Chapter 29: Phylum Myxomycota: True Slime Molds

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When you hear the word slime mold, what immediately pops into your mind?

Maybe some amorphous slimy blob?

Yeah, or maybe something out of a cheesy old sci -fi movie.

Exactly.

Well today, we are taking a deep dive into an organism that has truly puzzled scientists for centuries.

It defies easy classification and boasts a life history so extravagant it sounds like pure science fiction.

It's fascinating because, you know, your first instinct might be to think of fungi.

And historically, yeah, they were actually classified right alongside them for a very long time.

Right.

But here's the real kicker.

Modern science, with all its new tools and phylogenetic insights, has shown they are not fungi at all.

We're talking about the true slime mold, or mixomycota.

Mixomycota, which funnily enough kind of hints at that old confusion, right?

Mixo like slime, mycota like fungus.

Precisely.

It points right back to that long tangled taxonomic history.

So our mission today is to unpack the incredible world of these, well, these fascinating creatures for you.

We're going to explore their bizarre structure, their unique ways of living, reproducing.

And even their surprising importance, both out in nature and actually in the laboratory, too.

OK, so we'll guide you through their complex life cycle, figure out how they move and feed.

And really get why they continue to intrigue biologists from cell cycle experts all the way to geneticists.

It's a broad appeal.

Sounds great.

So let's start at the very beginning.

What exactly are true slime molds, beyond just being a blob?

OK, so at their core, mixomycota are commonly known as true slime molds.

You also hear plasmodial slime molds or S -cellular slime molds.

But the key thing, the defining feature, is their nutrition.

They're phagotrophic.

Phagotrophic, meaning they eat by engulfing, like an amoeba.

Exactly like an amoeba.

They literally wrap themselves around and ingest their food particles, bacteria, bits of organic stuff.

It's not like fungi, which secrete enzymes outside their bodies and absorb the digested nutrients.

Slime molds take the food inside.

Ah, OK, that's a fundamental difference right there.

It really is.

And you mentioned the classification confusion.

That sounds like it was a real mess for a while.

Oh, absolutely.

A proper taxonomic tangle.

I mean, the first recognizable description goes way, way back to 1654.

But it wasn't until Anton de Berry, you know, mid 19th century that we got really detailed studies of their life history.

And he actually put them in a group called Mysosoa, believing they were closer to protozoa, explicitly not fungi.

OK, so de Berry had it right early on.

He seemed to.

But then you had other really influential mycologists, like G .W.

Martin, arguing strongly that no, no, they belong with the fungi.

And for a long time, that view won out.

They ended up in mycology textbooks studied by mycologists.

So a real scientific debate.

But the modern view circles back to de Berry.

Pretty much, yeah.

New evidence, especially from molecular data, looking at their genes and some better grasp of evolutionary relationships, phylogenetic theory, it all confirms de Berry's original idea.

Tree slime molds are not fungi.

But being studied by mycologists probably kept them from being totally ignored, right?

That's a great point.

It's kind of an ironic twist of history.

That misconception might have actually saved the group from obscurity because mycologists were the ones out there finding them, describing them, cataloging them.

It's why even today, if you're out hunting for fungi, you're still quite likely to stumble across a slime mold.

Funny how science works sometimes.

So, OK, let's look at their life cycle.

What are the main stages?

You said it was complex.

It really is quite something.

There are basically four main phases or acts in their life story.

First, you have these tiny individual uninucleate cells.

Some can even swim with flagella.

Second, there's the phase most people picture, a much larger multinucleate stage called the plasmodium.

That's the blob.

Third, they have a super tough dormant stage called the sclerotium, kind of like a survival pod.

And finally, they have a reproductive phase where they transform into these stationary structures called scorophores, which produce and release the spores to start the cycle all over again.

OK, let's unpack those.

Starting with the spores, what are they like?

Tiny, I assume.

Oh, yeah.

Very small.

Typically, only four to 20 micrometers across.

So microscopic.

They're usually globe -shaped, and they have a definite thick wall.

And what about the surface?

Is it just smooth?

Ah, that's where it gets interesting.

Their surfaces can be incredibly varied.

Some are smooth, but others might be punctate, like covered in tiny dots or spiny, warty, or even form this intricate, net -like pattern or reticulum.

These patterns are actually really important for identifying different species.

Wow.

And you mentioned resilience earlier.

How tough are these spores?

Unbelievably tough.

Get this.

Some spores have been documented to germinate after being stored dry in a herbarium collection for, like,

61 to 75 years.

