The Origin of Oxygenic Photosynthesis

An evolution that terraformed the planet we know

Earth in the Archean Eon, 4,000 to 2,500 million years ago, appeared as an alien world. The planet’s crust was stabilizing, green oceans were filled with dissolved iron, and the atmosphere had negligible oxygen. Microbial life subsisted on chemical energy extracted from boiling plumes erupting miles deep in dark, hydrothermal vents, as it already had done for over a billion years. 

Then,
in the sunlit shallows, an evolution happened that terraformed the planet we know. Bacteria swapped genes through their membranes, birthing an entirely new innovation: microbes capable of surviving on Earth’s most abundant resource, sunlight.

The first of these organisms, the phototrophic bacteria, used red light, purple pigments, and the abundant iron and sulfur in the Archean ocean to fix a little dissolved carbon and supplement their sugar reserves. But before long another group learned to use a wider spectrum of sunlight to split the water itself for its electrons, fixing carbon with their negative charge and releasing oxygen as a waste product. They used the green pigment chlorophyll to absorb and channel the energy of sunlight for this electrolysis. These were the Cyanobacteria, the ancestors of the modern blue-green algae, and of the photosynthetic organs of all plants.

The evolution of these early cyanobacteria launched processes that eventually depleted the iron supply of their phototroph competitors, ensuring the global dominance of oxygenic photosynthesis for carbon fixation. At this point there were enough cyanobacteria, producing enough oxygen, to oxidize the methane in the atmosphere, converting it to CO2, a far less potent insulator. The result was sudden and dramatic global cooling: the Pongola Ice Age.

As Cyanobacteria proliferated, the oxygen they produced transformed the planet. The previously lime green oceans rusted to a dull red as the reactive oxygen bound their iron into its oxide, which then dropped from the water column as dust. This starved the previous generation of phototrophs of the iron and sulfur they needed, cementing the advantage of the water splitting cyanobacteria. Across the shallow continental shelves, their caustic waste product oxidized exposed minerals, transforming the colors of the rocks. Oxygen dissolved in the water up to its capacity, changing what had been a hospitable environment for Archean organisms into something analogous to what we would experience in a hydrogen peroxide bath. 

The carbon demands of the cyanobacteria increased exponentially as they expanded across the sunlit surface waters, drawing CO2 from the atmosphere to dissolve as carbonic acid into the oceans and finally be fixed as sugar in their cells. The seas, now saturated with dissolved oxygen, began to emit this new gas into the air. 

At this time, the young sun gave 20% less light than now, and Earth depended on a thick blanket of methane to trap enough heat to keep a climate similar to, or even warmer than, today's. The released oxygen reacted with this methane, oxidizing it to carbon dioxide and water. While carbon dioxide is now the primary warming gas in the atmosphere, it is far less potent than methane as an insulator. It was as though the Earth's blankets had been suddenly ripped off on a cold, dark night. The planet began to freeze. 

Ice sheets expanded from the poles, the planet's dimmest places, as the cyanobacteria pulsed in boom and bust cycles, not yet able to fully resist the toxicity of their own waste product, and periodically poisoned by their own success. In the banded iron formations laid down at this time, layers of deep red iron rich rock alternate with anaerobic black shales, as oxygen, and rust, levels rose and fell in waves in the suddenly flushed Archean seas, reddened by the same iron that now fixes oxygen in our blood.

Finally, enough methane was stripped from the atmosphere that the ice froze to the crust, and spread from the early continents to cover the global ocean, bringing the sunlit shelves into a cold darkness that would persist for 120 million years. This was the first ice age on Earth, the Pongola Glaciation.

While oxygen release had been quenched, volcanic eruptions continued releasing carbon dioxide at the same glacial pace as always. Eventually, enough warming gas accumulated to begin to thaw the ice in the tropics, and the dark water exposed strongly absorbed the sun's heat, rapidly melting across the whole face of the Earth, and revealing a transformed biosphere.

This post was part of our Deep Time Series. Continue reading the next post: Snowball Earth I