Chapter 6: Chytridiomycota

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Welcome to the deep dive.

You know, when we talk about fungi, most people think of mushrooms.

Absolutely, the ones you see popping up after rain.

Exactly.

But that's just, well, the tip of the iceberg.

There's this vast, ancient and often totally hidden kingdom playing essential roles all around us.

A whole world beneath our feet, really.

Today, we're diving deep into one of the oldest, maybe most overlooked groups,

the chytridiomycota.

Most people just call them chytrids.

Yep, chytrids.

We've gone through a key

chapter from Introduction to Fungi by Webster and Weber to try and unpack just how fascinating these little guys are.

And our mission, really, is to take all that dense science and make it clear, make it make sense for you.

Right.

So by the end of this, you'll have a really solid grasp of what chytrids are, how they live, why they matter, and, you know, without needing to stare at complex diagrams.

Think of it as the audio guide to chytrids.

Exactly.

Your shortcut to getting properly clued in on this amazing part of nature.

Okay, so let's start at the beginning.

What exactly are chytrids?

I mean, you said they're a whole group.

They are.

It's an entire phylum.

We're talking over 900 known species grouped into five main orders.

So yeah, they might be microscopic, but there's a huge amount of diversity there.

Wow.

Okay.

And where do they live?

Are they fussy?

Not really.

They're incredibly adaptable.

Most of them grow aerobically, you know, needing oxygen.

You find them in soil, mud, fresh water.

Standard places.

Pretty standard, yeah.

But perhaps surprisingly, you also find them in estuaries, even out in the sea.

Oh.

Their defining feature, though, and this is something we'll keep coming back to, is how they reproduce.

They made these unique little swimming cells called zoospores.

Zoospores.

Yeah, and these typically have a single flagellum at the back end, kind of like a tiny tail or whip propelling them along.

A little motor.

Exactly.

Although, interestingly, some of them, the Neo -Calamastigales, actually have multiple flagellas, which is pretty unusual.

Okay, that is different.

So this little swimming cell with its whip -like tail,

what do the chytrids themselves eat?

Well, many are saprotrophs.

Saprotrophs, meaning they eat dead stuff.

Precisely.

They feed on decaying organic matter, and they're really good at breaking down tough materials.

Things like cellulose from dead plants or chitin from insect shells, keratin from hair and skin and soil and mud.

Real recyclers.

So they're cleaning up?

In a way, yes, but then others are the complete opposite.

Some are mycoparasites.

Needing.

They actually feed on other fungi, like colicutrium, for instance.

That's pretty cool.

Parasites of their own kind, almost.

Can you grow these in the lab easily?

You can, yeah.

It's often done using quite simple baits.

Things like bits of cellophane or hair, shrimp shells, even pollen grains floating on water.

Simple stuff.

Surprisingly simple.

But here's the catch.

Trying to classify them just based on what they look like, their thallus morphology, can be really tricky.

Why is that?

Because their appearance can change quite dramatically, depending on whether they're growing in a pure culture in the lab versus out in their natural habitat.

Oh, okay.

So they look different depending on the conditions.

Exactly.

And that's why, looking at the fine details, the ultrastructure of the zoospores themselves are so crucial.

That structure is much more stable.

It's like their fingerprint for classification.

Gotcha.

So the zoospore's key.

What about their cell walls?

Anything interesting there, given how their shape can change?

Oh, yeah.

The cell walls are fascinating.

They commonly have chitin, that tough polymer you find in insect exoskeletons, and it's sort of a hallmark of most true shangai, the umicota.

Right, chitin.

But, and this is where it gets really interesting.

Some chitrids, like one called gonopogia, also have cellulose in their walls.

Cellulose, like plants.

Exactly.

Cellulose is more typical of things like water molds, the umicota, not usually fungi, and some related groups, the hyphocatrium icota, even have both chitin and cellulose.

Wow, that's a real mix.

Hints at their ancient history, maybe.

Definitely suggests a unique evolutionary path, maybe branching off quite early.

Okay, let's unpack this.

How do they actually grow?

What does their body,

their thallus look like?

It varies quite a bit.

You have some like olpidium or synchidrium, which are biotrophic, meaning they live inside a host cell.

In these, the entire thallus gets converted into reproductive structures.

Zoonospores or maybe gametes or resting spores.

The whole thing?

The whole thing.

No separate bits for growing and though, the thallus is eucarpic.

That means it's differentiated.

You have distinct parts.

There are rhizoids.

Like little roots.

Kind of, yeah.

Root -like structures that anchor the fungus and soak up nutrients.

And then you have separate reproductive organs like sporangia, which produce the spores.

Makes sense.

And these eucarpic ones can be monocentric, just making one sporangium or polycentric, where they develop several sporangia from a more extensive network.

And their relationship with the host, if they have one, are they always completely inside?

Good question.

That's another key difference.

Sometimes the sporangium develops outside the host cell.

That's called epibiotic.

Only the rhizoids go inside to get food.

Epibiotic on the outside.

Right.

Other times the whole setup, rhizoids and sporangium is completely inside the host cell.

That's endobiotic.

Endo inside.

Got it.

So those zoospores you keep mentioning,

they really seem central.

What makes them so special?

They really are.

I mean, the zoospore is just this tiny, incredibly efficient machine.

Its size is usually pretty consistent for a given species, but the number of zoospores made inside one sporangium, that can vary wildly.

How so?

Well, take Rhizophyllictus roseae.

If it's growing on media that's low in carbohydrates, a tiny sporangium might only make one or two zoospores, but give it plenty of cellulose to munch on, and a large sporangium can churn out hundreds.

Hundreds.

How do they all get out?

Is it like an explosion?

It can be pretty dynamic, yeah.

Pressure builds up inside, and then these little exit points, papillae burst open.

And get this, the flagellum is coiled up inside the zoospore like a tiny watch spring.

Yeah.

And there's also this sort of spongy material inside the sporangium that swells up really quickly with water, helping to push the first zoospores out forcefully.

After that, the rest just swim or kind of wriggle their way out through the exit tube.

That's quite a system.

You said their internal structure is stable, good for classification.

Exactly.

While the overall body shape can vary, the zoospore's internal blueprint, its ultra structure, is much more consistent.

Two really key features are the flagellular apparatus,

the motor basically, and something called the Microbody Lipid Globul Complex, or MLC.

Okay, tell me about that flagellum.

What makes it tick?

It's a whiplash flagellum, pretty similar structurally to those in other eukaryotes, the classic 9 plus 2 arrangement of microtubules inside.

At its base, there's the kinetzone, the engine that generates it.

But here's something really cool.

In some species, researchers have found a second dormant kinetosome right next to the active one.

Dormant, meaning?

Meaning it's there, but not currently generating a flagellum.

This strongly suggests that the ancestors of today's chytrids might actually have had two flagellas, and one was lost somewhere along the evolutionary path.

Wow, a little evolutionary echo.

And the MLC,

what's that about?

The MLC is this complex unit involving a micro body, a big droplet of lipid that's energy storage and some membranes.

Crucially, it often includes the structure called a rumposome.

Sounds like something out of Dr.

Seuss.

Ha, it does a bit.

It has this sort of honeycomb -like porous structure, and the current thinking is that it's involved in sensing the environment.

It helps relay signals from the outside membrane to the flagellum, possibly by managing calcium ions.

Changes in external calcium directly affect how the flagellum beats, and therefore which way the zoospore swims.

So it helps them steer based on chemical signals.

That seems to be the idea.

It helps them navigate towards food or a potential host.

So all these bits working together, what does their swimming actually look like?

It's often described as jerky or hopping.

They can make these really abrupt changes in direction.

Very useful for navigating complex environments like soil water.

And what fuels this?

They burn energy stored in glycogen patches and those lipid globules we mentioned.

And their mitochondria, the powerhouses, are often clustered right near the kinetoe, near the flagellar motor for efficient power delivery.

Many also have this prominent nuclear cap surrounding the nucleus packed with RNA and ribosomes suggesting they're geared up for quick protein synthesis ready for action.

That's an incredible amount of complexity packed into one tiny cell.

So they swim around,

then what?

What kicks off the next stage?

Well, their swimming time varies.

Could be just a few minutes, could be several hours, but eventually before it can start growing, the zoospore has to settle down.

It finds a spot, stops swimming, and then it insists.

Insists, like forms a cyst.

Exactly.

It either pulls its flagellum inside or just sheds it completely, and then it forms a protective wall around itself.

It hunkers down.

Okay.

And from that cyst, what happens?

It depends on the chytrid.

For those holocarpic parasites, the insisted zoospore might just stick onto the host surface and inject its contents directly inside the host cell.

Simple as that.

Direct injection.

Yep.

In many of the monocentric types, the cyst itself just starts to enlarge and becomes a sporangium.

Or sometimes it forms an intermediate stage, a prosporangium, which then develops into the sporangium.

For the polycentric ones, the ones with branching networks, the germinating cysts might form a little rhizoid system first, which then expands.

Is there a pattern to how they germinate from the cyst, like which side they grow out of?

Yes.

And that's actually another key feature used in classification.

Some germinate in a monopolar fashion.

Growth happens from just one point on the cyst wall.

That's typical for the order chytridiales.

Monopolar.

One pole.

Right.

Others show bipolar germination growth starts from two opposite points.

That's characteristic of another order, the blastocladeos.

So even the way they start growing tells you something about which group they belong to.

Okay, so that covers how they grow from a zoospore.

What about their full life cycles?

Is it always just asexual zoospore production?

Or is there sex involved?

Ah, good question.

While many chytrids seem to primarily reproduce asexually using those haploid zoospores, some groups, particularly in the blastocladeos, have much more complex life cycles.

They can exhibit a true alternation of generations.

Alternation of generations.

Like implants switching between different forms.

Exactly like that.

They cycle between a haploid stage, think of it as the sexual gamete producing stage called the gametothallus, and a diploid stage, the spore producing sporethallus.

