Algae produced oxygen for over a billion years before an oxygenated atmosphere appeared. What took so long?
This article is a re-write of a previous piece, “An oxygen tug-of-war between biology and geology.” The re-write responds to reader comments concerning good content but a stiff style. Hopefully, this article provides more pep.
Buried in the geological past is an enticing mystery about oxygen. The story starts about a billion years after the earth formed, and it ends a billion years later near the end of the Archean Eon. The protagonist in this story is the lowly, single-celled, blue-green algae (cyanobacteria). These humble little organisms started producing free oxygen about 3.5 billion years ago, but a permanently oxygenated atmosphere didn’t appear until over a billion years later. Where did all the oxygen go?
Perhaps a little background information is in order. Before blue-green algae appeared in earth’s oceans, chemosynthetic bacteria ruled the biosphere. The oceans and atmosphere had no free oxygen, and these clever, tiny lifeforms developed a knack for munching on inorganic molecules to get the energy needed for cellular metabolism. They feasted on a smorgasbord of ammonia, molecular hydrogen, sulfur, hydrogen sulfide, ferrous iron, and other delicious molecules. However, all good things must come to an end. When blue-green algae appeared on the scene, these chemosynthetic ecosystems drifted into oblivion in the world’s first mass extinction event.
The cyanobacteria (blue-green algae) developed a nifty trick, turning sunlight into food through photosynthesis. Then, they pooped out unneeded molecules like O2 (free oxygen). Unfortunately, free oxygen was toxic to the chemosynthetic bacteria and probably led to their demise. You might rightfully ask, “How do we know when these oxygen pooping algae arrived on the evolutionary stage?” Geologists come into play at this point.
Old rumpled rocks
Geologists have a wide variety of interests in earth processes, but they are particularly fond of wandering around the wilderness, looking at rocks poking out of the ground. During such wilderness adventures in Australia and Greenland, separate teams found rocks with stromatolites embedded in the stone fabric. Stromatolites get geologists very excited because they are the trace fossil remains from colonies of blue-green algae. These remains appear in the rocks as thin rumpled layers, or sometimes wiggly, rounded features that look like a sliced open cabbage head. But the exciting part was the age of the rocks. They were very old and provided evidence of algal colonies 3.4 to 3.7 billion years ago.
These geological discoveries pin down the date of first oxygen production to the early Archean Eon, about a billion years after the earth formed from cosmic debris. So, in the early Archean, cyanobacteria were hard at work, basking in the sun, photosynthesizing, reproducing, and excreting free oxygen. But nothing was happening. Ten million years passed. Then another hundred million years slipped by, and finally, after a billion years, the Great Oxygenation Event occurred. This event saturated the earth’s oceans and atmosphere with oxygen. This new, oxygenated world opened the gateway for endless cycles of evolution and extinction, eventually leading to a species intelligent enough to produce the website where this story resides.
The Great Oxygenation Event
Science is not always big and bold. Sometimes discovery lies in the small and unseen. When volcanic gases react with the atmosphere, the reaction produces certain sulfur isotopes, which eventually become incorporated in rocks. Atmospheric composition is important in determining the final form of these sulfur isotopes, and the presence of oxygen causes a variation in the isotope’s final chemical structure. So, in the geological record, an atmosphere with oxygen is distinguishable from an anoxic (oxygen-free) atmosphere by analyzing sulfur isotopes.
Wait a minute! If it only took ten million years for the transition to occur, what happened to the oxygen produced over the previous billion years? Concentrations of chemicals in the atmosphere, including oxygen, represent a balance between sources and sinks. An example of sources and sinks occurs when fossil fuels burn and become a source of carbon dioxide (CO2). However, trees use CO2 to grow through photosynthesis; thus, trees are a sink. The oxygen source (blue-green algae) was active in the early Archean, excreting lots of oxygen. If this oxygen wasn’t collecting in the atmosphere, then there must have been a chemical sink removing it.
Deep beneath the surface
A 2020 paper published in Nature (Shintaro Kadoya, et al.) proposes a solution to the missing oxygen mystery. The work by Kadoya indicates volcanic gases formed the sink responsible for the missing oxygen. But that theory is just the tip of an iceberg whose roots extend deep into the mantle of the earth. The mantle lies far below the earth’s crust and is the source of magma for volcanic hotspots like Iceland and Hawaii. Mantle plumes transport magma to the earth’s surface where lava flows from volcanos, and a broad range of noxious gases spew into the atmosphere.
Kadoya’s work picks up on research suggesting the Archean mantle’s composition was different from today, and the ancient earth’s mantle contained less oxygen. A less-oxidized mantle produces higher volumes of gases prone to react with oxygen. Gases, like hydrogen, naturally combine with free oxygen, thus removing it from the atmosphere. He suggests that oxygenating the mantle took almost a quarter of the earth’s history — a time during which a fierce tug-of-war between biology and geology took place. For a billion years, gases from active volcanoes gobbled up all the oxygen the biosphere could produce. Only after the mantle was fully oxygenated did the oxygen-reactive gases stop flooding the atmosphere. Then the evolutionary pathway opened for aerobic, oxygen breathing species, including Homo sapiens.
Science advances through hard work and much debate. Hypotheses require testing, and certainty arises from exploring all alternatives. Calling the mystery of the missing oxygen “solved,” is premature, but Kadoya’s solution is a satisfying potential answer.
The Icelandic Plume (Source: ArcheanWeb) – https://archeanweb.com/2020/03/25/the-icelandic-plume/ Also:
Yellowstone quakes and shakes, but will it blow? (Source: ArcheanWeb) – https://archeanweb.com/2020/06/12/yellowstone-quakes-and-shakes-but-will-it-blow/ Also:
Study pinpoints timing of oxygen’s first appearance in Earth’s atmosphere (By Jennifer Chu; MIT News Office) – http://news.mit.edu/2016/oxygen-first-appearance-earth-atmosphere-0513 Also:
Mantle data imply a decline of oxidizable volcanic gases could have triggered the Great Oxidation (By Shintaro Kadoya, David C. Catling, Robert W. Nicklas, Igor S. Puchtel & Ariel D. Anbar; Nature Communications) – https://www.nature.com/articles/s41467-020-16493-1 Also: