The Great Oxidation Event: How Earth’s Atmosphere Became Oxygen-Rich
By Helen Wong 王思齊
The history of life on Earth saw many pivotal moments, but perhaps none, other than the origin of life itself, is more significant than the Great Oxidation Event (GOE). Marking the period when the early Earth’s atmosphere started to fill with free oxygen, the GOE set the foundation for the rise of aerobic life and ultimately, present-day humans [1, 2].
Imagine traveling back to 4.5 billion years ago, when Earth had just formed. The atmosphere was vastly different from what we have today – it consisted of water vapor, carbon dioxide, and methane, but not oxygen. Consequently, the earliest life forms that emerged approximately 3.8 billion years ago were anaerobic.
But the entire game changed when a group of bacteria diverged from their anaerobic ancestors around 3.4 billion years ago [3, 4]. These unique microbes developed one of the most crucial innovations in the history of life on Earth – oxygenic photosynthesis – and evolved into what we now know as cyanobacteria (commonly called blue-green algae, although they are not technically algae) (Figure 1).
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Figure 1 A stromatolite fossil of cyanobacteria. The layered structure was formed from mats of cyanobacteria.
Photo credit: James St. John [5]
Through oxygenic photosynthesis, oxygen was generated as a by-product of water splitting. Initially, the oxygen levels in the atmosphere remained low, as the first oxygen released into seawater by cyanobacteria was quickly sequestered by chemical reactions with other elements, such as iron [2] (Figure 2). Over a period of 200–300 million years [1], seawater oxygen levels gradually increased, possibly due to a rapid expansion of cyanobacterial populations [3, 4], until the accumulated oxygen began to escape into the atmosphere. The escaped oxygen displaced the abundant methane, kicking off the GOE that took place between 2.4 and 2.1 billion years ago [1].
Figure 2 Banded iron formation as evidence of the GOE. Iron (II) ions in the ocean are thought to be oxidized and precipitated as red iron (III) oxides in the GOE [6].
Photo credit: Graeme Churchard [7]
The implications of an oxygenated atmosphere were immense for both Earth's climate and its inhabitants. Methane, a greenhouse gas, traps heat from sunlight and keeps the Earth warm enough for organisms to survive. Therefore, when methane was displaced by oxygen, global temperatures dropped, causing Earth to enter a series of ice ages known as the Huronian glaciation [8]. Meanwhile, ultraviolet radiation (UV) from the Sun split oxygen molecules (O2) into individual atoms, which then reacted with other oxygen molecules to create ozone (O3), forming the ozone layer that now protects life on Earth from harmful UV radiation.
The omnipresence of oxygen on Earth also fundamentally changed the planet’s biological landscape. To the anaerobic bacteria and archaea of the time, oxygen was toxic. This led to a mass extinction in which most anaerobes were wiped out.
However, some survivors found ways to adapt and even thrive in the newly oxygen-rich environment. They developed ingenious solutions in terms of oxygen binding, aerobic respiration, and oxygen detoxification. To protect themselves from oxygen, these anaerobic organisms made use of certain proteins to bind oxygen and incorporate it into other molecules they need such as melanin [9]. Scientists believe that some of these ancient proteins eventually evolved into oxygen-transporting respiratory pigments found in animal blood today [9, 10]. For example, hemocyanin was likely derived from the oxygen-binding protein tyrosinase. These organisms also harnessed the power of oxygen as the terminal electron acceptor in respiration, which releases much more energy than anaerobic respiration. On the other hand, they evolved more effective versions of detoxifying enzymes, including superoxide dismutase and catalase (footnote 1), to deal with the harmful reactive oxygen species resulting from aerobic respiration [1].
For those unable to adapt, alternative strategies were employed. Some chose to remain in anaerobic environments, while others “acquired” the ability to perform aerobic respiration by engulfing smaller aerobically respiring cells, as suggested by the famous endosymbiotic theory [11, 12]. The latter gave rise to the ancestors of eukaryotic cells, with the engulfed aerobically respiring cells eventually becoming today's mitochondria.
And the story of cyanobacteria did not end with the GOE – the endosymbiotic theory also suggests that they were engulfed by early non-photosynthetic eukaryotes [11] and became chloroplasts in modern plants and algae.
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Related Article Check out the following articles to learn more about mitochondria! Mitochondria: So Much More Than the Powerhouses of the Cell (Issue 020) |
1 Editor’s note: Superoxide dismutase converts harmful superoxide radicals (O2−•) to molecular oxygen (O2) and hydrogen peroxide (H2O2). Catalase further converts H2O2 to O2 and water.
References
[1] Aiyer, K. (2022, February 18). The Great Oxidation Event: How Cyanobacteria Changed Life. American Society for Microbiology. https://asm.org/Articles/2022/February/The-Great-Oxidation-Event-How-Cyanobacteria-Change
[2] Blaustein, R. (2016). The Great Oxidation Event: Evolving understandings of how oxygenic life on Earth began. BioScience, 66(3), 189–195. https://doi.org/10.1093/biosci/biv193
[3] Chu, J. (2021, September 28). Zeroing in on the origins of Earth’s “Single most important evolutionary innovation”. MIT News. https://news.mit.edu/2021/photosynthesis-evolution-origins-0928
[4] Fournier, G. P., Moore, K. R., Rangel, L. T., Payette, J. G., Momper, L., & Bosak, T. (2021). The Archean origin of oxygenic photosynthesis and extant cyanobacterial lineages. Proceedings of the Royal Society B: Biological Sciences, 288(1959). https://doi.org/10.1098/rspb.2021.0675
[5] St. John, J. (n.d.). STROMATOLITE [Photograph]. http://www.jsjgeology.net/Stromatolite.htm
[6] The Stephen Hui Geological Museum. (n.d.). O2 - Free Atmosphere - Banded Iron formation. https://www.earthsciences.hku.hk/shmuseum/earth_evo_03_arc04_3.php
[7] Churchard, G. (2014, January 24). Dales Gorge [Photograph]. Flickr. https://www.flickr.com/photos/graeme/12116315164/
[8] Bekker, A. (2015). Huronian Glaciation. In Gargaud, M., et al. (Eds.), Encyclopedia of Astrobiology (2nd ed.). Springer Berlin. https://doi.org/10.1007/978-3-662-44185-5_742
[9] Lutz, D. (2010, February). The Many Colors of Blood. ChemMatters. https://teachchemistry.org/chemmatters/february-2010/the-many-colors-of-blood
[10] van Holde, K. E., Miller, K. I., & Decker, H. (2001). Hemocyanins and Invertebrate Evolution. Journal of Biological Chemistry, 276(19), 15563–15566. https://doi.org/10.1074/jbc.r100010200
[11] Archibald, J. M. (2015). Endosymbiosis and Eukaryotic Cell Evolution. Current Biology, 25(19), R911-R921. https://doi.org/10.1016/j.cub.2015.07.055
[12] Sessions, A. L., Doughty, D. M., Welander, P. V., Summons, R. E., & Newman, D. K. (2009). The Continuing Puzzle of the Great Oxidation Event. Current Biology, 19(14). https://doi.org/10.1016/j.cub.2009.05.054