And sometimes these two stages can look almost identical.

That's called isomorphic alternation.

Wow.

So how does the sexual reproduction actually happen?

How do they fuse?

There are several fascinating ways.

One common method is gametogamy.

That's the fusion of specialized motile gametes, which are basically sexual zoospores.

Okay.

This can be isogamous, where the two fusing gametes look identical.

You see that in species like chytriomyces hyalinus or synchytrium endobioticum, the potato wart one.

Isogamous, identical gametes.

Right.

Or it can be anisogamous.

Here you have a smaller, usually more active male gamete fusing with a larger, more sluggish female gamete, like in the genus Allomyces.

Anisogamous, different size gametes.

Correct.

And then there's something really unique for fungi found in the monobul faradales, oogamy.

This involves a motile male gamete, like a little sperm, and a much larger, completely non -motile egg cell.

Very unusual in the fungal kingdom.

Oogamy.

Like animals and plants.

That is unique for fungi.

Are there other ways besides gamete fusion?

Yes.

Another method is somatogamy.

This is the fusion of just regular vegetative parts, like the tips of rhizoids from two different individuals.

It's well documented in chytriomyces hyalinus.

Rhizoid tips touch, fuse cytoplasm and nuclei, move to the fusion point, and they form a resting structure where the nuclei fuse, creating a deployed zygote.

So no specialized gametes, just bits of the body fusing.

Exactly.

And there's even one more.

Gametangio gametangiogamy, seen in zygorzidium planktonicum.

Here, a little tube grows from a smaller donor structure, holding the nucleus to a larger recipient one.

The nucleus migrates across,

fuses with the recipient nucleus, and forms a deployed resting spore.

Wow.

Quite the variety of strategies.

What's the end result of all this sexual activity?

Generally, the product is a thick -walled resting spore, or resting sporangium.

These things are They're incredibly durable.

Some can stay viable in soil or sediment for many, many years.

It's a key survival strategy for getting through harsh conditions.

But not all chytrids do this.

That's right.

It's worth remembering that similar thick -walled structures can also form asexually.

And honestly, for a lot of chytrid species, sexual reproduction has simply never been observed.

It might not even happen in some groups.

Okay.

So within this huge phylum, you mentioned the chytridialis is the biggest order, over half the species.

That's right.

It's the largest and perhaps most typical group.

Let's dive into some examples that you mentioned synchytrium earlier.

Yes.

Synchytrium.

These are all endobiotic living inside host cells and holocarpic, meaning their whole body turns into reproductive bits.

Sometimes it becomes a cluster of sporangia directly, or it forms a structure called a prosaurus first, which then develops into the sporangia cluster.

And this includes the potato wort fungus.

Right.

Synchytrium endobioticum.

That one sounds pretty serious.

It is.

Here's where it gets really interesting agriculturally.

This species causes black wort disease in potatoes.

It's a major pathogen, and frustratingly, you can't even grow it in the lab unless you have living potato cells.

So it's an obligate parasite.

What does the disease look like?

It causes these ugly dark brown sort of cauliflower -like growths on the potato tubers.

You can also get greenish distorted leafy galls on the shoots above ground.

If a crop is heavily infected, the yield loss can be significant.

Fortunately, there are immune potato varieties.

How does it cause those warts?

You mentioned resting spores.

Yeah.

The dark warts are basically galls areas where the fungus stimulates the potato cells to divide like crazy.

And packed inside these galls are loads of those thick -walled resting spores.

These are the things that can survive in the soil for over 40 years.

40 years!

That's unbelievable persistence.

How does it get going again from those spores?

When conditions are right, the resting spore germinates.

Its outer wall cracks, the inner wall balloons out, and forms a little vesicle where a sporangium develops.

Interestingly, scientists found that if the spores pass through the gut of a snail, it actually helps them germinate.

Maybe the snail's digestion scratches or weakens that thick wall.

Snails.

Nature is weird.

So the sporangium releases zoospores.

Right.

These zoospores swim around in the soil water.

They've got about two hours.

If one happens to land on a vulnerable spot on a potato, like an eye or a young tuber, before the skin gets tough, it quickly insists and injects its contents into a host cell.

And then what happens inside the potato cell?

Well, the host reaction depends on whether it was infected by a single haploid zoospore or a diploid zygote formed from fused gametes.

Ah, okay.

Difference there.

If a haploid zoospore gets in, the host cell and its neighbors tend to hypertrophy.

They just swell up, get bigger.

The fungus inside develops into that persaurus or a summer spore, which then releases more haploid zoospores.

This is the asexual cycle, spreading the infection rapidly during the growing season.

Got it.

Rapid spread.

But the zoospores can also act as gametes.

They fuse in pairs.

It's isogamous.

They look the same to form a biflagellate zygote.

Now, when the zygote infects a potato cell, it triggers hyperplasia.

Hyperplasia.

Meaning it causes the host cells to divide repeatedly.

This is what leads to those characteristic cauliflower -like warts, the galls.

The fungus gets buried deep inside layers of these rapidly dividing host cells.

