Chapter 2: Protozoa: Myxomycota (slime moulds)

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Welcome to the Deep Dive.

Today we're about to explore a really surprising corner of the biological world.

We're diving into a chapter you'd usually find in an introduction to fungi, but here's the twist.

Our subject isn't actually a true fungus at all.

Nope.

We're talking about the, well, truly fascinating, often misunderstood, and just completely unique group of organisms called slime molds.

That's them.

So our mission today, unpack the science behind these creatures, understand their kind of bizarre biology, and figure out why they were historically mixed up with fungi.

We want to paint a picture for you, you know, without needing any diagrams.

Right, it goes way back.

Early researchers, like Johann Link back in 1833, he coined Mixomycetes, and you can see why I write their fruit bodies.

They looked a bit like some fungi.

Ah, okay, so the name stuck.

Yeah, that mixomycetes suffix.

But even by 1887, scientists like de Berry were already thinking, hmm, maybe these aren't related to the true fungi, the Umicota.

So if they're so different, why were they always in the fungi textbooks, just hanging out there?

Well, it was mostly practical.

They live in the same places.

Mycologists studying fungi would just keep finding them on decaying wood in soil.

So convenience, basically.

Pretty much.

But biologically, physiologically, ecologically, they're playing a totally different game.

How so?

What's the big difference?

Okay, number one, how they eat.

Slime molds use phagocytosis.

Phagocytosis?

Yeah, they literally engulf their food bacteria, yeast, other little amoebae.

They wrap around it and digest it inside their cells in little sacs called vacuoles.

Wow, okay.

Not like fungi at all, then.

Not at all.

Fungi secrete enzymes outside, break stuff down, and then just absorb the nutrients.

Slime molds are predators on a microscopic scale.

Plus, their forms are so varied.

You get single amoebae, these huge multinucleate blobs called Plasmodia, even swimming stages with flagella.

Flagella, like little tails for swimming.

Exactly.

Their evolutionary history is still, well, let's say it's spread out, not one neat branch.

Okay, that definitely sets them apart.

So let's dive into that diversity.

Where should we start?

Maybe with the more individual ones.

Good place.

Let's start with the cellular slime molds.

First up, the acrosiomycetes.

Acrosiomycetes.

Got it.

These are a pretty small group, technically protozoa.

Their feeding stage is these cool limax -type amoebae.

Limax -type, like slugs.

Yeah, kind of.

Imagine a tiny cylinder moving like a slug.

It pushes out a single big pseudopod, a temporary foot at the front, and all its insides kind of trail behind.

Weirdly specific.

Where would you find these little slug amoebae?

Common on decaying plants, soil, dung.

But they're tiny.

You really need a scope to see them.

Any exceptions.

Anything visible.

Well, there's one across this rosea.

It can be orange or pink because it has carotenoid pigments.

So that one you might spot.

Okay.

And their life cycle, you said they're individualists.

Right.

So these single amoebae cruise around eating bacteria and yeast.

If things get tough, like it dries out, they form a little protective shell called a microsyst.

Okay, standard survival tactic.

But here's the cool part.

When they start starving, these independent amoebaes, they aggregate.

They come together.

Yeah.

They gather into a group called a pseudoplasmodium.

Pseudo, meaning false.

Why false?

Because even though they're all packed together in this common slimy sheath, each amoeba keeps its own identity.

They don't fuse.

They're just cooperating.

Ah, a temporary team.

Exactly.

And this team then builds a structure, a branched thing called a serocarp.

The amoebae climb up, round off, and become spores.

Interestingly, even the cells that make the stock can sometimes germinate later.

So they cooperate to reproduce.

Do we know what it tells them to get together?

For the acrises.

Still a mystery, actually.

We don't know the chemical signal.

And we haven't seen any sexual reproduction in this group either.

Fascinating.

Okay, from the individualists, let's go to the more social ones.

The Dictyosteleomycetes.

Yes, the social amoebae.

