Chris Kempes

And so if you think about the smallest bacteria, they have this membrane that is shrinking down. If you imagine a spherical cow, and here we really can imagine a spherical cow because the small bacteria are little spheres. So imagine the small spherical bacterium. And as it shrinks down, the membrane has its own finite thickness. And so imagine you have a really thick membrane.

And so if you think about the smallest bacteria, they have this membrane that is shrinking down. If you imagine a spherical cow, and here we really can imagine a spherical cow because the small bacteria are little spheres. So imagine the small spherical bacterium. And as it shrinks down, the membrane has its own finite thickness. And so imagine you have a really thick membrane.

tire, and you shrink down that tire, eventually the rubber is touching the rubber on all sides. And you don't have a tire anymore with something in the middle, you have just a hockey puck. The same thing is happening for cells. So they have this membrane that is very thin until you get to really small sizes, and then it starts to represent a large fraction of the cell size.

tire, and you shrink down that tire, eventually the rubber is touching the rubber on all sides. And you don't have a tire anymore with something in the middle, you have just a hockey puck. The same thing is happening for cells. So they have this membrane that is very thin until you get to really small sizes, and then it starts to represent a large fraction of the cell size.

And then you still have to inside of that in the remaining volume fit all of the stuff. So you have to put the DNA in there. You have to put a few functional proteins to actually run the metabolism. And you have to put in this amazing molecular device known as the ribosome, which is sort of this generic tape reading device that takes information from the DNA and turns it into functional proteins.

And then you still have to inside of that in the remaining volume fit all of the stuff. So you have to put the DNA in there. You have to put a few functional proteins to actually run the metabolism. And you have to put in this amazing molecular device known as the ribosome, which is sort of this generic tape reading device that takes information from the DNA and turns it into functional proteins.

And so in these small cells, the DNA starts to take up roughly half the cell volume. And so you're running into all of these space constraints where you almost can't fit in just the information of what the cell is. You know, you think about how strange that is. The storage system for information becomes sort of half the cell volume.

And so in these small cells, the DNA starts to take up roughly half the cell volume. And so you're running into all of these space constraints where you almost can't fit in just the information of what the cell is. You know, you think about how strange that is. The storage system for information becomes sort of half the cell volume.

And you get to a point where every single gene that you would eliminate kills the cell. So every gene knockout, as we call it, is fatal. And so you can't eliminate any more genes. You're stuck with this minimal genome.

And you get to a point where every single gene that you would eliminate kills the cell. So every gene knockout, as we call it, is fatal. And so you can't eliminate any more genes. You're stuck with this minimal genome.

And so the smallest cell is defined by this point where you have a minimal genome, a handful of ribosomes, and the functional proteins that do all the other metabolic and physiological aspects of the cell.

And so the smallest cell is defined by this point where you have a minimal genome, a handful of ribosomes, and the functional proteins that do all the other metabolic and physiological aspects of the cell.

What's amazing is that we write down energetic models of the cell and we think about what's the energy budget of the cell, what it has for running biosynthesis, the production of new stuff, and what it has for maintenance, which is just repairing and the repair and upkeep of existing stuff. then at these tiny cell sizes, the maintenance starts to take over the entire metabolism.

What's amazing is that we write down energetic models of the cell and we think about what's the energy budget of the cell, what it has for running biosynthesis, the production of new stuff, and what it has for maintenance, which is just repairing and the repair and upkeep of existing stuff. then at these tiny cell sizes, the maintenance starts to take over the entire metabolism.

So you start to look more and more like a cell that can only repair itself. It actually can barely grow any new stuff for replication. And this space constraint and this maintenance constraint, where maintenance metabolism becomes the whole metabolism, both happen at basically the same tiny cell size. And so we think it's this dual constraint that sets the smallest possible bacteria.

So you start to look more and more like a cell that can only repair itself. It actually can barely grow any new stuff for replication. And this space constraint and this maintenance constraint, where maintenance metabolism becomes the whole metabolism, both happen at basically the same tiny cell size. And so we think it's this dual constraint that sets the smallest possible bacteria.

amazingly, those predictions have agreed with all of the world record holders for smallest bacteria. And it's where you transition to seeing tiny viruses that crop up at these really small cell sizes. So we think this physical constraint tells you the smallest possible bacterium.

amazingly, those predictions have agreed with all of the world record holders for smallest bacteria. And it's where you transition to seeing tiny viruses that crop up at these really small cell sizes. So we think this physical constraint tells you the smallest possible bacterium.

And you could. And that's a really great question, Sean, because that's a game, especially in an astrobiological context, we like to think about a lot, which is, well, but the DNA takes up a certain volume. And we're committed to a certain chemistry there. So what if you change that a little bit? And the membrane is made up of this lipid bilayer.

And you could. And that's a really great question, Sean, because that's a game, especially in an astrobiological context, we like to think about a lot, which is, well, but the DNA takes up a certain volume. And we're committed to a certain chemistry there. So what if you change that a little bit? And the membrane is made up of this lipid bilayer.