Chapter 14: Fungi

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Welcome to the deep dive.

Okay, get this.

What if the largest living thing on earth wasn't a whale, wasn't a giant tree, but something, uh, you might just step on without even knowing?

And here's the really wild part.

It's actually more closely related to us, to animals than it is the plants.

That's nuts.

Okay.

So today we're taking a deep dive into, well, a truly fascinating and honestly often misunderstood world, the kingdom fungi.

We're using a really solid chapter from Raven Biology of Plants, the eighth edition, as our guide here.

Right.

And our mission today is basically to, you know, cut through the complexity.

We want to explore what makes fungi tick their incredible diversity and the vital, absolutely critical roles they play.

We're hoping you walk away with real clarity and maybe a few aha moments.

Exactly.

Because for ages, people thought fungi were like primitive plants.

They don't move.

They look kind of plant -like sometimes.

Sure.

But the molecular evidence now, it's crystal clear.

They are their own distinct kingdom, totally separate.

And closer to animals, you said?

Yep.

Closer to animals than plants, evolutionarily speaking.

It still surprises people.

And the scale we're looking at, wow, over a hundred thousand species named already.

And counting.

About 1200 new ones pop up every year.

But honestly, that's probably just scratching the surface, estimates.

They go up to over 1 .5 million species.

1 .5 million.

It's just this vast, mostly hidden kingdom under our feet.

It really is.

And that largest organism thing, it's a specific fungus, Armillaria solidipes,

causes root rot in trees.

People call it the humongous fungus.

Humongous fungus.

I like that.

There's one in Oregon.

It covers nearly 900 hectares.

That's like 2200 acres.

Whoa.

And they think it's maybe 2400 years old or even older.

Incredible.

There's another one, a relative, Armillaria gallica in Michigan.

Smaller, maybe 15 hectares, but still ancient.

At least 1500 years old.

And the crazy part, most of it is this underground network of tiny thread -like structures.

Just spreading unseen.

Exactly.

Only popping up as mushrooms sometimes.

Huge hit in life.

Okay.

So clearly massive, clearly widespread.

But what makes them so important?

Let's get into that.

They're immense importance.

You said foundational.

Absolutely foundational.

Fungi, alongside some bacteria, are the planet's primary decomposers.

Simple as that.

So the recyclers.

The ultimate recyclers.

Without them, dead stuff would just pile up.

They break down dead plants, animals, everything organic, releasing carbon dioxide back into the air and putting nitrogen and other crucial stuff back into the soil.

Making it available for new life.

Precisely.

It's this constant cycle.

Essential for plants and animals.

Earth's cleanup crew, keeping everything running.

And the amount of fungi doing this work is staggering, right?

Like tons per hectare.

Yeah.

In just the top layer of good soil, say the top 20 centimeters, you can find nearly five metric tons of fungi and bacteria per hectare.

It's dense with life.

Wow.

And they're not just on land.

There are hundreds of known marine species, lots in water.

Okay, but this decomposition, it doesn't always work in our favor, does it?

Ah, no.

That's the flip side.

Fungi are, let's say, enthusiastic decomposers.

They don't really care what they decompose.

Right.

So a dead log in the forest is fair game, but so is my hands post.

Exactly.

Or your cotton shirt, paint, leather, paper, even things like the coatings on camera lenses or CDs back in the day.

They have enzymes that can break down almost anything organic.

And our food, of course.

Oh, yeah.

A huge challenge for food producers.

We've all seen it, right?

That fuzzy gray stuff on strawberries, that's often rhizopus.

Or mold on bread, veggies, meat.

It spoils food, reduces nutrition, makes it taste bad.

And some are actually dangerous, producing toxins.

Mycotoxins.

Some fungi produce these really potent toxins on food, which can be seriously harmful if eaten.

And part of what makes them so effective, both good and bad, is their versatility.

Some grow below freezing.

Others love heat above 50 degrees C.

They adapt.

Which leads us straight into their medical and economic side.

As pests, pathogens, but also, surprisingly, producers.

Let's start with the negative, the pathogens.

Plants seem to get hit hard.

They really do.

Over 5 ,000 fungal species attack crops, garden plants, trees.

Think about diseases like anthracnose or that dogwood disease.

Even our humongous fungus, armillaria, isn't just eating dead wood.

It can attack living trees too.

And humans and animals aren't immune either.

No, definitely not.

Over 175 species cause serious diseases in us and other animals.

And worryingly, these infections seem to be increasing.

Why is that?

Partly because we have more people with compromised immune systems.

Think AIDS patients, people undergoing chemotherapy.

Fungi that wouldn't normally cause trouble can become really dangerous, like candida causing thrush or pneumocystis causing a type of pneumonia.

Okay, so definite downsides, but then there's the flip side, their value.

Huge value.

Take yeasts, for example.

Saccharomyces cerevisiae.

Tiny, single -celled fungi.

The baker's and brewer's yeast.

That's the one.

Essential for alcohol and wine and beer.