These are super important in research labs.

Dictyosteleum discoidium is the famous one, right?

That's the one.

Everyone studies dictyosteleum.

It's serocarp stock, for example, is multicellular.

Built from cellulose walls, the amoebae themselves secrete.

And where do they live?

Same places.

Yep.

Soil decaying plants.

Very common.

But their amoebae look different from the acrises.

They have these pointy phyllosodopods.

Okay, so pointy feet instead of blobby feet.

What about their life cycle basics?

Starts with spores, like the others.

They germinate into single haploid amoebae.

They eat bacteria by phagocytosis.

So engulfing again.

Right.

They divide asexually to make more amoebae.

And yes, they can form microcysts when things are bad, sometimes triggered by ammonia buildup.

Okay, similar pattern so far.

What about sex?

Ah, that's different.

It happens via these things called macrocysts.

Big cysts.

How do they form?

Two compatible amoebae fused together.

Just two.

This makes a giant cell.

The giant cell.

Yeah, much bigger.

And this giant cell then attracts other amoebae, unfused ones.

They all clump together, form a thick wall around the group.

And inside?

Inside, the nuclei from the original two fused amoebae finally merge.

That's karyogamy.

Now you have a diploid zygote.

Okay.

And this zygote then, well, it eats the other amoebae trapped inside with it.

Whoa, cannibalism.

Yep.

Feeds on them, grows, then undergoes meiosis, then mitosis, making lots of new haploid amoebae that eventually get released.

That's intense.

What triggers the initial fusion?

Interestingly, light and camp E actually inhibit it.

But ethylene gas emulates it.

Weird triggers.

But this leads us to the really famous part, right?

The aggregation.

Absolutely.

This is what makes Dictyostelium so fascinating for studying cooperation and development.

So what happens when the food runs out for these guys?

Not the sexual macrosyst, but just normal life.

They aggregate again, but this is a vegetative process.

No sex involved.

It's purely about survival and dispersal when starvation hits.

And how do they know where to go?

Chemical signals again.

It's chemotaxis.

They move towards a hormone gradient.

For Dictyostelium discoadium, that hormone is cyclic AMP or camp E.

Camp E, like the signaling molecule in our cells.

The very same.

Starving amoebae start pumping out camp LP.

Others sense it, change shape, and start moving towards the source, the highest concentration.

Imagine waves of amoebae pulsing inwards, drawn by the camp E, hundreds of thousands of them from maybe a square centimeter of soil.

Wow.

They stream together, stick to each other, secrete slime, and pile up into this dense bullet -shaped structure, the slug.

The slug.

Okay, so now we have this collective slug made of individual amoebae.

What does it do?

It moves.

It migrates, usually towards light.

It leaves a slime trail.

You can actually see it.

And the amoebae inside, still individuals.

Still individuals.

But they start to differentiate.

They're not all the same anymore.

You get pre -stock cells at the front.

They're bigger, secrete camp MP to guide the slug.

And pre -spor cells at the back.

So they have different jobs already.

What guides the slug's movement?

Environmental cues, light, like I said, but also oxygen levels, ammonia, temperature.

It's navigating its little world.

And then what?

It can't stay a slug forever.

No.

The migration ends with culmination.

The slug stops, rounds up, points upwards, and builds the final structure.

The pre -stock cells form the multicellular stock, and the pre -spor cells become the spores in the source at the top.

But wait.

The stock cells.

What happens to them?

Ah, that's the sacrifice.

About 80 % of the amoebae become spores the next generation.

The other 20%, the ones that built the stock, they die.

They sacrifice themselves for the others.

Essentially, yes.

They form the structure that lifts the spores up for better dispersal.

It's altruism on a cellular level.

Which immediately makes you think, could some cheat the system?

Exactly.

That's the cheater strain phenomenon.

If you're a stock cell, you die.

Your genes don't pass on directly.

So an amoeba that could somehow avoid becoming a stock cell, but still benefit from the stock built by others, it would have an advantage.

