Chapter 25: Phylum Labyrinthulomycota: Net Slime Molds

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Welcome curious minds to another deep dive.

Today we're plunging beneath the surface, way down into the watery parts of our planet.

We're exploring this hidden world, you know, teeming with life that often just slips under the radar.

Organisms that really defy easy labels.

They force scientists to, well, rethink how they even define life kingdoms.

Our subject for this deep dive, the phylum labranthilomycota.

You might sometimes hear them called net slime molds.

So our mission today, we're sifting through the source materials, actually a chapter from an introductory mycology textbook.

We want to pull out the really key insights about these pretty peculiar organisms.

We'll uncover their surprising classification story, their unique structures, how they reproduce, and yeah, their critical roles out there in the world.

Okay, let's unpack this then.

Why were these organisms so tricky?

Like what makes them stand out so much from everything else we think we know?

That's exactly the right question to start with.

For ages, I mean, decades, they were a real taxonomic headache, just bounced around different categories like a, well, a biological pinball, as you said.

But as we'll dig into, careful science, looking really closely, eventually gave us a much clearer picture of their, frankly,

fascinating biology.

We'll walk you through it.

So classification.

Why the confusion?

Well, yeah.

It's kind of wild to think about.

Yeah.

They were put in with plants at one point, then fungi, Protozoan's other protists.

It really shows you how hard defining life can be, doesn't it?

Even with modern tools.

It absolutely does.

Yeah.

For a time, they were grouped with Umicides, you know, water molds and their relatives.

That was mostly based on how their flagella looked, the little swimming tails.

Sparrow noted that back in 76.

But the real key, the thing that shifted the thinking, was looking at their ultrastructural morphology.

Ultrastructural morphology.

Okay.

Super detailed cell structure, right?

Like with powerful microscopes.

Exactly.

Getting down to the nitty gritty details of the cell components.

And that deep dive is what led to where we place them now, the classification that's pretty widely accepted.

They're considered straminopiles.

Staminopiles.

Okay.

That might be a new term for some listeners.

Yeah.

What does that actually group together?

Right.

So it's a really diverse bunch of eukaryotes organisms with complex cells.

Think everything from giant kelp, the huge brown seaweeds, down to microscopic diatoms.

What unites them are some very specific cellular features, especially related to their flagella having these tiny hair -like projections.

So it tells us something fundamental about their evolutionary history and cell design, even if the group looks really varied on the surface.

Okay.

So straminopiles.

That gives us their address in the big tree of life.

But what about the labyrintholomide coda themselves?

What are the defining features that pull them together as a distinct phylum?

What's their signature trait?

Yeah.

There are two really key things historically.

First and maybe the most visually striking is the ectoplasmic network.

Picture this intricate branching interconnected web made of these super thin filaments.

But importantly, they don't have rigid walls like a typical cell.

And this network extends outside the main cell body.

Outside the cell.

Wow.

How do they make it?

It's produced by the cells using a specialized spot on their surface.

It's called a bothrosome.

Sometimes you'll see the term cigenogen.

Think of it as a little indentation or maybe like a specialized port on the cell surface where this whole network originates and grows from.

It's like they build their own external highway system.

A highway system.

That's a great way to put it.

Okay, what's the second key feature?

The second thing is their cell walls.

They do have walls, but they're really unusual.

They're made of Golgi -derived scales.

You know the Golgi apparatus in the cell, the part that packages things up?

Yeah, like the cell's post office.

Right.

Well, these guys use it to build their walls out of tiny scales.

Very different from, say, the chitin in fungi or cellulose in plants.

And then there are their zoospores.

These are the little cells they use to move around and colonize new places.

Zoospores, okay.

The swimming ones.

Exactly.

They are hetero -cont and biflagellate.

Whoa, okay.

Break those down.

Sure.

Biflagellate just means they have two flagella, two tails.

Heterocont means those two flagellars are different.

They usually stick out from the side of the zoospore.

One is longer and it's called a tinsel flagellum because it's covered in these fine hair -like things.

The other one is shorter and smooth.

That's the whiplash flagellum.

This whole setup is pretty distinctive, though it does look a bit like the secondary zoospores you see in Oomycetes.

Okay, so the unique walls, this external network, and specific swimming cells with different flagella.

Interesting combo.

Where do they actually live with all this unique gear and what do they, you know, eat?

Good questions.

Their prime real estate is mostly estuarine and nearshore marine environments.

So think shallow coastal waters, places where rivers meet the sea, maybe a bit brackish.

You often find them living on, or at least associated with, things like decaying leaves of sea grasses or mangroves, bits of algae, and other organic debris floating around.

So decomposers mostly.

Primarily, yes.

Most are sap robes, meaning they feed on dead stuff, breaking it down.

They do this through absorptive nutrition, secreting enzymes out and soaking up the digested nutrients.

Some are considered weak parasites, maybe causing minor issues for algae or plants, but still feeding by absorption.

There's even some evidence suggesting some might live inside algae cells endobiotically, but the algae seem to have ways to keep them in check, defense mechanisms.

Fascinating.