Essential for CO2 and bread.

Also used in cider, sake.

We've even used genetic engineering to make better strains now.

They're workhorses.

And cheese.

You mentioned cheese earlier.

Oh, absolutely.

The distinctive flavors, the textures of cheeses like roefer, Danish blue, stilton, that's penicillium rocaforti, camembert and brie, penicillium camembert.

Brie, amazing.

And it doesn't stop there.

Fungi like aspergillus are key for making soy sauce, miso sake.

And then there are all the edible mushrooms we cultivate and eat, like the common button mushroom, agaricus bisporus.

And medicine.

Penicillin is the classic example, isn't it?

It truly is.

Discovered by Fleming, produced by penicillium notatum, it revolutionized medicine, saved countless lives during World War II, fighting bacterial infections.

Just incredible.

But not the only medical marvel from fungi.

Not at all.

There's cyclosporin, isolated from tulipicladium inflatum.

This drug was a game changer for organ transplants.

How so?

It suppresses the immune system to prevent rejection, but without the really severe side of earlier drugs.

It made transplants much safer, much more routine, a true wonder drug.

Wow.

And they have industrial uses too.

You bet.

Trichoderma fungi produce enzymes that degrade cellulose.

We use them to get that stonewashed looking genes.

Seriously.

And in laundry detergents.

It's also used as a biological control agent against harmful crop fungi.

Then you have fungi like phenylaceti, the white rot fungus, used in bioremediation, cleaning toxic waste, because it can break down really tough stuff like lignin.

Fungi are also used in research labs, right?

Heavily.

Yeast, especially Saccharomyces cerevisiae, is a model organism.

Easy to grow, simple eukaryote.

It was the first eukaryote to have its entire genome sequenced.

Huge for understanding genetics, metabolism, cell biology.

Okay, so they decompose, cause disease, make food, provide medicine, clean up waste.

They do a lot.

This brings up their relationships with other organisms.

Symbiosis.

Exactly.

They don't just act on things, they partner with things.

These mutualistic relationships are incredibly important.

Like mycorrhizas and lichens.

Those are the big two, yes.

Mycorrhizas are fungi partnering with plant roots.

Lichens are fungi partnering with algae or cyanobacteria.

And honestly, these partnerships fundamentally shaped life on land.

They were critical for plants first moving out of the water.

We'll come back to those.

But there are other partnerships too.

Oh yeah.

Some ants and termites cultivate specific fungi in gardens inside their nests.

The fungi break down tough plant material, essentially feeding the insects.

Wow.

And then there are endophytes.

These are fungi that live inside healthy plant tissues.

Invisibly.

And they help the plant.

Often, yes.

Many produce toxins that deter herbivores or fight off pathogens, protecting their host.

Like the ergot fungus?

That sounds dramatic.

Claviceps purpurea.

Yeah, crozon rye produces these potent alkaloids.

Historically, eating contaminated rye caused ergotism St.

Anthony's fire.

Horrible symptoms.

Gangrene, spasms, hallucinations.

Volucination.

Yeah, one of the alkaloids is related to LSD.

Some historians even speculate that ergot poisoning might have played a role in the Salem witch trials fueling the accusations.

That's a chilling connection.

Shows how deeply intertwined fungi are with, well, everything.

It really does.

And sometimes the symbiosis is even more complex.

There's this fungus, curvularia, living inside tropical panic grass.

It helps the grass tolerate incredibly high temperatures like 65 degrees C.

Get this.

The fungus itself needs to be infected with a specific virus, the curvularia thermal tolerance virus, to confer that heath tolerance.

A three -way partnership.

Fungus, virus, and plant.

Exactly.

A symbiosis within a symbiosis.

Just shows the layers of complexity we're still uncovering.

Okay.

Mind officially blown.

Let's pull back a bit and look at the fungi themselves.

What are their basic building blocks?

How do they work structurally?

Okay.

Basics.

Most fungi are filamentous.

They're made of these microscopic branching threads called hyphae.

Hyphae.

Got it.

A whole mass of these hyphae from one individual fungus is called a mycelium.

That's the main body, often hidden underground or inside whatever it's going on, like that armillaria network.

So this web -like structure, and they grow fast.

Incredibly fast.

A single fungus can churn out maybe a kilometer of new hyphae in just a day under good conditions.

Whoa.

And these hyphae, they have internal structures.

Some do.

Many have cross walls called septa.

Think of them like dividing walls, but they usually have a pore in the center.

So cytoplasm, organelles, even nuclei can still move between compartments.

The textbook's figure 14 -4 shows this.

Well, it's like rooms connected by a doorway.

But other fungi are aseptate or coenacidic.

No cross walls.

Just one long continuous tube filled with multiple nuclei.

And remember, fungal nuclei are usually haploid one set of chromosomes.

But not all fungi are threads, right?

What about yeasts?

Right.

Yeasts are basically just a growth form unicellular fungi.

It's not a strict taxonomic group.

