The Great Oxidation (Oxygenation) Event Powerpoint

1. The Great Oxidation (Oxygenation) Event Up until 2.4 billion years ago, there was no oxygen in the air. It took something big to change that – perhaps the biggest evolutionary leap of all

2. Intro • For the first half of our planet's history, there was no oxygen in the atmosphere. This life-giving gas only started to appear about 2.4 billion years ago. • This "Great Oxidation Event" was one of the most important things to ever happen on this planet. Without it, there could never have been any animals that breathe oxygen: no insects, no fish, and certainly no humans. • For decades, scientists have worked to understand how and why the first oxygen was pumped into the air. They have long suspected that life itself was responsible for creating the air that we breathe. • But not just any life. If the latest findings are to be believed, life itself was undergoing a tremendous transformation just before the Great Oxidation Event. This evolutionary leap forward may be the key to understanding what happened.

3. Earth was already 2 billion years old at the time of the Great Oxidation Event, having formed 4.5 billion years ago. It was inhabited, but only by single-celled organisms. • It's not clear exactly when life began, but the oldest known fossils of these microorganisms date back 3.5 billion years, so it must have been before that. That means life had been around for at least a billion years before the Great Oxidation Event.

4. Those simple life-forms are the prime suspects for the Great Oxidation Event. One group in particular stands out: cyanobacteria. Today, these microscopic organisms sometimes form bright blue-green layers on ponds and oceans. • Their ancestors invented a trick that has since spread like wildlife. They evolved a way to take energy from sunlight, and use it to make sugars out of water and carbon dioxide.

5. This is called photosynthesis, and today it's how all green plants get their food. That tree down your street is pretty much using the same chemical process that the first cyanobacteria used billions of years ago. • From the bacteria's point of view, photosynthesis has one irritating downside. It produces oxygen as a waste product. Oxygen is of no use to them, so they release it into the air. • So there's a simple explanation for the Great Oxidation Event. It was the cyanobacteria, pumping out unwanted oxygen, that transformed Earth's atmosphere. • But while this explains how it happened, it doesn't explain why, and it certainly doesn't explain when it happened.

6. The problem is that cyanobacteria seem to have been around long before the Great Oxidation Event. They're probably among the first organisms we have on this planet. • We can be confident that there were cyanobacteria by 2.9 billion years ago, because there is evidence of isolated "oxygen oases" at that time. • They might date as far back as 3.5 billion years, but it's hard to tell because the fossil record is so patchy. Geochemists from the University of Tübingen have discovered layers in South Africa's Pongola Basin which bear witness to oxygen production by bacteria as early as 2.97 billion years ago. That makes the Basin the earliest known home to oxygen-producing organisms -- known as an oxygen oasis.

7. That means the cyanobacteria were busy pumping out oxygen for at least half a billion years before oxygen started appearing in the air. That doesn't make a lot of sense. • One explanation is that there were a lot of chemicals around – perhaps volcanic gases – that reacted with the oxygen, effectively "mopping it up". • But there's another possibility. Maybe the cyanobacteria changed. Some evolutionary innovation in cyanobacteria helped them to become more successful and more important. Some cyanobacteria organise themselves into long strings

8. Some modern cyanobacteria have done something that, by bacterial standards, is remarkable. While the vast majority of bacteria are single cells, they are multicellular. • The individual cyanobacterial cells have joined up into stringy filaments, like the carriages of a train. That in itself is unusual for bacteria, but some have gone further. • Many cyanobacteria are able to produce specialized cells that lose their ability to divide. • This is the first form of specialization we see.It's a simple version of the many specialized cells that animals have, such as muscle, nerve and blood cells. • Multicellularity could have been a game-changer for Earth's early cyanobacteria. It offers several possible advantages.

10. On the early Earth, single-celled organisms often lived together in flat layers of gunk called "mats". Within each mat there would have been many different species of cyanobacteria, and a host of other things to boot. • A multicellular cyanobacterium would have one clear advantage compared to its single-celled rivals. It would find it easier to spread, because its larger surface area would mean it was better at attaching itself to slippery rocks. Such an organism would be less likely to wash away in the current. • Many modern multicellular cyanobacteria can move around within their mats. They're not extremely fast but they can move. That suggests the primordial ones could as well. • Moving could have helped them survive. At the time the Earth was being bombarded with harmful ultraviolet radiation from the Sun, and there was no ozone layer to keep it out.

11. In modern mats, cyanobacteria will turn around and appear vertical instead of horizontal to protect themselves from excess sunlight. You have also movement between layers. It might be these multicellular cyanobacteria had the ability to position themselves optimally within the mat. • It's a neat idea. But for it to be true, cyanobacteria must have evolved multicellularity before the Great Oxidation Event. All life on Earth is one big family, from bacteria to humans

12. Scientists have spent the last few years trying to figure out when cyanobacteria first evolved multicellularity. • The clues lie in their genes. By examining genes that all cyanobacteria share, and identifying tiny differences between them, they could figure out how they are all related – essentially drawing up a family tree of cyanobacteria. • With that tree in place, they could then home in on the multicellular cyanobacteria, and estimate roughly when they first became multicellular. Nostoc, a multicellular cyanobacterium

13. The origin of the air we breathe

14. Cyanobacteria can form green "blooms" on water A cyanobacteria bloom in Guatemala’s Lake Atitlán created a vast dead zone in the otherwise fecund lake, showing that billions of years later these bacteria can still cause grief.

