Earth experienced a rise of oxygen levels in its shallow oceans and in the atmosphere during the Paleoproterozoic Era or about 2.5 billion years ago. The long-spanning phenomenon changed the chemical composition of the atmosphere and also altered the makeup of the persisting biosphere. It lasted between the Siderian Period and the Rhyacian Period of the Paleoproterozoic. This event is now called the Great Oxidation Event and it has been considered as one of the most important turning points in the history of the Earth.
A Discussion of the Importance of the Great Oxidation Event: How It Triggered Mass Extinction and Paved the Way for the Emergence of Complex and Diverse Life Forms
There was no abundant amount of free oxygen on Earth during its first three to four billion years. The biosphere was also dominated by anaerobic organisms. These included ancient prokaryotes which were grouped into bacteria and archaea. Several studies have also suggested that photosynthesis first transpired around 3.5 billion years ago with the emergence of anoxygenic photosynthetic bacteria. These organisms used photosynthesis to produce organic molecules using light energy but do not produce oxygen as a waste product.
Nevertheless, from these anoxygenic photosynthetic bacteria, other studies have also suggested that oxygenic photosynthetic bacteria later emerged. These are organisms that release oxygen as a waste product during photosynthesis. Several hypotheses have attempted to explain the emergence of oxygenic photosynthesis. These include gene transfer from early pre-oxygenic photosynthetic bacteria, duplication and modification of genes for anoxygenic photosynthesis, or environmental pressure due to increasing levels of carbon dioxide, decreasing levels of hydrogen sulfide, or increasing solar radiation.
It is important to note that cyanobacteria or blue-green algae eventually emerged from these oxygenic photosynthetic bacteria. Their appearance around 2.7 billion years ago did not immediately cause a critical rise in free oxygen levels. It took several hundreds of millions of years for it to accumulate in shallow oceans at levels that would be released into the atmosphere. Nonetheless, about 2.5 billion years ago, the overall buildup of oxygen in ancient oceans and atmosphere led to the Great Oxidation Event.
The propagation of the population of cyanobacteria over a period of hundreds of millions of years is one of the most accepted causes of the Great Oxidation Event. Remember that the earliest appearance of this specific bacteria did not immediately result in critical oxygen accumulation. This is due to the fact that the element is reactive to other reducing elements and compounds in the environment. For example, due to its reactivity with iron, it turned ancient oceans red and formed banded iron formations on ocean floors.
Nevertheless, over time, the explosion of the cyanobacteria population produced enough levels of oxygen that depleted reducing elements and compounds such as iron or ferrous compounds, hydrogen sulfide, and atmospheric methane. The absence of reducing compounds allowed oxygen to accumulate in ancient oceans until its release and accumulation in the atmosphere. Take note that this entire process took hundreds of millions of years. The bistability hypothesis explains how oxygen from cyanobacteria accumulated over time.
The bistability hypothesis specifically suggests that the accumulation of oxygen was a result of raising its levels beyond a moderate threshold that led to the formation of the ozone layer. The absence of this layer depleted lower levels of oxygen because of its reaction with methane. However, with the formation of the ozone layer, methane oxidation decreased because the layer shielded the Earth from some ultraviolet radiation. This allowed the further accumulation of free oxygen and increased its level further to a stable state of 21 percent or more.
Factoring in the population explosion of cyanobacteria is also essential in understanding the causes of the Great Oxidation Event. Take note that these organisms have a rapid multiplication rate under favorable conditions. Researchers have identified several possibilities that supported their population growth. These include volcanic and other geological activities that released more nutrients in the oceans and the depletion of anaerobic-specific nutrients like nickel that starved anaerobic organisms and allowed aerobic organisms to prosper.
Another proposition called the increasing photoperiod hypothesis that links oxygen to the transition of the rotational period of Earth from about six hours 4.5 billion years ago to about 21 hours by 2.4 billion years ago. Experiments showed that cyanobacteria tend to consume nearly as much oxygen at night as they produce during the day. Other experiments showed that mats of cyanobacteria produce a greater excess of oxygen with longer photoperiods. The longer rotational period of Earth became conducive for oxygen production.
The exact cause of the Great Oxidation Event remains up for debate. What is clear is that this occurrence has been considered one of the most critical events in the history of the Earth. Some researchers call it the “Oxygen Catastrophe” or the “Oxygen Crisis” because of how it changed the ancient hydrosphere, atmosphere, and biosphere. The abundance of free oxygen reacted to different elements and compounds. It specifically oxidized atmospheric methane to carbon dioxide and water. This caused planetary cooling.
Furthermore, because oxygen is toxic to anaerobic organisms, it could have possibly resulted in the mass extinction that wiped out different species and populations across the anaerobic biosphere of the Earth. The same toxicity of oxygen also drove the evolutionary transformation of an archaeal lineage into the first eukaryotes due to selective pressure. Hence, apart from its possible contribution to the extinction of ancient anaerobic organisms, a positive effect of the Great Oxidation Event was the emergence of diverse multicellular life.
It is important to underscore the fact that oxygen helped earlier multicellular organisms to grow larger and more complex than single-celled organisms. The element supports several cellular metabolic processes that help in specializing cells for different tasks. Aerobic respiration or cellular respiration in the presence of oxygen is also more efficient than anaerobic respiration when it comes to energy production. It supports differentiated cellular functions and has allowed multicellular organisms to evolve and grow.
Some studies have also explained that the accumulation of oxygen helped create a stable environment that was conducive for multicellular life to thrive. The atmosphere of the Earth before the Great Oxidation Event was unstable and prone to changes in temperature and acidity. The rise of oxygen levels resulted in different oxidation processes that created an atmosphere that was more hospitable for multicellular organisms. Oxygen also provided some protection from UV radiation via the ozone layer and cellular oxygen molecules.
The accumulation of oxygen also triggered an explosive growth in mineral diversity. Remember that free oxygen is reactive. Hence, when it became abundant, it enabled various elements to occur in one or more oxidized forms in minerals in the near-surface environment. The presence of oxygen specifically increased the number of possible oxidation states for many elements and the minerals that could be formed. Estimates suggest that its abundance has been responsible for more than 2500 of around 4500 known minerals found on Earth today.
FURTHER READINGS AND REFERENCES
- Catling, D. C., Zahnle, K. J., and McKay, C. P. 2001. “Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth.” Science. 292(5531): 839-843. DOI: 1126/science.1061976
- Claire, M. W., Catling, D. C., and Zahnle, K. J. 2008. “Biochemical Modeling of the Rise in Atmospheric Oxygen.” Geobiology. 4(1): 239-269. DOI: 1111/j.1472-4669.2006.00084.x
- Goldblatt, C., Lenton, T., and Watson, A. 2006. “Bistability of Atmospheric Oxygen and the Great Oxidation.” Nature. 443(7112): 683-686. DOI: 1038/nature05169
- Holland, H. D. 2006. “The Oxygenation of the Atmosphere and Oceans. Philosophical Transactions of the Royal Society B: Biological Sciences. 361(1470): 903-915. DOI: 1098/rstb.2006.1838
- Klatt, J. M., Chennu, A., Arbic, B. K., Biddanda, B. A., and Dick, G. J. 2021. “Possible Link Between Earth’s Rotation Rate and Oxygenation.” Nature Geoscience. 14(8): 564-570. DOI: 1038/s41561-021-00784-3
- Tanai, C. 2008. “Early Archean Origin of Heterodimeric Photosystem I.” Heliyon., 4(3): e00548 DOI: 1016/j.heliyon.2018.e00548