75 years.

That's astounding.

Like a seed, but even more patient.

Exactly.

It's a biological time capsule just waiting for the right conditions.

Oh, how do they get around once they're ready to disperse?

Mostly wind and water carry them.

But they're also great hitchhikers.

Animals, especially tiny insects like arthropods, can get covered in them.

Picture a little beetle crawling over a spore for, it gets dusted with spores, and then carries them off somewhere new.

A microscopic delivery service.

Okay, so a spore lands somewhere damp, maybe after decades.

What happens next?

Does it just inflate into the blob?

Not quite so fast.

When a spore germinates, usually needs a bit of moisture, like rainwater, it cracks open and releases either one or more mixamubi.

Those are the little amoeba -like cells, or flagellate swarm cells.

And what determines which type comes out?

It often depends on how much water is around.

If there's plenty of water, you're more likely to get the swarm cells, the ones with flagella that can swim.

If it's just damp, maybe you get the mixamubi that crawl.

Swarm cells.

So they actually swim around.

What's that like?

It's quite dynamic.

Under a microscope, you see them moving with this rapid rotary motion kind of spinning, but also contracting their bodies like an amoeba.

They usually have one long, obvious flagellum whipping around.

And sometimes there's a second shorter one kind of pressed against the cell body, harder to see.

And what are they doing while they're swimming or crawling around?

Eating.

Both the mixamubi and the swarm cells are phagotrophic, remember.

So they're actively hunting and engulfing bacteria, yeast cells, little bits of organic debris, even some fungal spores.

They apparently have a sticky rear end that helps them trap food particles.

Sticky rear ends, okay.

And what if conditions get bad again quickly?

Can these little cells tough it out?

They can.

The mixamubi, if things dry out or get unfavorable, can round up, form a protective wall, and become a microsyst.

It's like a mini -dormant stage just waiting for things to improve again.

Okay, so we have these individual amoeboid or swimming cells feeding, maybe taking little breaks in cysts.

How do they become the big plasmodium?

The main event, so to speak.

Right.

This is the really unique part.

These individual cells, the mixamubi or swarm cells, actually function as gametes.

When two compatible ones meet, they fuse together to form a zygote.

Like sexual reproduction.

Exactly.

It's a sexual fusion.

Interestingly, it seems they don't just fuse instantly on contact.

There's often an induction period needed, maybe several hours, and sometimes they need to reach a certain density, a critical mass of cells, before fusion happens efficiently.

Huh.

Okay, so a zygote forms, then what?

Then the growth phase begins.

The zygote starts feeding and growing, and its nucleus divides by mitosis.

But here's the crucial bit.

The nucleus divides, again and again, synchronously with all the other nuclei, but the cell itself doesn't divide.

Wait,

so millions of nuclei dividing at the same time, inside one continuous bag of cytoplasm?

Precisely.

You end up with this massive, amorphous blob of protoplasm, packed with potentially millions of genetically identical nuclei, all enclosed within just a thin outer membrane and a slime sheath.

That's the plasmodium.

One giant multinucleate supercell.

That's mind -bending.

What does it look like, typically?

Is it always slimy and blob -shaped?

It can be.

Often brilliantly colored, bright yellow, orange, red, sometimes violet, or even black.

It might be a thick, cushiony blob, or it could spread out incredibly thin, like a living lace network over logs or leaves, covering quite large areas.

And how does this giant supercell move around to find food?

This is maybe the most mesmerizing part.

Protoplasmic streaming.

If you watch it under a microscope, it's unbelievable.

The internal cytoplasm flows rapidly, like a river, in one direction, sometimes up to 1 .35 millimeters per second, which is super fast at that scale.

Wow.

Then it slows down, stops, and reverses, flowing back in the other direction.

This rhythmic shuttle streaming is constant.

It's driven by actin and myosin filaments, the same proteins that power our muscles, orchestrated by calcium levels.

So it's literally pulsing its way across surfaces.

Exactly.

It creeps along, engulfing food particles, more bacteria, fungi, protozoa, decaying bits.

It also secretes enzymes to help break down food externally before engulfing or absorbing the nutrients.

It's a very efficient feeding machine.

And you mentioned different types.

Yes.

Based on their appearance and streaming patterns, we classify them roughly.

There's the fanoroplasmodium, which is the big, visible fan -shaped type many people see, like in phiserum.

Then there are others like the affenoplasmodium, which is very thin and transparent, almost invisible, and the protoplasmodium, which remains microscopic throughout its life.

And these different plasmodias, say from different species, they don't mix.

