and oxygen, using sunlight as energy.
As a result of this process, the concentration of oxygen in the atmosphere increased significantly, especially during the Great Oxygenation Event, which occurred around 2.4 billion years ago. This event marked a substantial increase in atmospheric oxygen levels, fundamentally altering the environment and paving the way for the evolution of aerobic (oxygen-requiring) organisms.
In summary, the activity of photosynthetic bacteria increased oxygen concentration in Earth's atmosphere. There was no significant increase in hydrogen concentration, nor did these processes lead to a decrease in oxygen or hydrogen concentration.