Right.

It gets its spores dispersed without paying the price.

And these cheater strains exist.

In nature.

In the lab.

Some only cheat a bit.

Some only cheat if non -cheaters are around.

Some can't make a stock at all.

Wow.

So studying Dikti teaches us about the evolution of cooperation and how social systems deal with cheaters.

Absolutely.

It's a fantastic model system for those kinds of big evolutionary questions.

How does cooperation evolve and remain stable?

Beyond social stuff, you mentioned human health.

How does Dikti connect there?

Well, remember phagocytosis?

How Dikti eats bacteria?

Yeah.

Engulfing them.

It turns out the way Dikti interacts with certain bacteria that also infect humans like pseudomonas or legionella is strikingly similar to how our own immune cells, our phagocytes, interact with them.

Really?

Yeah.

Some of these pathogens can actually kill Dikti after being eaten, just like they can sometimes survive inside human immune cells.

Studying these interactions in Dikti is giving us clues about infectious diseases.

That's amazing.

Using a slime mold to understand human infections.

It's a powerful tool.

Okay.

Incredible.

Let's shift gears now to the plasmodial slime molds.

You mentioned the protosteliomyces kind of bridge the gap.

Exactly.

Protosteliomyces.

They're found everywhere, literally pole to pole.

Like Dikti, their amoebae have those pointy, fallow pseudopods and they eat by phagocytosis.

But what makes them a bridge?

Some of them can form small plasmodia.

Not the huge ones yet, but definitely multinucleate structures.

So they show links to both the cellular types and the bigger plaque module types.

Okay.

And how do they make spores?

It's delicate.

A single feeding amoeba or maybe a small plasmodium rounds up, it becomes a prespoor cell, then it secretes a stalk.

But this stalk is a cellular.

It's not made of cells, it's just a secreted tube.

A non -living stalk.

Right.

The cell climbs up the stalk as it's being built and forms usually just one spore, maybe a few, at the very top in a tiny sporangium.

Can you give us a sense of scale, like protostelium?

Okay, picture this.

A stalk may be 75 micrometers long, thinner than your hair, and perched on top.

A single, tiny, spherical spore.

It's microscopic elegance.

How does it even build that stalk?

It's pretty neat.

The amoeba shapes this hollow tube from its own internal material.

The protoplasm actually flows up the inside of the tube as it extends, driven by actin and myosin, like tiny muscles.

Then pop, it forms the spore at the tip.

Amazing mechanics.

Any other notable ones in this sort of intermediate zone?

Well, there's serratia mixofridiculosa.

Its classification is a bit debated, which tells you something about slime mold evolution being tricky.

What's special about it?

It forms spores externally, all over its surface, not inside a sack.

And these release swimming swarmers with flagella.

It does have a proper diploid plasmodial stage too.

What does it look like?

You might see it on wet rotting wood.

It looks like little patches of white or translucent frost, almost like tiny pillars.

Okay, interesting.

Now let's get to the main event for plasmodial types.

The mixomycetes.

The true plasmodial slime molds.

Yes, the mixomycetes.

This is the biggest group and probably what most people think of if they think of slime molds at all.

And their defining feature?

The plasmodium.

A big, free -living, multi -nucleate mass of protoplasm.

No cell walls dividing it up.

Just one giant, creeping, feeding supercell.

Wow.

Where do we find these?

Very common on moist decaying wood, leaf litter, bark mulch.

Sometimes they look like, well, like colorful slime.

Yellows, oranges, reds.

You said they can be big.

Are there different kinds of plasmodia?

Yeah, we classify them based on appearance of structure.

You have protoplasmodia.

These are microscopic, really simple, and usually just turn into a single sporangium later.

Hard to spot.

Okay.

Then affenoplasmodia.

Affenote meaning hidden or indistinct.

These form a very thin, transparent network of strands, like a delicate, almost invisible net.

You'd need a scope again.

Right.