And inside, the zygote develops into one of those super -tough, thick -walled resting sporangia, ready to survive for decades.

Nuclear fusion actually happens in the zygote before it even penetrates the host cell.

Wow.

So the type of infection determines the outcome rapid spread versus long -term survival structures.

How do farmers manage this?

The main strategy is resistance.

Breeding and planting potato varieties that are immune.

These varieties might kill the invading fungus quickly or just wall off the infection so it can't spread.

But you said new strains are emerging.

Exactly.

It's a constant battle.

New pathotypes or races of the fungus pop up that can overcome resistance of previously immune varieties.

It's a serious threat, potentially undoing decades of plant breeding work.

Other methods, like fungicides, are costly and not always effective.

Any other clever approaches?

There's an interesting one from Newfoundland using crab shell meal.

They put it in the soil above the seed tubers.

It significantly reduces the disease.

The idea is that the crab shells, which are rich in chitin, encourage the growth of soil microbes that naturally break down chitin, and these microbes then attack the chitinous walls of the fungus's resting spores.

Boosting the soil's natural defenses.

That's neat.

Okay, moving beyond potatoes.

What about rhizophytium?

You mentioned they attack algae.

Yes, rhizophytium.

It's a huge genus, about 100 species found pretty much everywhere, soil, freshwater, marine environments.

Many are just saprotropes breaking down dead stuff.

But others are significant pathogens of algae, and they can cause massive epidemics.

Algal epidemics.

How does that work in a lake, for example?

A well -studied case is rhizophytium planktonicum.

It parasitizes a specific type of diatom called Asterian eleformosa, which forms these beautiful little star -shaped or cartwheel -like colonies.

The chytrid zoospores are chemically attracted to the diatoms.

They latch on, insist, and then send their rhizoids in, usually between the plates of the diatom silica shell.

What does the infection do to the diatom?

It causes the diatom to lose its photosynthetic pigments, essentially starving it, and eventually kills it.

So how do these outbreaks, these epidemics, happen?

Well, studies have shown it's quite complex.

You can find infected diatoms pretty much all year round.

But a full -blown epidemic, where a large percentage of the diatom population is infected, usually only kicks off when the diatom numbers reach a certain high density.

Enough hosts around for the parasite to really take off.

Makes sense.

What about environmental factors?

Light and temperature play crucial roles, but in a really fascinating way.

The zoospores actually need light exposure to become infective, but they don't actively swim towards light.

Both very low light and low phosphorus levels in the water can limit how many zoospores the fungus produces.

But the interaction between light and temperature is the really surprising part.

At warmer temperatures, say above 6 degrees Celsius, high light intensity actually encourages epidemics.

More light, more disease.

But when the water is colder, below 5 or 6 degrees Celsius, it flips.

Lower light intensities actually promote epidemics under cold conditions.

That's weird.

Why would that be?

It's not entirely clear, but it explains why you can get major outbreaks of this chytrid disease both in the summer, with high light and warm water, and in the winter, under ice perhaps, with low light and cold water.

It's also a very specific relationship this particular chytrid seems to only infect Osterianella, and maybe only certain genetic strains of it.

Highly specialized.

Okay, so we've had parasites of potatoes and algae.

What about the more straightforward decomposers?

You mentioned cladocatrium.

Right, cladocatrium.

These are common saprotrophs, breaking down dead aquatic plants.

They're eucarpic and polycentric, so they form quite extensive branching networks of rhizoids, really permeating the material they're decomposing.

Anything visually distinctive about them.

One species, cladocatrium replicatum, has these really bright orange lipid droplets inside its sporangia.

The color comes from carotenoid pigments like lycopene, the same stuff that makes tomatoes red.

Even the zoospores have a single orange droplet.

An orange chytrid.

Does light affect it?

Yes.

Light seems to encourage the development of the sporangia.

We don't know for sure if it reproduces sexually, but it makes both regular thin -walled for quick zoospore release,

and also thicker -walled resting sporangia for survival.

Interestingly, these resting sporangia can be smooth or spiny, even within the same species.

Okay, and noacowskella,

another decomposer.

Yep.

Noacowskella elegans is another common one on decaying aquatic plants.

It also forms a spreading rhizomycelium, often with these characteristic swollen sort of top -shaped cells called turbinate cells.

Turbinate cells.

Any quirks with this one?

Particularly how it releases spores.

Well, its sporangia often have an operculum, a little preformed lid that pops off to release the zoospores.

Like a perculum, like a little cap.

Exactly.

But, and here's another case showing how tricky classification can be, some strains, like one called n -perfusa, which is probably just a variant of elegans, can actually open up in three different ways.

Sometimes the lid pops off to the outside, exooperculate.

Sometimes it breaks off, but stays inside the opening, endoperculate.

Sometimes there's no lid at all.

It just dissolves a poor endoperculate.

All three ways in potentially the same species.

Apparently so.

It really shows you can't always rely on one single feature like that for identification.

It highlights the variability within these groups.

Definitely complicates things.

Okay, let's shift gears slightly.

Another order,

the spialomycetails.

What makes them different?

The main thing that sets the spialomycetails apart is their zoospores.

Unlike the ones we've mostly talked about with a single lipid droplet, their zoospores contain multiple lipid droplets, and they can also do a bit of amoeboid movement, sort of crawling, in addition to swimming with their flagellum.

Multiple fat droplets and a bit of crawling.

Okay.

What's a key example, maybe one that affects us?

A really important one is olpidium brassicae.

It's super common, found on the roots of cabbage plants, hence brassicae, but it infects a whole wide range of other plants too.

Now the fungus itself doesn't usually cause much direct damage to the plant.

So it's not a major pathogen itself?

Not usually, no, but its zoospores have a nasty habit.

They act as vectors, carriers for several important plant viruses.

Ah, so it transmits disease even if it doesn't cause much itself?

How does that work?

That's the crucial question.

There are two main ways it transmits viruses.

First, there are the non -persistent viruses like tobacco necrosis virus, TMV, or cucumber necrosis virus.

These viruses are basically acquired externally.

The virus particles just stick onto the outside membrane of the zoospore and its flagellum while it's swimming around in the soil water.

Just hitching a ride on the outside.

Essentially, yes.

Then when the zoospore finds a root, insists, and injects its contents into the plant cell, the virus particles that were stuck to its surface get carried in too.

But importantly, these viruses are not carried inside the fungus's resting spores, so transmission relies on active zoospores.

Okay, that's non -persistent.

What's the other type?

The other type involves persistent viruses like lettuce big vein virus, LBVV.

These are acquired in vivo, meaning the virus actually gets inside the fungal phallus while it's growing within the plant host cell.

Inside the fungus itself.

Yes.

And here's the truly remarkable and worrying part.

LBVV can survive inside the chytrid's air -dried resting sporangia, and not just for a little while, for 18 to 20 years.

20 years inside a dormant fungal spore.

Incredible, isn't it?

The chytrid essentially acts as a long -term, highly protected reservoir for the virus in the soil.

When that resting spore finally germinates years later, the zoospores it releases are already carrying the virus internally, ready to infect new plants.

That makes controlling that virus incredibly difficult.

Absolutely.

It's a major challenge.

The opidium zoospores themselves are tiny tadpole -shaped things.

They swim for maybe 20 minutes, find a root hair, stick on, dissolve a hole in the plant cell wall, and inject their contents.

Within days, new fungal thalli develop, churning out more zoospores.

They also form those thick -walled resting sporangia for long -term survival, though we don't actually have evidence of sexual fusion in this particular species.

Okay, another one from this order.

You mentioned Rhizoflictus roseae earlier, the one that makes variable numbers of zoospores.

That's right.

Rhizoflictus roseae.

It's a monocentric saprotroph, a decomposer.

It plays a really significant, but often overlooked, role in breaking down cellulose in soils worldwide.

It's incredibly tough.

How tough?

It can survive for long periods in completely dry soil.

It can even survive being heated to 90 degrees Celsius for two days.

In fact, if you want to isolate it from a soil sample, the best way is often to air dry the soil first that kills off a lot of other microbes, but Rhizoflictus survives.

Heat resistant and drop resistant.

How does it grow and spread?

When it grows on cellulose -rich stuff, its sporangia turn this lovely bright pink color, again due to carotenoid pigments.

It produces these quite coarse rhizoids that really aggressively penetrate and break down the substrate.

You can almost see the evidence of the powerful cellulase enzymes it's secreting.

A real cellulose crusher.

Definitely.

Its zoospores can swim for several hours searching for new material.

And structurally, they also have that striated rhizoplast connecting the nucleus,

similar to olpedium.

It also makes thick -walled resting sporangia for survival, though whether these are formed sexually isn't definitively known.

There's some hint it might need compatible mating types, suggesting it could be heterothallic.

Okay, let's move to a really different group now.

The Neo -Calamassioles.

You call them rumen fungi.

That sounds intriguing.

Oh, they are absolutely fascinating.

This is a group of obligately anaerobic zoosporic fungi.

Obligately anaerobic, meaning they cannot survive with oxygen.

Correct.

They die in the presence of oxygen.

And they live exclusively in the digestive tracts, primarily the rumen, of herbivorous mammals.

Think cows, sheep, deer, goats, horses, even elephants and kangaroos.

In the gut.

How do they survive there without oxygen?

Well the rumen, that big fermentation chamber in cows and sheep, is naturally an anaerobic environment.

All the bacteria and protozoa living there use up any available oxygen very quickly through their own respiration.

So it's the perfect spot for these fungi.

If people didn't know fungi could live without oxygen.

For a long time, no.

Obligately anaerobic fungi just weren't thought to exist.

Microbiologists studying the rumen actually used to see the zoospores of these fungi swimming around and just assumed they were some kind of protozoa.

Mistaken identity.

Totally.

It wasn't until the 1970s that their true nature was discovered.

A researcher named Orpin made this incredible observation in sheep rumen.

Shortly after the sheep were fed, there was this massive, extremely rapid increase in the number of these flagellates.

