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Let's dive into the branching weirdness of the pulsating slime molds!

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SOURCES:
https://pdfs.semanticscholar.org/d0f7/712487248808b08f4077b97759a4506add16.pdf
http://plantclinic.cornell.edu/factsheets/slimemolds.pdf
https://www.ncbi.nlm.nih.gov/pubmed/8457802
https://herbarium.usu.edu/fun-with-fungi/slime-molds
https://www.sciencedirect.com/topics/immunology-and-microbiology/mycetozoa
https://books.google.com/books?id=iPBeCwAAQBAJ
https://academic.oup.com/femsre/article/40/6/798/2400841
https://www.popsci.com/the-blob-slime-mold/
https://www.ncbi.nlm.nih.gov/pubmed/2398347
https://warwick.ac.uk/fac/sci/lifesci/outreach/slimemold/facts/
https://www.theatlantic.com/science/archive/2016/12/the-brainless-slime-that-can-learn-by-fusing/511295/
https://royalsocietypublishing.org/doi/full/10.1098/rspb.2016.0446
https://royalsocietypublishing.org/doi/10.1098/rspb.2016.2382
https://www.pnas.org/content/107/12/5267
https://science.sciencemag.org/content/327/5964/439.abstract
This is slime.

You might have expected that from the title of the video. Even if you didn't, you might have just looked at it and thought, "Yeah, yeah that looks like slime." Slime has had a bit of a cultural journey in the last few decades.

Some of us grew up watching green slime getting dumped on the heads of game show contestants. And there's also been a bit of a slime renaissance recently, with entire YouTube channels and. Instagram accounts dedicated to teaching people how to make their own gooey, sticky blobs at home with chemistry and a few simple ingredients.

Of course, the most important step in any of these slime projects is saying, "It's aliiiiiiiive," while you watch it spread, even when you know that, of course, it’s not actually alive. The microcosmos, on the other hand, got to its experiments long before we did, giving us slime molds: a slime that not only lives, it learns. a slime that fuses microbes together to create a living thing that grows into something we can observe with our own eyes. You may have seen slime molds before, their bodies sometimes laden with spores as they grow on plants and decaying wood.

They look kind of like fungi, and for that reason, that is how they were initially classified. Except that one of the defining characteristics of fungi is their inability to absorb and digest their food internally. Slime molds, however, are plenty capable of that.

They do phagocytosis all the time. Where we see decaying matter, slime molds detect a cornucopia of microbes. Plus, fungi do not move, and slime molds--in their own unsettling way, do.

We know now that slime molds are eukaryotes, and they’re made up of amoeboid organisms. But they don’t fit neatly into any of our definitions of plant, animal, or fungus, leaving them in the more ambiguous Protista kingdom and clustered under the label Mycetozoa. However, that label is a, get this, polyphyletic grouping, and that means that the organisms we’ve lumped together as slime molds aren’t necessarily all related.

They’ve just found their way, over the course of evolution, into a set of similarities that is both convenient and misleading in our attempts to categorize the natural world. There are three major types of slime molds. There are the Protostelids, which are the least studied.

Then there are the Dictyostelids, also known as cellular slime molds, and then there’s the Myxomycetes, also known as the "true" or "plasmodial" slime molds. Scientists love studying slime molds, particularly Dictyostelids and Myxomycetes, for reasons that we will discuss soon. In fact, the slime molds we're going to show you today come from a lab, though we are on the hunt to see if we can gather any of our own in our sampling trips.

This species is Physarum polycephalum, a well-studied example of Myxomycetes, making it a plasmodial slime mold. We feed it oatmeal and barley flakes, keeping it in the dark because it's not particularly fond of light. We're observing our Physarum here on petri dishes because it's hard to transfer them to glass slides.

This, in turn, limits our magnification, so we won’t be able to see as deeply into their bodies as we might be able to with other organisms. But they are remarkable to observe as an entity, with their sprawling, networked branches. Now, we also don't have samples of the other types of slime molds right now, so we're going to be focusing our discussion mostly on Physarum and plasmodial slime molds.

