The first organisms that oxygenated the atmosphere were
a. cyanobacteria
b. phototrophic organisms
c. anaerobic organisms
d. all of the above
The correct answer and explanation is:
The correct answer is a. cyanobacteria.
Cyanobacteria, also known as blue-green algae, were the first organisms to oxygenate Earth’s atmosphere. They are capable of photosynthesis, a process that converts light energy into chemical energy, using carbon dioxide and water to produce glucose and oxygen. This ability to perform oxygenic photosynthesis was a key factor in the rise of oxygen in Earth’s atmosphere.
Before the emergence of cyanobacteria, the Earth’s atmosphere was anoxic, meaning it lacked free oxygen. The early Earth’s environment was dominated by anaerobic organisms, which did not require oxygen for survival and could only perform processes like fermentation or anaerobic respiration. Cyanobacteria, however, introduced a significant change by using sunlight to split water molecules (H2O) during photosynthesis, releasing oxygen as a byproduct.
The oxygen produced by cyanobacteria over millions of years led to the gradual buildup of oxygen in the atmosphere, a phenomenon known as the Great Oxygenation Event (also called the Oxygen Catastrophe), which occurred around 2.4 billion years ago. This event had profound effects on the Earth’s atmosphere and life. As oxygen levels rose, it caused a mass extinction of many anaerobic organisms that could not survive in the newly oxygenated environment. At the same time, it paved the way for the evolution of aerobic (oxygen-dependent) organisms, which use oxygen in cellular respiration to produce energy more efficiently than anaerobic organisms.
While phototrophic organisms and anaerobic organisms were part of Earth’s early life forms, cyanobacteria were specifically responsible for oxygenating the atmosphere through their photosynthetic activity. Thus, cyanobacteria are considered the primary contributors to Earth’s current oxygen-rich atmosphere.