Cyanobacteria have a deceiving sense of the understated about them.

Compared to some of the complex organisms we've featured on Journey to the Microcosmos, these blue-green bacteria seem almost basic in their morphology and habits. They glide, divide, and photosynthesize.

Oh, and they get eaten a bunch too. But you do not need to be complicated to be important. Ecosystems are built on primary producers like cyanobacteria.

The energy they translate from the sun into chemical stores gets passed up the food chain, sustaining life of all sizes. And long ago, some eukaryotes gained their own photosynthetic abilities by absorbing cyanobacteria, an endosymbiotic event that set the stage for the evolution of plants. And plants are a pretty big deal.

Cyanobacteria, however, are more than just a prologue for the story of complex lifeforms. You might say they are the creative forces behind that tale. Billions of years ago, these tiny, seemingly innocuous organisms brought something new to the planet, an innovation that would create massive changes in their environment and set the stage for eons of remarkable life to come.

This is the story of the Great Oxidation Event, and how in the process of destroying the world as it existed, cyanobacteria created the Earth as we know it now. The bulk of this tale takes place about 2.5 billion years ago, at the end of the Archean eon that had started about 1.5 billion years before. This vast stretch of time included huge milestones for our planet, including the early days of the microcosmos.

But the Archean Earth looked very different from ours. The planet’s crust was stabilizing, and the oceans were filled with dissolved iron. Most importantly though, the world was anoxic, meaning it had almost no oxygen.

And because life reflects the environment it fills, the earliest creatures on our planet were anaerobic microorganisms, built to withstand and thrive in an oxygen-less environment. Some of these organisms may have been phototrophic, but their photosynthetic machinery relied on iron and sulfide in place of water, similar to bacteria like thiospirillum that thrive in anoxic conditions today. But for all the dramatic changes in Earth and in life that took place during the Archean eon, the real transformation was only just beginning.

We don’t know quite when cyanobacteria emerged, just that it’s some time before 2.5 billion years ago. And when they did, cyanobacteria brought something new to the photosynthetic mix: they used light to split water into its chemical components: hydrogen, which could then be used to make other energy-storing compounds, and oxygen, which was released into the environment as a waste product. This simple chemical reaction is the source of the Great Oxidation Event, which you might also hear called the Great Oxygenation

Event: an accumulation of oxygen in a previously anoxic world. That reaction would go on to become one of the most important ones to life, the source of the very oxygen you are breathing now. And remarkably, cyanobacteria were not just the first organisms to evolve oxygenic photosynthesis; as far as we know, on our planet anyway, they are the only organisms that do so. Other photosynthetic bacteria don’t produce oxygen.

And the eukaryotic organisms that gained the capacity for photosynthesis did so by co-opting the talents and machinery of endosymbiotic cyanobacteria. And as wonderful as that is now, back at the end of the Archean eon, oxygenic photosynthesis didn’t just make cyanobacteria unique, it made them catastrophic. Oxygen is great--if you are evolved to tolerate it and to use it.

But for the anaerobic microorganisms specifically evolved for a world without oxygen, this newfangled molecule was toxic. The species that survived the Great Oxidation Event likely did so by finding oxygen-less sanctuaries. And meanwhile, cyanobacteria continued pumping out oxygen.

Yes, they are tiny, and the amount of gas they produce is even tinier. But with hundreds of millions of years and many many organisms, all of those reactions added up, affecting not only the lives of their fellow microbes, but also the chemistry and climate of the entire planet. For one, cyanobacteria might be responsible for setting off a series of ice ages.

As they consumed carbon dioxide and produced oxygen that reacted with methane, they cut down on the planet’s stock of greenhouse gases and dropped temperatures to glacial levels. As you might imagine, these early ice ages were not particularly hospitable for life, including for cyanobacteria. Coupled with the anaerobic microbes that died due to all of that oxygen in the atmosphere, the Great Oxidation Event is responsible for an extinction so ancient that we don't have the tools or physical evidence to even fully describe it.

But life still managed to endure. And as it did, the oxygen produced by cyanobacteria would go on to form our planet's ozone layer, shielding the survivors from dangerous ultraviolet wavelengths of light. Moreover, the availability of oxygen in the water and air would go on to make aerobic metabolism possible, providing organisms more energy than anaerobic metabolism ever could and thus the capacity for more complex life.

This intertwined story of destruction and evolution is built on evidence scientists have gathered from geological and biological sources. Some parts are etched in rocks, like the banded iron formations formed by oxygen reacting with iron in the ocean. And other parts are encoded in the genes of modern day cyanobacteria, whose sequences and machinery elucidate what their ancient counterparts may have looked like.

But characterizing the lives of microbes that existed billions and billions of years ago is really, really hard, especially because we haven’t uncovered many Archean fossils. One recent paper begins with the following confession: “Writing about early microbial evolution is a daunting task, so it was possibly unwise to agree to do it.” And as you dig through the research on the subject, that honest evaluation rings true. There are many gaps in our understanding of the Great Oxidation Event, gaps built on unanswered fundamental questions like, “how did photosynthesis even evolve?” We alluded to one of the most important gaps earlier.

It’s the question of when cyanobacteria first appeared, and that question is key to understanding how they caused the Great Oxidation Event. If that event began shortly after the evolution of cyanobacteria and oxygenic photosynthesis, then it all seems relatively straightforward: the cyanobacteria produced oxygen, the oxygen accumulated, and catastrophe ensued, but some evidence suggests that cyanobacteria emerged well before the accumulation of oxygen on Earth, and if that’s the case, then we have to ask, “What took so long for the Great Oxidation Event to begin?” Maybe there was a geological delay of some sort. Or maybe oxygenic photosynthesis wasn't the only cyanobacterial innovation: maybe after their first appearances on the planet, they developed some other trait that bumped their oxygen production up even more.

One group of scientists investigated that very possibility and came to a possible explanation that we don’t often associate with bacteria: multicellularity. Some modern cyanobacteria species form filaments of individual cells, a few of which even take on specialized functions like nitrogen fixation. These scientists traced the origins of that trait to before the Great Oxidation Event.

So maybe multicellularity allowed cyanobacteria to not only better contend with the challenges of their environment, but to produce more oxygen than before. And perhaps this was the added trait needed to, say, allow a new kind of bacteria to take over an entire world...a claim over our planet that they have, in many ways, never given up. Of course, that idea is scientific conjecture built on the available evidence, which means it is still subject to debate.

But the seemingly simple question underlying it, the question of when cyanobacteria evolved, reflects what’s so important about the Great Oxidation Event in general: that questions of biology and geology and climate are so intertwined that to understand the history of this planet, we have to piece together the history of the microcosmos. And if something as small as cyanobacteria can set off a catastrophe of extinction and ice, well, then what will the planet look like when we’re through with it? Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

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