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This round little unicellular creature came to us via a plankton net, a mesh with tiny, microscopic holes through which we ran hundreds and hundreds of liters of water, letting us collect anything too large to pass through.

We haven’t been able to identify this species yet, making it a bit of a mystery. But the bigger mystery is still to come because this little creature is about to undergo that most universal and unknowable experience of all—death.

Death comes to the microcosmos in many forms. Like this Stentor Polymorphous, slowly expelling the contents of its once trumpet-like body into the surrounding environment. Or this dead larva, whose exoskeleton is now an inanimate host to two unicellular organisms.

Even the mighty tardigrade, which has survived as a species through multiple mass extinctions, is not immune to death. This is, of course, the natural order of things. Predators hunt, and their prey attempts to survive…with varying levels of success.

This is Loxophyllum meleagris, a large unicellular organism that we’ve shown before eating a rotifer. This one is practically stuffed with those multicellular creatures. We counted five rotifers inside of it.

But sometimes the predator becomes the prey, and even the Loxophyllum meleagris has to find ways to ensure survival when other species come after it. This seemingly unlikely threat is the Lacrymaria olor. Its name, in Latin means “tears of a swan”, a name that suits both its teardrop shape and its neck-like extension, which gets up to 8 times longer than its body in search of prey.

Sometimes, we can see its neck poking out of the dirt on our microscope slide. But even knowing that, you’d be forgiven for thinking that it’s unlikely that something so small could pose a problem for those larger Loxophyllum. And yet, the Lacrymaria manages to take quite a chunk out of the Loxophyllum.

The Loxophyllum though survives thanks to its ability to regenerate the piece that was taken, but not all prey gets so lucky. Here, this rotifer has been killed by a heliozoan, destined to become food, a fate that this flagellate is about to share as it becomes captured by a heliozoan that is in the middle of cell division. The flagellate has been trapped by those long extensions, called axopods, that radiate out from the heliozoan’s body.

As the flagellate comes further in, it will be engulfed by the cells into its own food compartment called a vacuole. There, it will be lysed open and its contents digested by the heliozoa. In the end though, the natural order comes for predators too.

Here, another heliozoan’s dying cellular body attracts the various decomposers of the microbial world. Aside from predators, there are many other factors that lead a single-celled organism to die. Changes in temperature, oxygen concentration, pH, water quality, so much more.

This single-celled organism is swollen because the water surrounding it is entering the cell via osmosis. Many organisms have water pumps called contractile vacuoles that they use to push water back out and prevent that swelling. But as in the case of this organism, sometimes those contractile vacuoles stop working, and when that happens the cell swells and explodes.

Other times, the cause of death is harder to determine, like this Paradileptus that spent several hours swimming before going still, its shape beginning to change until it melts away, seeming to kill not only the Paradileptus but this small green cell swimming nearby…but leaving other smaller flagellates seemingly unaffected. And this brings us back to the beginning, with our mystery organism that is about to undergo a death laden with even more mysteries. At first, the cell looks like it’s just melting away, dissolving into something that resembles a microbial Milky Way…except that for a few seconds, it almost looks like the cell membrane is able to close itself back up.

We think, though we can’t know for sure, that some of the mechanisms inside the cell are still working, and that the organism is trying to recover. But alas, survival is not in the cards. Its membrane goes through lysis, releasing its insides to the surrounding environment.

This death is unlike any other kind of death we have observed under our microscope, and we’re still not sure what caused it. Perhaps there were so many organisms in the sample that they depleted the oxygen, and the organism could not continue cellular respiration, but perhaps it was something else. Death at every size holds its own mysteries, but it also reveals.

The observations we make, even the guesses we come up with, tell us about the way these microbes interact with their environment, the way their bodies work, and the connections that exist between them. It is only ever in the mysteries that knowledge is waiting to be found. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

If you want to see more from our Master of Microscopes, James, check out Jam and Germs on Instagram where he is constantly posting cool stuff. And if you want to see more from Journey to the Microcosmos here on YouTube there's always a subscribe button somewhere nearby.