Oxygen is the third most common element in the universe, following hydrogen and helium. It is also the most abundant chemical element by mass in the biosphere of the Earth. The crust of the planet is actually made up of 49.2 percent of oxygen while the atmosphere has 20.8 percent.
The most abundant forms or allotropes of oxygen on Earth are trioxygen or ozone, which has its highest concentration in the stratosphere, and dioxygen or diatomic oxygen, which is essential in biological reactions, particularly in cellular aerobic respiration.
Nevertheless, the abundance of this element on the planet poses some fundamental questions: Where does the oxygen on Earth come from? How do living organisms and human activities play a role in the consumption and production of oxygen?
Understanding the Sources of Oxygen on Earth
The First Accumulation of Oxygen: Hypothesized Causes of the Great Oxidation Event
Most scientists believe that free oxygen gas was nearly nonexistent on Earth before the emergence and evolution of photosynthetic archaea and bacteria. Researchers Sean A. Crowe et al. noted that there is a wide assumption that atmospheric oxygen concentrations remained persistently low or 5.14 thousand times less than the present level for about the first 2 billion years since the formation of the Earth.
The study of Crowe et al. suggested that the free oxygen first appeared in significant quantities during the Paleoproterozoic Era that transpired between 3.0 and 2.3 billion years ago. They explained further that an occurrence called the Great Oxidation Event transpired during the Paleoproterozoic Era that changed the atmosphere of the Earth from a weakly reducing atmosphere to an oxidizing atmosphere. The exact causes of this event remain up for debate.
Several hypotheses have been presented to explain the possible causes of the Great Oxidation Event. Take note of the following:
1. Increasing Flux: One of the major hypotheses centers on the notion that the event resulted from an increase of the source of oxygen. More specific hypotheses argue that the Great Oxidation Event was either the immediate consequence of photosynthesis or that it immediately transpired upon the emergence of oxygenic photosynthetic organisms. However, based on the timeline, there is a significant delay amounting to hundreds of millions of years between the first appearance of cyanobacteria and the onset of the Great Oxidation Event.
2. Decreasing Sink: Another group of hypotheses collectively suggest that oxygen accumulation was a consequence of decreasing sinks. To be specific, with the emergence of oxygenic photosynthetic organisms and the eventual explosion of cyanobacteria population, the released oxygen eventually depleted reducing elements and compounds such as ferrous compounds and methane. Oxygen production from photosynthesis decreased oxygen sinks and surpassed the availability of reducing materials in the environment.
3. Tectonic Trigger: Changes in Earth driven by tectonic events could also be a contributor to oxygen accumulation. Evidence suggests that landmass collisions can break down water molecules into hydrogen and oxygen. This process is called the stoichiometric decomposition of water. Other tectonic events such as volcanic eruptions and underwater volcanic activities released nutrients to the ocean. These nutrients are essential for supporting and promoting further the population growth of oxygen-producing photosynthetic organisms called cyanobacteria.
4. Nickel Famine: Early chemosynthetic organisms called methanogens produced methane. They also require nickel as an enzyme cofactor. Methane oxidizes carbon dioxide and water in the presence of ultraviolet radiation. This traps molecular oxygen. The cooling of the crust resulted in a decrease in the supply of volcanic nickel. This could be a factor that allowed cyanobacteria to overtake methanogens. K. O. Konhauser et al. found out that the rate of nickel deposition declined steadily from 2.2 to 2.4 billion years ago.
Continued Accumulation and the Cycle of Oxygen: Major Sources of Oxygen On Earth
A number of people would agree that trees and the entire vegetation are one of the primary sources of oxygen on Earth. Plant life essentially produces oxygen as a byproduct of photosynthesis and respiration. One of the most cited examples is the contribution of the Amazon Rainforest. Estimates suggests that it provides around 20 percent of oxygen on the planet. This figure can be misleading.
The study of Yadvinder Malhi published in the Journal of Ecology showed that most forests consume almost all of the free or atmospheric oxygen they produce. The living species in a particular forest ecosystem use up most of it. The collective species release only about 1 percent to 6 percent of oxygen in reserve into the atmosphere.
Nearly all of the breathable oxygen specifically came from the oceans. Scott Denning, professor of atmospheric science, explained that the current supply of oxygen came from millions of years of build-up left by a tiny imbalance between growth and decomposition of ocean plant matter. He noted further that the build-up has hovered around 21 percent of the volume of the atmosphere, and there is enough oxygen to last for millions of years.
It is important to highlight the role of plant-like organisms called phytoplankton as a major contributor to the modern oxygen reserve. These organisms that reside in oceans also undergo photosynthesis and thus, release oxygen as a waste product. Scientists estimate that at least 50 percent of atmospheric oxygen has been produced by these organisms collectively. These experts, of course, remain unsure about the exact figure because of limitations in estimations.
Modern Consumption and Depletion of Oxygen
Remember that nearly all of the oxygen produced on land through photosynthesis is consumed by aerobic organisms and in some cases, localized wildfires. In other words, the net production of this gas by land vegetation is close to zero because land animals and microbes consume it. Denning noted that less than one percent of oxygen would be consumed if all organic matter on Earth would burn at once.
He also added that oxygen accumulation in the atmosphere is only possible if the organic matter produced by plants through photosynthesis is rendered unavailable for consumption, particularly if they are buried in places devoid of oxygen, such as deep-sea muds.
Anthropogenic fossil fuel combustion is the largest contributor to atmospheric oxygen consumption. Researchers J. Huang et al. noted that this activity consumed 2.0 Gt/a in 1900 and has increased to 38.2 Gt/a by 2015.
Estimates calculated using a Representative Concentration Pathways scenario revealed that approximately 100Gt of this gas would be removed from the atmosphere per year until 2100. The oxygen concentration would decrease from its current level of 20.946 percent to 20.825 percent.
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: 10.1126/science.1061976
- Crowe, S. A., Døssing, L. N., Beukes, N. J., Bau, M., Kruger, S. J., Frei, R., and Canfield, D. E. 2013. “Atmospheric Oxygenation Three Billion Years Ago.” Nature. 501(7468): 535-538. DOI: 10.1038/nature12426
- Denning, S. “Amazon Fires are Destructive, But They Aren’t Depleting Earth’s Oxygen Supply.” The Conversation. Available online
- Huang, J., Huang, J., Liu, X., Li, C., Ding, L., and Yu, H. 2018. “The Global Oxygen Budget and Its Future Projection. Science Bulletin. 63(18): 1180-1186. DOI: 10.1016/j.scib.2018.07.023
- Konhauser, K. O., Pecoits, E., Lalonde, S. V., Papineau, D., Nisbet, E. G., Barley, M. E., … Kamber, B. S. 2009. “Oceanic Nickel Depletion and a Methanogen Famine Before the Great Oxidation Event.” Nature. 458(7239): 750-753. DOI: 10.1038/nature07858
- Malhi, Y. 2012. “The Productivity, Metabolism, and Carbon Cycle of Tropical Rainforest Vegetation.” Journal of Ecology. 100(1): 65-75. DOI: 10.1111/j.1365-2745.2011.01916.x