Like 15 to nearly 300 times more almost instantly.

How could they multiply that fast?

It was way too fast for normal cell division.

The explanation turned out to be ingenious.

The fungi live most of their lives as sedentary thali, anchored by rhizoids to bits of partially digested plant fiber in the rumen.

When fresh food comes in, soluble stuff like hame compounds

triggers these anchored fungi to release a massive, synchronized flood of preformed zoospores all at once.

A coordinated release triggered by food.

Exactly.

These newly released zoospores then quickly swim around, find fresh bits of plant material that just arrived, attach, and germinate.

The whole cycle takes about 30 hours.

That's amazing.

So what makes their zoospores different living in this unique gut environment?

Well, for one thing, while some have the typical single flagellum, others, like the generin neocalimastics or pinomyces, are multiflagellate.

Their zoospores can have 8, 16, or even more whiplash flagella.

Wow, superpowered swimmers.

Maybe.

And internally, they're also quite different.

Because they live without oxygen, they lack the mitochondria that aerobic organisms use for energy.

They also don't have typical goldy bodies.

Instead, they have organelles called hydrogenosomes.

Hydrogenosomes.

What do they do?

These are really the key to their anaerobic lifestyle.

They take the place of mitochondria.

They metabolize sugars like glucose, but instead of using oxygen, they ferment them, producing things like acetic acid and formic acid.

And crucially, as a byproduct, they release hydrogen gas H2.

Hydrogen gas.

Inside the cow.

Yep.

And this hydrogen gas doesn't just float away, it's immediately snapped up by another group of microbes in the rumen, the methanogenic archaea.

These use the hydrogen to reduce carbon dioxide, also present in the rumen, into methane CH4.

Ah, so the fungi make the hydrogen and the archaea make the methane.

Exactly.

It's a close partnership.

And that methane is then famously released in large quantities by the rumen and animal burping.

These hydrogenosomes are thought to have actually evolved from mitochondria, but lost their own DNA and adapted completely to life without oxygen.

That is wild.

So ecologically, what's their role in the gut?

Are they important for the animal?

They play a really vital role, especially in the early stages of digesting tough plant fiber.

They are among the first microbes to colonize the freshly eaten plant material.

They produce a whole arsenal of powerful enzymes capable of breaking down complex carbohydrates, things like xylene, cellulose, starch,

stuff that's hard for the animal itself to digest.

So they kickstart the digestion process.

They do.

By physically penetrating the plant tissues with their rhizoids and chemically breaking down the fiber with their enzymes, they likely make it much easier for the bacteria to come in afterwards and continue the digestion process.

They're key players in fiber breakdown.

How do they survive outside the animal to get passed on?

They can form resistant cysts or thick -walled thali that get passed out in the feces.

When feces dry out, these structures can survive for some time.

Transmission to young animals probably happens mainly through licking and grooming behavior between mothers and offspring, or contact with contaminated environments.

So next time I see a cow burp, I'll think of hydrogenosomes.

Aha!

There you go!

Tiny anaerobic fungi doing their part.

Okay, let's move to the blastoclaudias.

Another order, what's their deal?

The blastoclaudias are mostly saprotrophs again, found in soil and water.

But this group also includes some significant pathogens, parasites of plants, aquatic invertebrates like insects and crustaceans and even other fungi.

Most need oxygen, they're aerobes, but a few, like blastoclaudia, can actually manage without it.

If necessary, they're facultatively anaerobic.

And life cycles.

You mentioned alternation of generations earlier.

Yes, this is the group where you really see that complex alternation of generations between haploid gamete producing and diploid sport producing stages, most clearly.

It really makes you wonder how such relatively simple organisms manage such sophisticated life cycles.

Any examples that have caught researchers' eyes, maybe for practical reasons?

Definitely.

There's a genus called Coamomyces.

These are all obligate parasites of insects, mainly mosquito larvae.

Mosquito parasites, that sounds useful.

Potentially, yes.

And what's incredibly cool about their life cycle is that it requires two different animal hosts.

The diploid sport producing stage lives in the mosquito larvae,

but the haploid gamete producing stage lives in a completely unrelated tiny crustacean, a copod, that lives in the same water.

It needs both a mosquito and a copod to complete its cycle.

Isn't that amazing?

The fungus has to alternate between these two very different hosts.

Because of this, people are very interested in trying to use Coamomyces species for biological control to naturally reduce mosquito populations.

Two -host fungal weapon against mosquitoes.

Yeah.

Very clever.

What about their zoospores?

Anything special there?

The blastoclaudial zoospore also has that single posterior whiplash flagellum.

Like some others, it often has that dense crescent -shaped nuclear cap full of RNA and ribosomes hugging the nucleus.

But a real defining feature, especially in well -studied species like blastoclaudial emersoniae, is the presence of a single large prominent mitochondrion.

Just one big one.

Yep.

One large mitochondrion, typically positioned right near the base of the flagellum, the kinetosome.

There are these striated bodies or rootlets connecting the kinetosome to this

mitochondrion, probably involved in transferring energy for swimming or maybe anchoring everything together.