Generally no.

If two plasmodia from different species meet, they usually won't fuse together.

This incompatibility has actually been used sometimes to help figure out species boundaries.

That makes sense.

A biological recognition system.

Now, the synchronous nuclear division you mentioned,

that sounds useful for research.

Incredibly useful, especially in species like phiserum polycephalum, which is common and relatively easy to grow in the lab.

Because all the nuclei divide at the exact same time, it provides scientists with a massive, naturally synchronized population of nuclei to study the cell cycle mitosis, DNA replication, all the controls.

It's a fantastic model system for fundamental cell biology,

protoplasmic movement, even things like aging.

OK, so the plasmodium is the feeding, growing, moving stage.

But life isn't always easy.

What happens if conditions get really bad, like a drought or freezing temperatures?

Right, it can't just keep streaming along if it dries out or freezes.

That's where the sclerotium comes in.

The entire plasmodium can convert itself into this hardened,

irregular dormant mass.

So the whole blob just hardens up.

Pretty much.

It differentiates into small, walled compartments called macrosysts, each containing a few nuclei.

This sclerotium is highly resistant to adverse conditions.

It allows the slime mold to survive long periods like overwintering in cold places.

It's their ultimate pause button.

And when conditions improve.

It can wake up.

The sclerotium can absorb water.

The macrosysts break open and it reforms into an active streaming plasmodium again, ready to pick up where it left off.

Amazing adaptability.

OK, so it survives the tough times.

What triggers the final act?

The reproduction, the spora force.

That's usually triggered by some environmental cue.

Often when the plasmodium has grown large enough, but maybe starts running out of food or encounters changes in moisture, temperature or light, it's a signal that it's time to switch from feeding and growing to reproducing.

And this change, the conversion into spora force or fruiting bodies, is irreversible.

The whole plasmodium is consumed in the process.

The whole thing transforms.

And you said these spora force come in different shapes.

Yes, quite a variety.

And these forms are really key for identifying them.

They can be quite beautiful and intricate.

What are the main types we might see?

OK, four main categories.

The most common type is the sporangium plural sporangia.

These are typically small individual structures, often on tiny stalks, each with its own outer wall, the peridium.

Think of tiny little lollipops or balls on sticks.

Then you have the ephthalium plural, ephthalia.

This is where the entire plasmodium basically mounds up into one large cushion shaped fruiting body without fully dividing into separate little sporangia.

It might have a cortex on the outside, but inside it's more continuous.

The famous example here is phalligosceptica.

Ah, the blob, the bright yellow one.

That's the one sometimes called dog vomit slime mold, unfortunately.

It can form these really large startling yellow masses, sometimes several feet across, appearing seemingly overnight on lawns or mulch.

I remember hearing about that panic in Texas back in 73.

People thought it was alien.

Exactly.

That was phalligosceptica.

It looks alarming, especially when it's big and foamy looking, but it's completely harmless to plants, pets, and people, just unsightly perhaps.

Good to know.

What are the other types?

There's the pseudoethelium.

This looks a bit like an ethelium because it's a cluster of sporangia packed very tightly together, looking like a single unit.

But if you look closely, you can usually still make out the individual sporangia that make it up.

Okay, sort of halfway between the two.

Kind of, yeah.

And the last type is the plasmodium carp.

This one is really distinctive because the fruiting body retains the shape of the major veins of the plasmodium it came from.

So it often looks like a raised network or branched, elongated structures following the pattern of where the plasmodium was streaming.

Like a fossilized network?

Very cool.

And inside these structures, what's going on besides just spores?

Inside, along with the millions of spores, you often find other intricate structures.

There might be a collimella, which is like a central sterile pillar extending up from the stalk.

And very importantly, there's usually a capillitium.

Capillitium?

What's that?

It's a network of sterile threads kind of mixed in among the spores.

These threads can be simple or branched, smooth or spiny.

And they're often hygroscopic, meaning they move in response to changes in humidity.

They move.

Why?

To help disperse the spores.

As the capillitium twists and flicks around with humidity changes, it helps to break up the spore mass and fling the spores out into the air currents.

It's like a tiny built -in catapult system.

A spore catapult.

Nature thinks of everything.

And the spores themselves, that's where meiosis happens, the genetic shuffling.

Typically, yes, the reduction division, meiosis, usually occurs within the developing spores, often about 18 to 30 hours after they've initially formed inside the sporafor.

This restores the haploid state and creates genetic variation for the next generation.

