December 10, 2024

Tracy Huynh 

11th Grade

Fountain Valley High School

If you are reading this article, then your body is most likely facilitating cellular respiration right now. Cellular respiration is the process of taking in oxygen, glucose, and other molecules in the body and turning these resources into valuable energy in the form of ATP, also known as adenosine triphosphate, as well as water and carbon dioxide. 

Now, you probably aren't reading this if you are a cyanobacteria, mainly because these organisms are “aquatic and photosynthetic” bacteria meaning they live in the water and create their own energy through the process of photosynthesis. However, the cyanobacteria we are talking about photosynthesized differently than the modern plants we see around us today. The cyanobacteria from over 2 billion years ago facilitated photosynthesis by absorbing the sun’s infrared light, dissolved sulfides, and hydrogen in order to create their usable energy. As a result of this reaction, the cyanobacteria produced sulfuric acid and solid yellow sulfur as byproducts. During this crude era of photosynthesis, the surrounding environment was toxic, cruel, and definitely not the best-smelling. 

In school, we learn that photosynthesis, similar to cellular respiration, is the process taking in water, carbon dioxide, and energy (typically from the sun) in order to produce glucose, a usable form of energy and oxygen as a byproduct. These two processes are connected to each other due to the fact that each cellular process produces the other process’ required reactants. Without any organisms photosynthesizing, there would not be enough oxygen for us to breathe in. In a scenario where one cellular process exists while the other does not, the whole world would be thrown out of balance.

Unfortunately, this scenario has occurred in the distant past, approximately 2.4 billion years ago and is now known as the “Great Oxygenation Event” or “Great Oxidation Event”. This event gets its name from oxygen, a product of photosynthesis which may seem harmless to us now but sent the Earth into its first mass extinction.

As cyanobacteria evolved, so did their method of photosynthesis. The toxic, sulfur producing reaction transitioned into a safer, simpler reaction. This new reaction is one which involves the absorption of red and blue light, water, and carbon dioxide. This method of photosynthesis is the same one used today by green plants and algae, the same one that produced the villain of our story: oxygen. 

As photosynthesized oxygen increases in concentration, it chemically reacts with iron in the water, producing iron oxide as a product. This product is known as rust and with its accumulation over time, changed the color of the ocean from its normal blue, to a deadly red. Back then, over two billion years ago, the organisms living on Earth were unaccustomed to oxygen as it had been a scarce resource at the time. This meant that nearly all living beings on Earth were not adept at processing oxygen or protecting themselves from it. During this time period, there were no organisms who facilitated cellular respiration either. This meant that there was no way for the oxygen levels to be controlled as they kept rising. As a result, the cyanobacteria were suffocated by the very oxygen they produced. 

Despite the harshness of their conditions, there were a few bacteria who avoided the oxygen suffocation. Those who survived would then repopulate the ocean and continue to photosynthesize. Once their population grew too large and oxygen concentrations rose too high, their populations would once again dwindle down and then the survivors would repopulate. As this oscillating trend continued as time went on, the surviving bacteria developed resistance to oxygen and soon had no limit to how much they could photosynthesize. As a result, oxygen concentrations skyrocketed as the ocean continued to house the element. While oxygen spread throughout the ocean, it reached other bacterial colonies which had not developed a resistance to oxygen due to not needing the ability in the past. These colonies would soon die out as more would have the same fate. 

After wiping out the earth of virtually all of its other bacteria colonies, the cyanobacteria continued to photosynthesize at the peak of its population. At this point in time, oxygen had spread throughout the ocean and couldn’t be contained in its waters anymore. Because of this, the photosynthesized oxygen was then released into the earth’s atmosphere where it reacted with the methane filled atmosphere. At the time, methane was the Earth’s primary insulator which kept the Earth warm enough to live in. However, oxygen reacting with methane produced carbon dioxide which became the Earth’s new insulator. But, carbon dioxide proved to be not as capable as an insulator as temperatures dropped significantly, sending the planet into one of its most significant ice ages. This ice age was predicted to have wiped out 99% of living life on Earth, including the cyanobacteria as the planet turned into a giant snowball, ice covering every surface it could access.

The first ever mass extinction of our planet was one with a hopeful beginning filled with discovery as bacteria first began photosynthesizing and ended tragically as cyanobacteria greedily photosynthesized, consuming the vital resources of the Earth. The remaining 1% of survivors would then go on to repopulate our planet and evolve into animals and plants, forming ecosystems built to withstand the once toxic oxygen.