Okay.

And germination.

You mentioned bipolar earlier.

Yes.

That's our key feature here.

When the zoospore insists, it shows bipolar germination.

It puts out a narrow germ tube from one side, which develops into the anchoring rhizoids, and a wider germ tube from the opposite side, which develops into the main body or hyphae.

This two -point germination is a hallmark of the blastoclaudials, different from the single -point unipolar germination common in the chytridiales.

Got it.

So within this group, Allomyces is a big name.

Oh, yes.

Allomyces is probably the most famous genus in this order.

You find them in soil and mud, especially in warmer regions, tropics and subtropics.

They're a real favorite for researchers, a model organism almost.

Why is that?

Because they're relatively easy to grow in the lab on simple media.

This means scientists have been able to study their entire life cycle in detail, including all the genetics and the factors controlling development.

And their life cycle shows that alternation of generations clearly.

Beautifully.

The classic U.

Allomyces type, seen in species like A.

arbuscula and A.

macroginus, has a perfect isomorphic alternation.

The deployed sporophyte generation looks very similar to the haploid gamophyte generation.

Okay, walk me through it.

The deployed stage.

Right.

The deployed sporothallus.

It produces two kinds of sporangia.

Thin -walled mitosporangia release deployed zoospores, mitospores, that just grow into more identical deployed thalli.

That's the asexual multiplication loop.

Okay, cloning itself.

Exactly.

But it also produces thick -walled, brownish -resistant myosporangia.

These are resting sporangia.

Inside these, meiosis happens.

Meiosis.

So the chromosome number is halved.

Precisely.

When these myosporangia germinate, they release haploid zoospores or myospores.

And these haploid spores grow into the sexual stage.

You got it.

These haploid myospores germinate and develop into the gametalli.

These are monoetius, meaning one thallus produces both male and female reproductive structures, called gametangea.

Male and female parts on the same body.

Are they different?

Yes, visibly different.

The male gametangea are usually smaller and located towards the tips of the branches.

And they are bright orange because they're packed with beta -carotene.

The female gametangea are larger, located below the male ones, and they're colorless or grayish.

Orange boys, colorless girls.

And they release gametes.

They do.

The male gametangea release small, very active, orange male gametes.

The female gametangea release larger, paler, much more sluggish female gametes.

So this is clearly anisogamy, different size gametes.

Okay, so how do the fast orange ones find the slow, pale ones in the water?

Ah, this is where it gets really elegant.

Chemical signaling.

The female gametangea, and the released female gametes themselves, secrete a specific hormone called sirenin.

Sirenin.

Like the sirens of mythology.

Exactly like that.

It's named because it lures the males.

Sirenin is a bicyclic sesquiter pin, and it's an incredibly potent chemoattractant for the male gametes.

They can detect and swim towards it, even at vanishingly low concentrations like 10, 10, or 10, 11 molar.

Wow, extremely sensitive.

Do the males signal back?

They do.

The male gametes secrete their own hormone called paracin, which seems to attract the female gametes, although maybe over shorter distances.

A two -way chemical conversation.

Pretty much.

The male gamete eventually fuses with the female gamete, forming a diploid zygote.

This zygote has two flagella initially, swims for a bit, then insists.

Nuclear fusion happens inside the cyst, and it immediately starts developing into a new diploid spora thallus, completing the entire cycle.

That's a beautifully complex cycle.

You mentioned genetics.

Yeah.

Anything else interesting there?

Polyploidy.

Yes.

Studies back in the mid -20th century showed that Allomyces species exhibit polyploidy.

They can have multiple sets of chromosomes beyond the basic diploid number.

For instance, A.

arboscula has a basic haploid number, N of 8, but strains exist that are N16, N24, even N32.

A.

macrogonus is basically N14, but you find strains that are N28 or N56.

So they can have many copies of their genome.

Does this affect anything?

It does.

And you can even hybridize different species like arboscula and macrogonus.

The hybrids show really interesting genetic phenomena, variations in chromosome numbers, even changes in how the male and female gametanja are arranged on the thallus in subsequent generations.

It shows there's complex genetic control over all these developmental steps.

A real playground for fungal geneticists.

What about Blastocladiella?

You mentioned B.

emersonia and its single big mitochondrion.

Right, Blastocladiella.

These are generally simpler, monocentric forms, maybe looking a bit more like some of the chytridialis members.

Their rhizoids are known to be chingotropic, meaning they grow towards nutrients, which is obviously vital for absorbing food.

Their life cycles tend to follow the same basic pattern as Allomyces, with alternation of generations.

But Blastocladiella emersoni is particularly famous for its remarkable responses to environmental cues.

How so?

What triggers changes?

Well, under standard lab conditions, it mostly just goes through the asexual cycle, producing thin -walled sporangia that quickly release zoospores.

Ordinary cycle.

But, and here's the really cool part, if you simply add about 10 millimolar bicarbonate to the growth medium,

essentially yes, or related salts like potassium chloride, sodium chloride, even ammonium chloride at high concentrations,

or, alternatively, just expose the cultures to a bit of UV light.