And then the stars.

Phaneroplasmodia.

Phanero meaning visible.

These are the ones you can actually see easily.

What do they look like?

Large sheets or networks of thick veins, often brightly colored.

And the amazing thing is you can see the protoplasm streaming inside the veins.

Yeah, rhythmic back and forth pulsing.

It flows one way for 60, 90 seconds, pauses, then flows back the other way.

It's called shuttle streaming.

What causes that?

It's driven by actin and myosin again, interacting with calcium ions.

It's how the plasmodium moves and circulates nutrients through its huge body.

Incredible.

Okay.

So walk us through a typical mixomycete life cycle.

Let's use vicerum polycephalum, the lab rat one.

Okay.

So the big visible thing is the plasmodium and it's diploid.

It creeps around engulfing food phagocytosis again.

Right.

When conditions are right, it decides to reproduce.

It forms a spore for a spore -bearing structure.

Inside this, meiosis happens, producing haploid spores.

And the spores get out.

Dispersed by wind, maybe insects.

If they land somewhere good, they germinate.

Into what?

Either a little amoeba called mixomobae, or swimming cells with two flagellas called zoospores or swarmers.

Amoebae or swimmers, can they switch?

Yep.

They can interconvert depending on moisture.

And both can divide to make more of themselves.

If times get tough, they can also form those protective microsists.

Okay.

So we have these haploid amoebae or swarmers.

How do we get back to the big plasmodium?

Sex.

Two compatible haploid cells, either two mixomobae, two swarmers, or one of each have to find each other and fuse.

That makes the zygote.

Makes the diploid zygote.

And the zygote then grows, its nucleus divides over and over without the cell dividing, and it develops into the large multinucleate plasmodium.

A very different path to a big body.

What about survival for the plasmodium itself, if conditions get bad after it's formed?

Ah, they have another trick.

The whole plasmodium can transform into a sclerotium.

Sclerotium?

It's a hardened, dormant mass.

The plasmodium breaks up internally into little compartments, each surrounded by a wall.

It can survive drought, cold, starvation like this for ages.

And then come back?

Yeah.

When conditions improve, the protoplasts inside emerge, fuse back together, and reconstitute the plasmodium.

Very resilient.

And when it finally makes spores, does it leave anything behind?

Often, yes.

When the plasmodium converts into sporangia, it frequently deposits a thin layer on the surface it was sitting on, called the hypothallus, like a footprint.

What triggers the decision to make spores instead of just becoming a sclerotium?

Usually environmental cues, again.

Things like running out of food, changes in moisture, and very often, light is a key trigger.

Now, these sporophores, the fruiting bodies,

they're not all the same shape, are they?

Not at all.

Lots of variety.

The most common is the sporangium, basically a little stocked container full of spores.

But sometimes, the entire plasmodium just mounds up and converts into one big, cushion -like structure.

That's an athelium.

Like the dog vomit one.

Exactly.

Phyllogosceptica, the bright yellow blob, sometimes seen on mulch, that's a classic athelium, a giant communal spore structure.

Any others?

You also get pseudo -athelia, which look like fused sporangia, and plasmodio carps, where the spore -bearing structure retains the shape of the plasmodial veins it formed from.

So much variety just in the spore structures.

Any cool dispersal tricks?

Oh yeah.

Some genera, like Trichia, have this network of threads mixed in with the spores inside the sporangium.

It's called the capillitium.

Capillitium.

Yeah.

When the sporangium breaks open, these threads absorb moisture and twist and writhe hygroscopically, meaning in response to humidity changes.

Like little springs.

Kinda.

They act like elaters, fluffing up the spore mass and helping to release the spores gradually over time, not all at once, ensures better dispersal chances.

That is clever.

And you mentioned spores surviving for ages.

Incredible resilience.

There are documented cases of mixomycetus spores from herbarium specimens germinating successfully after more than 50 years of dry storage.

50 years?

Wow.

