Photosynthesis – the process of capturing a photon of light and through a series of complicated electron exchanges, exhaling oxygen – is the base on which most life on the planet lives. Yeah, there are some really awesome bacteria living on toxic hydrothermal vents on the bottom of the ocean, and yeah, there are some bacteria who eat rocks and excrete sulfuric acid, and yeah, there are some really, REALLY awesome bacteria living off of arsenic, but we are alive because of photosynthesis.
However, photosynthesis has not always been around, and oxygen wasn’t always the most abundant element in our atmosphere. Don’t believe me? What, have you got your own TARDIS now?
Yeah, no, that’s what I thought.
Once upon a time, on an Earth millions of years before humans, plants were not green. They weren’t even plants yet. They were bacteria, and they did not photosynthesize. They did not take energy from the sun and water from Earth and convert it into sugar and oxygen. For the first 2.7 billion years of Earth’s history, photosynthesis did not exist. This was a time back when everything, from one-celled organisms to multi-celled bacteria (because really, that’s all there was) lived in the sea, a time when the methane-rich air was toxic to any living thing, a time when volcanoes periodically spewed nickel-rich lava into the oceans.
These conditions are known as anaerobic, or, lacking oxygen. The source of life for these bacteria (and even still for some bacteria today) was a process called sulfate reduction, which is long, boring, and incredibly complicated – but the bottom line is, they basically “breathed” sulfur, rather than oxygen.
And then, about 2.5 billion years ago, the ocean floor rusted.
Rust, if you recall from your high school chemistry classes, is the product of oxygen binding to iron.
And if there’s iron on the bottom of the ocean, that meant that somewhere, something was producing oxygen.
Enter cyanobacteria: the primitive and over-complicated beginning of the photosynthesizing centers found in green plants today. Also known as the ancestors of chloroplasts, the organelles in a plant cell that carry out photosynthesis. Cyanobacteria were the first organisms to absorb visible light and produce oxygen. Around the rise of cyanobacteria, a band of iron-oxide was deposited on the seafloor, suggesting a reaction between oxygen and iron in the seawater.
Scientists also noticed that a short time after the appearance of cyanobacteria, there was a change in sulfur-oxygen ratio in the atmosphere. It was a change that suggested that oxygen was becoming our atmosphere’s dominant element (which they call The Great Oxidation Event )
How did cyanobacteria take over so quickly? A study published in Nature proposed that the rise of cyanobacteria had more to do with the decline of nickel in the oceans than a survival-of-the-fittest reason. Remember those nickel-rich oceans? This was prime real estate for a microorganism called a methanogen. These methanogens require nickel for life and secrete methane. When the Earth started to cool off, the nickel-spewing volcanoes let up, and methogens started to die. This allowed those fancy blue-green, light-capturing, oxygen-exhaling cyanobacteria to take over and change the Earth’s atmosphere, giving rise to the planet we know and love today.