Any of these triggers will cause the fungus to switch pathways.

Instead of making the ordinary sporangia, it starts developing the thick -walled, brown -resistant resting sporangia.

Just by adding salt or bicarbonate or a bit of UV, it switches its whole developmental program.

Exactly.

You can flip a switch in its metabolism and morphology just by changing the environment.

For instance, when bicarbonate is absent, it uses a standard metabolic pathway, the tricarboxylic acid cycle.

But when bicarbonate is present, part of that cycle actually reverses.

It switches to alternative ways of processing carbon.

It even starts using a different enzyme for respiration, a polyphenol oxidase instead of cytochrome oxidase.

And it ramps up production of melanin, the brown pigment, and chitin for that thick resting spore wall.

That's incredible metabolic flexibility.

What's the bicarbonate actually doing?

It seems the effect is actually due to the increased concentration of dissolved carbon dioxide, CO2, that results from adding bicarbonate.

Wow.

Any other weird environmental responses?

Yes.

It also shows something called lumosynthesis.

It actually fixes CO2, incorporates it into its own biomass more rapidly when it's in the light compared to when it's in the dark.

Light -driven CO2 fixation.

Like photosynthesis.

Sort of, but without chlorophyll.

It's mainly responsive to blue light.

This light stimulation leads to larger thallia and more nuclear division.

It suggests a complex light -sensing system that isn't fully understood yet, but it clearly affects its growth and metabolism.

So sensitive to CO2, light salts,

a very responsive fungus.

Okay, one last group.

The moddable Faradales.

You said they were unique because of oogamy.

That's right.

This is a relatively small group, only about 20 known species, but they hold a really special place because where we know their sexual reproduction, it's Remind me again what that means.

It means they have a large non -modal egg cell, an oosphere, and a small motile male gamete, a spermatozoid that has a single posterior flagellum.

This fusion of a motile sperm with a non -modal egg is very common in animals and plants, but extremely rare among fungi.

That makes the moddable Faradales stand out.

Where do these rare oogamous fungi live?

You find them in very specific habitats,

quiet, clear freshwater pools or streams, typically where there's not much silt.

They often grow on submerged, waterlogged twigs, especially twigs that still have bark on them like birch, ash, or oak.

So clean water and woody debris, what do they look like?

Their thallus is eucarpic, usually forming delicate, branched, thread -like filaments.

It's not a very robust structure.

And asexual reproduction.

That happens via zoospores released from sporangia, much like other groups we've discussed.

The zoospores are oval -shaped with a long whiplash flagellum, and they can apparently change shape slightly, showing some amoeboid movement.

They have distinct internal regions, too.

But the main event is the oogamy.

How does that work in a species like monobliferous?

Well, in monobliferous macrandor, for example, the thallus develops distinct male structures called antheridia, which release the tiny uniflagellate spermatozoids.

It also develops female structures, oogogonia.

Inside each oogonium, a single large spherical egg cell,

matures.

When the egg is ready, a little receptive spot or papilla on the oogonium wall breaks down, creating an opening.

A spermatozoid swims in and huses with the oosphere.

Fertilization occurs.

After fertilization, the resulting zygote, now called an oospore, quickly secretes a thick, tough, often golden -brown wall around itself.

In some species, this resting oospore stays inside the old oogonium wall.

In others, it actually moves to the opening of the oogonium and remains attached there, sometimes developing a warty or bumpy surface.

These oospores then enter a dormant period before germinating to grow into a new thallus.

A truly unique process for fungi.

So wrapping this all up, what does this deep dive into the chytridium icoda tell us?

Well, I think it shows just how incredibly diverse and ancient this fungal group is.

We've gone from their signature flagellated zoospores, those little hopping swimmers, to seeing their huge range of ecological roles.

Yeah, decomposers breaking down tough stuff.

Parasites causing major plant diseases like potato wart, vector -spreading viruses, pathogens hitting algae, partners living inside insect guts and making hydrogen.

They're involved in so many different processes, often completely unseen.

We've seen their life cycles, from the really simple holocarpic ones where the whole body just becomes spores, to the incredibly complex alternation of generations and allomyces, complete with chemical signals like sirenin.

We've seen how they can survive for decades as resting spores, how they respond dramatically to environmental cues like CO2 or light.

And that microscopic world inside the zoospore with its kinetosomes, rumposomes, nuclear caps, just amazing engineering.

It really is.

Their adaptability is just stunning.

Surviving heat, drought, anaerobic guts, it really underscores the incredible resilience and evolutionary creativity of life.

These aren't just obscure microbes, they are genuinely powerful forces shaping ecosystems.

So perhaps the next time you see a potato with a funny wart, or watch a cow chewing its cud, or even just walk past a pond.

You might just remember this hidden world of ancient swimming fungi, the catrids, and the surprising complex roles they play all around us.

Definitely gives you a new perspective.

Thank you for joining us on this deep dive.

And a warm thank you from the Last Minute Lecture Team.