What would happen if every microbe in the world just...disappeared.

Could we live? Could anything live?

And for how long? First, let’s talk about mass extinctions. The first microbes on this planet appeared billions of years ago.

In the long history of life since then, researchers have estimated using fossil evidence that there have been five major mass extinctions. These extinctions were catastrophic mixes of geological, biological, and of course on at least one occasion astronomical issues that, over the often protracted period they occurred, killed off a large percentage of extant species. The end-Permian extinction, which is considered to be the largest mass extinction, occurred over 252 million years ago, the result of volcanic activity that produced greenhouse gases and warmed the planet.

But more recently, researchers have identified another potential culprit: microbes. Specifically: methanogens, a type of methane-producing archaea whose numbers may have increased as they fed on dying plants and animals, creating even more methane that warmed the planet further, killing more organisms, producing more food for methanogens. A classic positive feedback loop that changed the planet too fast for many species to cope with.

Now, this idea that microbes may have been involved in the largest mass extinction is a hypothesis, rooted in scientific observation but still not proven but, as we look at what a world without microbes might be like, remember that they may have, at least once, done living systems a good deal of damage all on their own. But a world without them would do far more damage. With this thought and perhaps our own biases in mind, the collective scientific understanding of microbes feels a little bit like being in love.

It wasn’t long ago that we didn’t know they existed. But now, it hard to imagine a world without them. Now, this has not stopped scientists from trying to imagine such a world.

In 2014, Jack A. Gilbert and Josh D. Neufeld published a paper in PLOS Biology called Life in a World without Microbes, envisioning a hypothetical world where in a single moment, all microbes vanish.

Their thought experiment uses what we’ve learned of microbes to take on a belief put forth by Louis Pasteur, that, “Life would not long remain possible in the absence of microbes.” and that may have been the case then, but we have more technology now. There are many ways to explore this hypothetical world without microbes, and in their work, Gilbert and Neufeld consider what we might broadly group into two questions. What would an absence of microbes do to the world around us?

And what would an absence of microbes do to the world inside of us? Let’s start with that second question: the internal one. If all microbes, single-celled and multi-celled, were to vanish, there would be a large number of diseases that would just cease to exist.

Now, in this hypothetical, mitochondria and similar endosymbiotic prokaryotes that have long been integrated into multicellular, eukaryotic bodies such as our own, are treated not as microbes, but as part of our bodies. If all mitochondria disappeared, do not fear, you would be dead that very instant. But even without endosymbionts that leaves many, many microbes inside of us, particularly bacteria in our gut that aid in digestion and metabolism.

It’s unlikely that we’ll ever know for sure what would happen to our bodies if all of those microbial residents were just suddenly evicted. The examples we can take from various types of germ-free animals that have been raised since the end of the 19th century suggests that life, at least inside of us, will still go on—it’s just not clear what that life would look like for us. We would likely have to rely on synthetic nutrients to compensate for changes in how our body processes food.

Meanwhile, our own physiology might change. We would, for example, be smaller, having access to fewer nutrients. leaving us to wonder what of our body belonged just to us after all. But of course there are many, many microbes, in fact the vast majority of them, existing outside of our bodies as, and a hypothetical microbial banishment would create some clear challenges.

You might think that, without the work of diatoms and other photosynthetic microbes, we’d be at a risk of dying from a lack of oxygen pretty quickly. And yes, it would be an eventual problem. But it turns out there is enough atmospheric oxygen to sustain life for a pretty long time.

Gilbert and Neufeld note this as more of a centuries-long back-up plan while we can maybe figure something else out. But, if we're being honest with ourselves, we probably wouldn't make it that long. There are larger problems.

Some bacteria, like Nostoc, convert diatomic nitrogen in the atmosphere into other nitrogen-containing compounds that plants can actually use. Their disappearance would severely reduce photosynthesis, and yes, limit the further release of oxygen into the atmosphere, but also, it would just make it much harder to grow plants which, you will remember, we need for food. Meanwhile, macroscopic decomposers would have to take on much more of the burden of converting death into new ingredients for life.

And we would also feel the loss of other cycles that rely on microbes, like the processes that move phosphorous across different layers of life. The interlocking nature of these microbe-based processes within their environments mirrors our relatively new understanding of how our microbiome dictates the interplay between microbes and our bodies. In fact, scientists have begun studying what they call the Earth Microbiome. not just a catalogue of the diversity of microbes across the planet, but a broader understanding of how the microbial composition is its own dynamic thing that shapes and responds to the world around it.

There is an ongoing conversation about the possibility that we are somewhere in the midst of a sixth extinction event driven primarily by our own human actions. The characteristics of this extinction are much more readily observable on animals and plants than on microbes, but they are an important piece of that extinction puzzle—both in terms of their future and our own. Many microbes, after all, seem readily capable of survival—like the trusty tardigrade, the mascot of all microbes who do what we would all love to do when conditions are bad, they curl up in a protective cyst and lie dormant.

The question, if that really is what it will to take to survive, what will the world look like when they wake up. At the end of “Life in a World Without Microbes,” Gilbert and Neufeld describe a world where nutrients and resources become increasingly scarce--a world where we would need to rapidly fill the void left by microbes with manufactured alternatives designed to fill in cavernous cracks. They argue that we might actually do okay for a few days, and maybe a small number of humans would manage to last even for centuries.

But in the long term, for us humans at least, the prognosis is bleak. But they also point out that while microbes play a massively important role in our world right now, there is a hypothetical where other organisms adapt or evolve to reconfigure those essential niches. Perhaps we would be gone, but life exceeds us.

And so does its capacity for invention. And then, we suppose, Earth would just have to get ready for some new species to come along, develop technologies and look into the fossil record to find what caused our mass extinction, and then make an internet...and then internet videos about us, and how weird our world was, back when there were microbes. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

And also thank you to all of these people, our patrons on Patreon, who allow us to go on these deep dives and understand more of how much we rely on these little organisms. So various and so delightful. If you want to see more from our Master of Microscopes James Weiss, you can check out Jam and Germs on Instagram.

And you want to see more from us, here at Journey to the Microcosmos, you can find us at YouTube.com/microcosmos