Picture yourself in space, with no space suit on. You're floating above our blue dot, flying out towards mars in zero gravity. You land on the red planet, and it's quiet. You are alone. It's so peaceful.
Just kidding, it isn't. It's hot, hotter than you've ever experienced, and the UV index is so high that your skin is burned red and peeling before you can even say, "Martian". Your throat is parched, so you run past the black smoking hydrothermal vents spewing out charcoal-coloured clouds, towards a river you can see glimmering in the distance. You dip your hands in to cup some water… But the water is acid. You already wish you'd stayed on Earth – how could anything ever live here?
Indeed, on our own, humans can't. But compared to microbes, humans are weak.
Today's Microbial Monday is inspired by my inspiring friend Shane, who has been opening my eyes to the world of astrobiology – biology in space, and the subfield of biology that investigates the origins, evolution, distribution, and future of life not only on our blue dot but all across the universe – since I met him in a undergraduate neurobiology class. Today, I hope I can begin opening your eyes to this field, too.
Microbes are incredibly important to the field of astrobiology because, unlike humans, they can survive incredibly extreme environments. In fact, there is a word to describe the collection of microbes, which include bacteria, archaea, and eukaryotes, that can survive in places that, to humans, seem completely inhospitable: extremophiles. This word roughly translates to "extreme-loving". These microbes are the biggest daredevils of all, taking on places that are so acidic or so basic that our skin would burn off, or so hot that our blood would boil, or so salty that our organs would just give up. The study of these bad-ass microbes helps scientists understand the limits of where life can go. So far, those limits are pretty impressive.
As an example, I want to talk a little about the lifestyle of my favourite extremophile: the tardigrade, or "water bear". These tiny animals, which can be as small as 50 μm in length, are impressively complex for their size. Tardigrades have a complete digestive system, a nervous system with a basic brain and eyes, and a reproductive system based on eggs. And, as the authors of this paper put it, "There is no denying that tardigrades are one of the toughest animals on our planet and are the most unique in the extremophiles group."
Me & my people with a larger-than-life tardigrade at the Micropia museum in Amsterdam
Tardigrades are found all over our planet, from dry soils to mossy riverbeds to salty seashores. This is impressive to biologists, because it means that they have to be able to survive all sorts of "stresses": from dessication (drying out) and freezing temperatures to lack of oxygen and excessive salt. There aren't too many things out there that we know of that can handle all of those without batting an eye.
The way that tardigrades survive these crazy environments is by entering into a special state that protects them against all sorts of extreme stresses: cryptobiosis. This is kind of like what happens to Austin Powers when he is frozen to catch the future Dr. Evil. When tardigrades encounter a stress (like the stress of Dr. Evil escaping in the '60s), they are able to pause all of their bodily functions – like a cross between being dead and frozen, but they can zombie back out of it. It's not yet fully understood how tardigrades accomplish this, but we do know that tardigrades and other animals that survive extremely cold environments make special molecules, like the sugar trehalose, which seem to protect their cells from dying.
It was this ability of tardigrades which made them a sort of poster child, or model organism, for space research. Tardigrades have been brought into space several times now, to research things like how cosmic radiation can damage DNA (the tardigrades were pretty much fine with the cosmic radiation), and if tardigrades can survive in the vacuum of space (they can).
Extremophiles like tardigrades are one of the reasons why the "panspermia" theory of life emerged in science. This theory holds that life on earth didn't start out here, but was transported from somewhere else in the universe. This theory originated from the study of these microbes, which are able to adapt so well to new, and extreme, conditions, like what may have been the norm on Earth at its very beginning. It's been suggested that life may have come to Earth during the period of time between about 4 and 3.8 billion years ago, at the end-tail when our planet was enduring an extraterrestrial blitzkrieg of meteors. The earliest signs of life we've found seem to date from just over 3.8 billion years ago, so the timing is more or less right. However, the panspermia theory doesn't account for how life originated in general - just how it got to Earth. It's also by no means the only theory of the origin of life on Earth. For a good read on an alternate theory that we'll definitely touch on in the future, you can check out this book by Dr. Nick Lane.
In the meantime, we're not really sure how life originated. It's hard to look back so many billions of years, after all. However, extremophiles like tardigrades are pushing the limits of where we thought life could survive, and that's pretty cool already. Go, tardigrades, go!
Until next week, respect the water bears!
~ Alex