Okay, let's really focus on phasor and polycephalum.

You said as a lab star, why?

Well, for several reasons.

First, that shuttle streaming we talked about.

The pulsing cyplasm.

Yeah.

It's a fantastic system to study cell motility, the role of actin, myosin, calcium ions.

And interestingly, in phasorum, higher calcium inhibits the contraction, unlike an animal muscle where it stimulates it.

It's more like cytoplasmic streaming in plants.

Okay.

Fundamental cell mechanics.

What else?

Synchronized mitosis.

This was huge.

Phasorum was one of the first organisms where the cell cycle was really demonstrated.

How so?

Because it's one giant cell with millions of nuclei, and all those nuclei divide at exactly the same time.

Perfectly synchronized across the entire plasmodium.

All at once.

How?

It's regulated by a protein kinase, a master switch.

And that kinase turned out to be basically the same as a key cell cycle regulator discovered in yeast.

Showed a deep evolutionary connection in how cells control division.

Amazing.

What about behavior?

You hinted at something.

Oh, the smart network stuff.

This is mind bending.

Put a phasorum plasmodium in a maze at the exit.

It will explore the maze, and the plasmodial tubes will eventually reorganize to form the shortest possible path between the start and the food.

It solves the maze.

It seems to reinforce pathways that efficiently transport nutrients and prune back ones that don't.

It optimizes its network.

You can give it multiple food sources scattered around, and it will connect them with a network of veins that approximates the most efficient network, mathematically speaking, balancing connection length and breaks.

That sounds like intelligence.

Or problem solving.

It certainly looks like it.

It raises huge questions about how complex adaptive behaviors can emerge from seemingly simple biological rules without a brain or nervous system.

It's used as a model for network design, even for things like transport systems.

Wild.

Anything else from the phasorum lab?

One more key thing.

Plasmodial incompatibility.

What happens when two different phasorum plasmodium eat?

Do they merge?

Sometimes.

If they're genetically compatible, their veins fuse, they become one bigger plasmodium.

That's a compatible reaction.

And if they're not?

Then you get a lethal reaction they might touch and pull away.

These are often both plasmodia.

They reject each other.

Like tissue rejection in animals.

Exactly.

It's controlled by specific genes, similar to immune system compatibility or blood groups in humans.

It's a fundamental biological process, recognizing self versus non -self playing out in a slime mold.

But that's strange, isn't it?

Because earlier you said fusion between different amoeba is needed for sex.

Ah, you caught the paradox.

Yes.

At the haploid stage, fusion between genetically different mating types is required for sexual reproduction.

It promotes diversity.

But at the diploid plasmodial stage, fusion between genetically different individuals is prevented.

It maintains the integrity of the individual plasmodium.

So encourage mixing for sex, but prevent mixing during vegetative growth.

Fascinating biology.

It really highlights the different selective pressures at different life stages.

Well, this has been an absolutely mind -extending dive into the world of slime molds.

We've seen they're definitely not fungi.

Fundamentally different.

Phagocytosis, unique life cycles, motility.

We went from single amoebae, some cooperating, some sacrificing themselves.

Like the social amoebae Dictyostelium, teaching us about cheating and cooperation.

To these incredible giant pulsating plasmodia like Pfizerum.

That can solve mazes, build efficient networks, and show us fundamental principles of cell biology, like synchronized division and non -self recognition.

What's the big takeaway for you?

For me, it's just the sheer unexpected complexity and, frankly, cleverness you find in these organisms we often just overlook.

I agree.

They demonstrate how incredibly diverse life solutions can be.

And how studying these seemingly simple organisms can give us profound insights into really complex biological processes, from cell movement to social behavior to maybe even computation.

Makes you wonder what else they, or other overlooked microscopic life, might still have to teach us, doesn't it?

Absolutely.

There's always more to discover.

Well, thanks for diving deep with us today into the weird and wonderful world of slime molds.

Until next time, keep exploring, keep questioning, and stay curious.