OK, now the outline mentions two main families within this phylum, labyrinthylaceae and throstocytriaceae.

This is where it gets even more specific, right?

That's right.

Porter recognized these two main families back in 1990, labyrinthylaceae and throstocytriaceae.

They're related, definitely, but they've specialized in slightly different ways.

The labyrinthylaceae family is simpler, taxonomically speaking.

Just one main genus, labyrinthola, with maybe 30 or so known species.

The throstocytriaceae are more diverse, seven genera listed in our source.

With throstocytrium being the most common genus, it holds about half the species in that family.

Let's tackle labyrinthylaceae first.

The true net slime molds.

Now, you mentioned the net part makes sense with the ectoplasmic network, but the slime mold bit, you said that causes confusion.

Oh, absolutely.

Big time confusion.

Yes, labyrinthola species produce this ectoplasmic network, and it can look slimy and net -like under the microscope.

You can picture it, right?

This sort of glistening mesh.

But calling them slime molds lumps them in with like five other completely different groups that people also call slime molds, like the big plasmodial ones you see on logs or the cellular ones.

Labyrinthola is not closely related to any of them, so it's a descriptive nickname that's stuck.

But taxonomically, it's misleading.

Got it.

So besides the potentially confusing name, what makes labyrinthola species distinct within the phylum?

Well, their main vegetative cells, the ones doing the growing and feeding, are typically spindle -shaped, kind of elongated, pointed at both ends.

They also produce those zoospores we talked about.

But uniquely in labyrinthola, these zoospores often have an eye spot.

An eye spot.

Yeah.

Like to sense light in a microscopic organism like this.

Exactly.

A little photoreceptive organelle, pretty sophisticated.

And some labyrinthola species are known to reproduce sexually, which isn't seen in the other family.

Plus, their ectoplasmic network tends to be really extensive, often completely surrounding and enveloping the spindle cells moving within it.

Okay, let's talk reproduction then.

How do they make more labyrinthola?

Asexualy, the main way is simple binary fission.

One cell just splits into two.

It's usually a transverse split across the middle that then kind of rotates to look diagonal.

When they form zoospores for dispersal, it's through something called successive bipartition.

That means the nucleus divides and then immediately the cell divides.

Nucleus divides, cell divides, it rapidly increases the number of cells.

And if bits of the network break off with cells inside, those can start new colonies too.

And the sexual reproduction, you said, that's special.

It is.

It's been studied in detail in labyrinthola vitalina.

What happens is a bunch of these spindle cells clump together, they round up, get bigger and become sporangus structures that will produce spores.

And here's the cool part.

Researchers actually observed synaptonemal complexes during the first nuclear division inside these structures.

Synaptonemal complexes, that sounds technical.

What does that tell us?

It's a very specific protein structure that forms between paired chromosomes during meiosis, the type of cell division that halves the chromosome number for sexual reproduction.

Seeing those complexes is really strong evidence that meiosis is happening.

They even counted the haploid chromosome number as nine in that species.

It's quite something.

This kind of meiosis, followed by that successive bipartition to form the spores, is rare in fungus -like organisms and doesn't happen in true fungi at all.

It really sets them apart.

Where exactly fertilization happens, though, plasmogamy and karyogamy, that's still a little bit of a mystery.

Wow.

And those zoospores produce sexually.

Yeah.

They have eye spots too.

Yep.

The zoospores, these non -walled swimming cells with their heterochont flagella, each have an eye spot.

This allows them to show positive phototaxis.

They actively swim towards light sources.

Then when they find a good spot, they settle down, differentiate back into those spindle -shaped vegetative cells, and they actually lose the eye spot, the flagella and the centrioles involved in making them.

Incredible complexity in such tiny things.

Okay, this isn't just academic biology, though.

You mentioned real -world impact.

This is where the eelgrass story comes in.

Exactly.

This is probably their most famous or infamous ecological role.

It involves labyrinthal azosterae and the eelgrass wasting disease.

Eelgrass zoster marina is super important.

It's a marine flowering plant, forms these huge underwater meadows.

Critical habitats, right?

Like nurseries for fish and shellfish.

Absolutely vital.

Nurseries for shrimp, scallops, fish.

They stabilize sediment, improve water quality, really crucial coastal ecosystems.

But back in the 1930s, there were these massive catastrophic die -offs of eelgrass on both sides of the Atlantic, just wiped out huge areas.

It was devastating.

And labyrinthala was suspected.

It was.

Species of labyrinthala were always found on the diseased plants.

But proving it was the cause was tricky.

Definitive proof only came much later, in 1988.

Then they identified the specific culprit as a new species, labyrinthal azosterae, described in 1991.

And here's where the biology connects.

That ectoplasmic network seems to be key to its pathogenicity.

How so?

The highway system helps it attack.

It seems that way.

The network appears to be involved in secreting enzymes that break down the eelgrass cell walls.

This basically clears a path, allowing the spindle -shaped labyrinthala cells to glide right into the host tissue, causing the characteristic lesions and the wasting disease.

Wow.

So the network isn't just for moving around and eating dead stuff.