OK, so we've traced this incredible journey from spore to amoeba to plasmodium to sclerotium and finally to sporafor and back to spore.

Where do these amazing organisms actually live?

Are they rare?

Not at all.

They are truly cosmopolitan, found all over the world in pretty much any terrestrial ecosystem you can think of.

Moist forests are classic habitats on decaying logs, leaf litter, bark, but also tropical forests, grasslands, even alpine zones and deserts.

In deserts, you might find them on decaying cactus tissues, for example.

Well, even in my backyard.

Quite possibly, especially if you have mulch like wood chips.

Certain species like phiserum scenarium commonly show up on lawns, sometimes forming those

patches or fuligo on mulch beds.

So yeah, they could be right under your nose.

OK, so they're everywhere.

But ecologically, what's their main role?

Do they do anything important?

Well, they don't have huge direct economic importance for us humans, not like fungi used for food or medicine.

But ecologically, they're definitely important players.

They are major consumers of bacteria, yeasts, protozoa and fungal spores.

So they're part of the decomposition process helping to cycle nutrients.

And they themselves are food for other creatures, especially small arthropods like insects and mites.

So part of the micro food web.

Exactly.

And as we mentioned, their appearance can sometimes cause alarm, like fuligo the blod or the phiserum on lawns, but mostly harmless, just doing their thing.

The advice for the lawn patches is usually just to mow it or rake it up if it bothers you.

It doesn't harm the grass.

Good to know.

Easy fix.

Now, you've hinted at their scientific importance several times.

Let's circle back to that.

Why are they such superstars in the lab?

It really comes down to that clasmodium stage, especially in species like phiserum polycephalum.

That natural thievery of millions of nuclei dividing at once is just an unparalleled gift for cell biologists studying mitosis.

A living experiment.

Totally.

It lets you study the biochemistry, the structural changes, the genetic controls of the cell cycle in bulk in real time.

It's also fantastic for studying protoplasmic streaming, how cytoplasm moves, the mechanics of it.

And it's been used to study cell differentiation, how this seemingly simple blob can transform into complex sporophores, and even basic learning or habituation in cellular aging.

Learning in a slime mold.

Well, maybe habituation is a better word.

They show chemotaxis moving towards food sources like sugars and away from repellents.

Studies have shown they can kind of remember paths or learn to ignore repeated unpleasant stimuli.

It challenges our ideas about intelligence needing a brain.

That is fascinating.

And you mentioned something about salmonella.

Oh, right.

Yeah.

There was research showing that phiserum polycephalum plasmodia could actually distinguish between different strains of salmonella bacteria isolated from poultry based on chemical cues, which is pretty remarkable chemical sensitivity.

Wow.

So scientific workhorse, potential allergen, any other connections to humans.

You didn't mention food again.

Ah, yes.

The food question.

Believe it or not, there are reports.

The young developing athelia of one species, enteridium lecopridone, are apparently collected and eaten fried in parts of Veracruz, Mexico.

Fried slime mold.

What do they call it?

The local name is reportedly cacadeluna.

Cacadeluna.

Moon poop.

Doesn't sound too appetizing, does it?

Slight chuckle.

Yeah.

No, I think I'll pass on the moon poop, thanks.

Anything else?

Allergies?

Yes, that's a possibility.

The spores are very small, easily airborne, and they have been implicated as potential aero allergens.

So for sensitive individuals, they could contribute to respiratory allergies, though it's probably not super common compared to pollen or fungal spores.

Okay.

So to wrap up then, we've journeyed through the world of the myxomycota, the true slime molds, not fungi, despite the history.

Right.

We've seen their incredible life cycle, the resilient spore, the crawling or swimming amoeba, the amazing multinucleate plasmodium that streams and feeds.

The Tufts chlorotium for survival, and those diverse, often beautiful sporophores for reproduction, complete with spore catapults.

And we've touched on their global presence, their ecological roles in decomposition and food webs, and their surprising stardom as model organisms in biological research, helping us understand fundamental processes of life.

It really makes you think, doesn't it, if an organism that seems so simple on the surface, like a slime mold, holds these incredibly complex,

adaptable, almost intelligent biological systems?

Yeah.

What else are we missing?

What other fundamental rules of life can we uncover by looking more closely at these less obvious, often overlooked corners of the natural world?

A really great question to ponder.

It definitely gives you a new appreciation for that slimy patch on a log.

Hopefully.

They're truly wonders.

Well, thank you for joining us on this fascinating deep dive into the world of true slime molds.

Until next time, keep exploring the hidden wonders of our world.