15. Two Unresolved Questions • The first is, did multicellularity really offer them the advantages the scientists thinks it did? We don't know, but we could find out: by testing how modern single-celled and multicellular cyanobacteria cope with different situations. • The second question is harder: why did it take so long for cyanobacteria to become multicellular? If it is so advantageous, why did they not evolve it sooner, and trigger an earlier Great Oxidation Event? • Now scientists need to find out which genes are responsible for multicellularity in cyanobacteria. Then they could say why it took that long and why didn't it evolve earlier. • Whatever caused the Great Oxidation Event, it's clear that it is one of the most important things to ever happen on this planet.

16. Oxygen would have been lethal for many bacteria. It's hard to prove, because from the fossil record we don't have a lot of deposits from that time but we can assume we had a lot of bacteria dying at that point. • Those first multicellular cyanobacteria triggered the evolution of complex life • But in the longer term, it allowed a whole new kind of life to evolve. Oxygen is a reactive gas – that's why it starts fires – so when some organisms figured out how to harness it, they suddenly had access to a major new source of energy. • By breathing oxygen, organisms could become much more active, and much larger. Moving beyond the simple multicellularity developed by cyanobacteria, some organisms became far more intricate. They became plants and animals, from sponges and worms to fish and, ultimately, humans. • To sum it up, those first multicellular cyanobacteria most likely triggered the evolution of complex life, including us, by producing oxygen on a global scale.

17. Poisoned Planet • Most of the bacteria thriving on Earth, especially in oceans were anaerobic, literally thriving and metabolizing their food without oxygen. • Aerobic means 'with air' and refers to the body producing energy with the use of oxygen. • Cyanobacteria are photosynthetic. They convert sunlight into energy and produce oxygen as a waste product. • Back then, the Earth’s atmosphere didn’t have free oxygen in it as it does today. It was locked up in water molecules, or bonded to iron in minerals.

18. The cyanobacteria changed that. But not at first: For a while, as they produced free oxygen as their waste, iron would bond with it and the environment could keep up with the production. • At some point, though, as cyanobacteria flourished, the minerals and other sinks became saturated. They could no longer absorb the oxygen being produced. It built up in the water, in the air. • To the other bacteria living in the ocean—anaerobic bacteria, remember—oxygen was toxic. The cyanobacteria were literally respiring poison. • A die-off began, a mass extinction killing countless species of bacteria. It was the Great Oxidation (Oxygenation) Event. But there was worse to come.

19. Up until this time, the atmosphere was devoid of the reactive molecule. But as oxygen abundances increased, some of it combined with methane to create carbon dioxide. Methane is a far more efficient greenhouse gas than CO2, and this methane was keeping the planet warm. As levels dropped, the Earth cooled. This triggered a massive glaciation event, a global ice age that locked the planet in its grip. • Things got so bad the cyanobacteria themselves were threatened. Their own numbers dropped, along with nearly all other life on Earth. The mass extinction that followed was vast. • But there was an exception: Some organisms could use that oxygen in their own metabolic processes. • Combining oxygen with other molecules can release energy, a lot of it, and that energy is useful. It allowed these microscopic plants to grow faster, breed faster, live faster.

20. The anaerobic species died off, falling to the oxygen-burning plants, which prospered in this new environment. Certainly, anaerobes didn’t vanish from the Earth, but they were vanquished to low-oxygen environments such as the bottom of the ocean. They were no longer the dominant form of life on Earth. • It was perhaps the first of the mass extinctions life would face on our planet, and its impact resonates through the eons (and of course there is quite a lot of detail to this story). To this day, our atmosphere is rich in oxygen, with most multicellular life on Earth descended from the upstart oxygen breathers, and not the anaerobes.

21. Evidence for Rise of Oxygen Evidence from Rock Record of Low O2 until 2.2 Ga Ga = One billion years • Rocks provide evidence of the oxidation state of the atmosphere/ocean • Presence of detrital minerals, uraninite and pyrite • These minerals are insoluble (can’t be dissolved) in absence of oxygen • Uraninite and pyrite disappeared after 2.3 Ga • Banded iron formation • Marine sedimentary rocks consisting of layers of iron-rich minerals and chert • Iron is only soluble in seawater in its reduced form (Fe2+)- indicating low O2 • BIFs become scarce after ~2.2 Ga BIF

22. BANDED IRON FORMATION -- BIFsCOMPOSED OF REDUCED IRON MINERALS

23. Summary & Video • The Great Oxygenation Event occurred when cyanobacteria living in the oceans started producing oxygen through photosynthesis. As oxygen built up in the atmosphere anaerobic bacteria were killed leading to the Earth's first mass extinction. The change in diversity and the arrival of appreciable atmospheric oxygen (as evidenced by the red bands in the rocks) can be analyzed to see what happens when a resource that was scarce becomes very abundant. • https://www.youtube.com/watch?v=dO2xx-aeZ4w