It's a weapon in this case.

Can you elaborate a bit more on how that network and movement actually works?

It sounds amazing.

It really is.

So those filaments are produced right from the bothrosomes on the cell's plasma membrane.

The filament itself is basically an extension of the cell's outer membrane bordering the environment.

But inside the network, there's another plasma membrane that encloses the actual spindle -shaped cells, keeping them sort of isolated within pockets inside the filaments.

Porter suggested back in 72 that this network is crucial for everything.

Motility, getting nutrition, sticking to surfaces, maybe even cell -to -cell communication.

And the cells themselves, these spindle -shaped things, described as having an aerodynamically sound design.

Which is kind of cool.

They exhibit this gliding motility within the network's filaments.

Gliding.

How fast?

Pretty fast for their size.

Up to 100 micrometers per minute.

That's rapid transit on a microscopic scale.

The mechanism isn't fully understood, but it likely involves some kind of contractile system.

Maybe using actin -like proteins, pulling the cells along inside their network tracks.

Okay.

That covers the labyrinthalaceae.

The spindle cells, the big network, the eye spots, and that impactful eelgrass disease.

Now let's switch gears to their relatives.

The throstichytraceae.

What's their story?

Right.

The throstichytrids.

They're also found pretty much everywhere, in marine and estuarine waters globally.

Their roles are varied, too.

Many are really important suprobic decomposers, breaking down organic bits and pieces, just like labyrinthala.

But some are known necrotrophic parasites.

They attack already weakened or dead tissues, particularly on things like shell -less mollusks, nudibranches, octopus' squids.

They've also been found in the guts of sea cucumbers and living on sponges, though what they're doing there isn't always clear.

Interestingly, you rarely see them directly in nature.

Most of what we know about them comes from lab studies, often using pollen grains as bait to isolate them from water samples.

Pollen grains.

That specific bait.

Yeah.

Apparently they like it.

Helps researchers grow them.

Now structurally, they look quite different from labyrinthala.

Instead of spindle cells, they usually form a small, almost spherical structure described as globose.

This sphere -like body, or thallus, is surrounded by a wall made of those ultra -thin Golgi -derived scales, similar to labyrinthalaceae.

The exact chemistry can vary.

Some have algalactose, others sulfated galactan.

But here's a major difference.

Their ectoplasmic network is much less extensive.

It doesn't envelop the main cell body.

Okay, so where is it, then?

It typically extends out from the base of the globose thallus, primarily acting like an anchor, attaching the organism to whatever it's growing on, like a piece of debris.

In older books, you might see these attachment filaments called rhizoids, but they are still derived from a bothrosome.

Got it.

Less of a highway system, more of an anchor system.

What about reproduction and throstocatreds?

This is another key difference.

Asexual reproduction is basically the whole story for them, as far as we know.

Crucially, neither sexual reproduction nor eye spots have ever been documented in any throstocatred.

Big contrast to labyrinthala, most species just reproduce asexually by making those biflagellate heterochont zoospores inside the main globose body, which functions as a sporangium.

So the sphere swells up, makes spores, releases them.

Pretty much.

The zoospores swim off, find a new spot, settle down, lose their flagella, and then enlarge to become a new globose thallus.

And they can do this incredibly fast.

Porter noted some life cycles can be completed in less than 24 hours.

Wow, rapid turnover.

Any exceptions to the swimming spore rule?

Yes.

Interestingly, there are eplenopitreum and labyrinthaloids that don't make flagellated cells at all.

Instead, they produce spores or amoeboid -like cells that actually move on the surface of individual ectoplasmic network filaments.

It's described as a kind of jerky, gliding motility in labyrinthaloids.

So even within this family, there's variation on how they get around and reproduce.

Evolution, finding different paths again.

Fascinating.

So, summing up the throstic hydrates.

Globose decomposers are parasites.

Less network, no sex or eye spots known, mostly asexual spores,

but still ecologically important.

Definitely.

Vital decomposers, parasites, and some research even shows them involved in bioerosion, breaking down calcium carbonate shells.

So yes, ecologically significant in multiple ways.

Okay.

So let's pull this all together.

So we started this deep dive looking at organisms that were just, well, a taxonomic mess for a long time, bouncing between kingdoms.

And now we have this much clearer picture of their really unique biology, right?

Exactly.

We've seen how they fit as stramenopiles, but with their own very distinct toolkit.

That ectoplasmic network, produced by the Bothersome, is really the hallmark.

Then there are the Golgi -derived scales for cell walls, the unique hetero -cont flagellated zoo spores, sometimes with eye spots, sometimes not.

And crucially, their varied roles.

Essential marine decomposers, sometimes parasites, and even majorly pathogens like labyrinthal azostrea causing that devastating eelgrass wasting disease.

They're definitely not just slime molds.

Absolutely.

They are microscopic players with a massive impact.

It really makes you think.

Given the astonishing and often hidden complexity of organisms like these labyrinthal macota playing such critical roles, especially in marine ecosystems,

what other overlooked microbial players might be out there, quietly shaping our world in ways we're only just beginning to uncover.