But both cellular and plasmodial slime molds come up often in the news because scientists have observed them doing very impressive things, but let’s start out by quickly explaining the differences here. Cellular slime molds, the dictyostelids, are made up of organisms that will likely spend most of their lives as singular amoebas. But if the conditions around them become bleak, these amoebas team up to create a multicellular thing called a "slug" that coordinates the various cells so that they can all move to a good spot and then morph into a fruiting body that releases spores.

This is wild, right? They’re single cells and then they get some signal that they should all come together to form an organism and spore out to create the next generation. But while cellular slime molds are made up of many cells, true slime molds like Physarum polycephalum are actually made up of only one cell.

Yeah, all of that branching weirdness, that is all one cell. The life of this Physarum started with the sporangia, a black globular body made by another Physarum. The sporangia holds spores that spread and eventually germinate into either an amoeba or a flagellate.

At this unicellular stage in their lives, as you might expect, the Physarum only has one nucleus. But then the amoeba finds another amoeba to mate with. And if you've heard about a slime mold that has several hundred sexes, this is where that comes in.

Each of the little cells produced by the spores has two copies of three sex genes, and each of those sex genes has their own variants across the species. Taking into account the number of different combinations of those sex gene variants that are possible, you end up with an organism whose sex is just one of hundreds, which increases the number of possible mating partners for any individual physarum amoeba. When they do find their mate, the two become one, literally.

They fuse together all the way down to their nucleus. And after this point, the new organism doesn’t divide anymore. It grows.

The plasmodium formed by the merged Physarum cells expands, sometimes up to two feet in diameter if the conditions are really good. But, the nuclei inside will continue dividing and dividing and dividing, their count ending up in the millions. Physarum have actin and myosin, or muscle proteins, the same ones we have, that act as a part of a network to contract and relax the cytoplasm, creating that stream you see moving through the body of the organism.

This process moves the slime mold, which can reach a speed of around 4 centimeters/hour, and it also helps distribute nutrients around that ever-widening, increasingly gigantic body. And here's the thing: whatever it is these amoebas have come together to create, it is very, very smart. Listing off the physarum talents that have been unearthed by scientists is enough to inspire a sense of inadequacy.

They can learn to manage uncomfortable stimuli and pass on their knowledge to other Physarums by fusing with them. They can also solve mazes and make good nutritional choices, which, like, I can’t. In one of the most well-known Physarum experiments, scientists placed oat flakes down on an agar plate.

These flakes were distributed in a pattern similar to the arrangement of cities around Tokyo. And then they watched as a Physarum that started at the central Tokyo equivalent eventually morphed itself into a route that resembled, shockingly closely, the Tokyo rail system. The Physarum’s optimized hunt for food resembled the efficiency that human engineers seek when transporting people around cities.

If you grew up watching Saturday morning cartoons or even their blockbuster adaptations, you've seen plenty of robots that assemble together into some kind of mega-robot. The bodies are metal and they’re full of circuits, transforming the individual, fantastical powers of those robots into a bigger stomping, punching thing. Nature, as it inevitably seems to, got there first, beating even our imaginations, and with a touch that is so much lighter than ours, yet still so incredible.

Instead of imagined mechanical frames, it's the organic bodies of amoeba assembling together to create a bigger living, learning thing. The contours are smooth, the movements streaming. The slime mold doesn't need to stomp or punch, it just spreads and consumes and fruits, expanding the microcosmos in its own body as it stakes a claim in our world.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. Thank you to all of our new viewers who are with us and thank you especially to all of our patrons on Patreon, who make this show possible. If you’ve just stumbled across Journey to the Microcosmos, these people on the screen right here are the ones you really have to thank.

If you want to see more from our Master of Microscopes, James Weiss, you can find him at Jam & Germs on Instagram, and if you want to see more from us, you can go to youtube.com/microcosmos where we have a new episode up every week.