The Oxygen Disaster

My previous post was in praise of carbon, that versatile element—wonderful, flexible carbon, which came from an exploding star. Ninety-four elements came from that star, and among them was oxygen.
     Oxygen is highly reactive—it quickly combines with other elements to form compounds called oxides, such as carbon dioxide (CO2). You can see this reaction taking place anytime you have a fire.
     During the first couple of billion years of the Earth’s existence, oxygen was pretty much tied up in oxides all the time. Any free oxygen, or O2, would quickly combine with other elements. Earth’s atmosphere is thought to have been mostly carbon dioxide, methane, and water.
     ​Around 3 billion years ago, a variety of life forms began to appear in the shallow oceans that covered Earth. We know very little about them, because they barely even made fossils. We know only what scientists can infer from microfossils, from presumed modern descendants, and from the mineral composition of deposits laid down way back then, when they can be found.

​   These earliest life forms were single-celled or existed as colonies of cells. They were anaerobic, meaning that they did not need oxygen for their life processes. (Of course not! There was no free oxygen to be had at that time.) Not only did these cells not require oxygen, but they also could not tolerate it.
​​     And then, around 2.4 billion years ago, one of the cell types developed a new pathway. These ancestors of modern cyanobacteria could use the energy of sunlight to combine carbon dioxide with water to make sugar and release free oxygen. 
​     Photosynthesis had begun.

Modern cyanobacteria in colonies. Taken from a mat of growth in Baja California. NASA, public domain.

2.1 billion year old rock containing black-banded ironstone. The bands are thought to have been created by increased oxygen that led to "rust." Photo by Andre Karwath, (aka) Aka. CC BY-SA 2.5.

   At first, free oxygen was no problem because plenty of elements and compounds were available for oxygen to grab. As time went on—maybe a hundred million years or so—all the mineral “sinks” for oxygen filled up.
​  Gradually, the concentration of oxygen in the seas and atmosphere began to rise. This was a disaster for anaerobic organisms—even for some of the cyanobacteria, who were essentially producing their own poison.​

     It’s thought that free oxygen wiped out a vast number of life forms globally. This was a slow-motion extinction compared to the asteroid impact that killed the dinosaurs, but it was still disastrous for life at that time. Because there is so little direct evidence, scientists don’t technically consider it a “mass extinction.” 
     The life forms that survived were able to detoxify oxygen, and even used it to power biochemical processes. As time went on, some of these cells engulfed the early cyanobacteria. Instead of being broken down for energy, some cyanobacteria continued to live inside an engulfing cell—a process termed endosymbiosis that eventually led to the rise of true plants. Chloroplasts are descendants of those ancient cyanobacteria. 
     Plants not only produce oxygen; they also consume carbon dioxide. On land, large forests are particularly good at this. Reforestation and preservation of existing forests could help offset carbon dioxide produced from burning fossil fuels. ​​

     In the oceans, oceanic plankton and other plants capture enormous amounts of carbon. Marine diatoms, microscopic plants living at the ocean’s surface, absorb an amount of carbon equal to what is captured yearly by all of the world’s rainforests. Seagrasses, growing on only 0.2% of seabeds, account for 10% of the ocean’s capacity for storing carbon.

Diatoms through the microscope. NOAA, public domain.

     With proper strategies, the rise in atmospheric CO​2 and the resulting greenhouse effect and climate change could be slowed or stopped. If we do nothing, who knows? Depending on how severe climate change gets, recovery could take a few hundred million years, more or less—but the Earth has plenty of time. 

For Further Exploration

[The articles listed below are somewhat technical. Wikipedia actually contains some fairly solid, basic information on the Great Oxidation Event.]

Robert E. Blankenship, ​“Early Evolution of Photosynthesis.” Plant Physiology, 2010 Oct; 154(2): 434–438. Click here.

Roger Buick, “When did oxygenic photosynthesis evolve?” Philosophical Transactions of the Royal Society B: Biological Sciences, 2008 Aug 27; 363(1504): 2731–2743. Click here.

Michael W. Gray and Keith G. Kozminski, “Lynn Margulis and the endosymbiont hypothesis: 50 years later.” Mol Biol Cell. 2017 May 15; 28(10): 1285–1287. [This gives a summary of endosymbiosis as described by Lynn Margulis.] Click here.

Katharine Rooney, “These tiny plants and giant animals are helping to store vast amounts of CO2 in our oceans.” World Economic Forum, May 19, 2021. [This is a mostly non-technical article that talks about diatoms, but also about whales.] Click here.