You find yeasts in different fungal phyla.

They mostly reproduce by budding a little outgrowth, just pinches off.

Like a little clone.

Pretty much.

And some fungi are cool because they're dimorphic.

They can switch between being a yeast and being filamentous, depending on the environment.

Very adaptable.

And their cell walls are different from plants.

Totally different.

Plat walls are cellulose.

Cudfungal walls are primarily chitin.

Chitin, like an insect.

Exactly the same stuff.

That tough, resilient polymer you find in insect exoskeletons and crab shells gives them strength and makes them hard to break down.

Okay, so structure, hyphae, or yeasts, chitin walls, how do they eat?

You said they absorb.

Yes, they're heterotrophic absorbers.

Because of those rigid cell walls, they can't engulf food like an amoeba.

Instead, they secrete digestive enzymes, exoenzymes, out onto their food source.

Digesting it outside their body.

Yep.

Break it down into smaller molecules, then absorb those molecules to the cell wall and membrane.

They can be saprotrophs, eating dead stuff, parasites, feeding on living hosts, or mutualists in those partnerships we talked about.

And they store energy like animals.

Primarily as glycogen, yes.

Just like animals and bacteria.

Also as lipids.

Some parasitic ones develop specialized structures called hostoria, little hyphal projections that penetrate into host cells to absorb nutrients directly.

Figure 14 -6 shows this, like a little straw tapping into the host cell.

Clever, if a bit invasive.

Okay, what about reproduction, making more fungi?

It's complex.

Their nuclear division during mitosis and meiosis is often unique.

The nuclear envelope,

the membrane around the nucleus, frequently stays intact during division.

They use things called spindle pole bodies, not centrioles like animal cells, except in one group, the chytrids.

Interesting.

And sex.

Sexual reproduction usually involves three key stages.

First, plasmogamy, the fusion of the cell contents, the protoplasts.

Second, curiogamy, the fusion of the nuclei.

And third, meiosis, which brings it back to the haploid state.

Okay.

But here's a fungal twist.

In many groups, curiogamy, the nuclear fusion, is delayed.

After plasmogamy, you get cells with two distinct haploid nuclei, one from each parent coexisting.

This is called the decarion stage, often written as N plus N.

So two nuclei just hanging out together.

Exactly.

They can even divide together for potentially a long time before finally fusing to form the diploid zygote nucleus.

That diploid stage is usually very short -lived though, followed quickly by meiosis.

The decarion stage sounds unique, but they reproduce asexually too, right?

You said that's more common.

Often, yes.

The most common way is through spores, usually non -modal little packets of genetic material, except, again, in those flagellated chytrids.

And spores are good for spreading.

Perfect for it.

They're tiny, often dry, light, easily carried by wind over huge distances.

Some are sticky and hitch rides on insects.

And then there's pylobolus.

The dung cannon fungus.

Yeah.

It grows on herbivore dung.

It has this swelling right below the sporangium, the spore case.

It acts like a lens, focusing light, and builds up fluid pressure.

Bang.

It shoots the whole sporangium spores in all, maybe two meters or more, away from the dung pile, hopefully onto fresh grass, where another herbivore will eat it.

It's ballistic dispersal.

Pretty cool mechanism.

That is seriously cool.

Evolutionarily, you mentioned they're in the epistoconta group with animals.

Right.

Closely related.

They branched off from a shared ancestor, probably some kind of flagellated protest, maybe 1 .5 billion years ago.

And the earliest fungi kept those flagella.

The chytrids did, yeah.

That scene is a primitive trait they retained.

Other fungal groups seem to have lost flagella quite early on.

The fossil record is patchy, but we see evidence of early fungi, and importantly, those early mycorrhizal fungi, the glomeromycota, associated with the very first land plants.

Suggesting that partnership was key to plants conquering land.

It really seems that way.

Essential for nutrient uptake in those early, probably poor, soils.

Okay.

Fantastic overview.

Now, maybe we can walk through the six major groups you mentioned earlier.

Just the highlights.

What makes each distinct?

Sure thing.

Let's do a quick tour.

First, microsporidians.

Tiny unicellular parasites of animals.

Used to be classed elsewhere, but now seen as very early, highly specialized fungi.

Key thing.

They lack mitochondria and use a unique polar tube to inject their contents into host cells.

Think of it like a tiny hypodermic needle.

They infect lots of animals, including humans.

Okay, parasites with a unique weapon.

Second, titrids.

Mostly aquatic, some in soil.

They're big distinction.

They're the only fungi with motile cells, zoospores, and gametes that swim using a single posterior flagellum.

That's a link to their ancient ancestry.

Some are decomposers, some are parasites, like the one causing devastating amphibian die -offs.

The swimming fungi.

Got it.

Third, zygomycetes.

These are the classic fuzzy molds, like the black bread mold, Rhizopus.

Mostly filamentous, coenocytic, hyphae, no cross walls.

They reproduce asexually with spores and sacs, called sporangia.

Sexually, they form that characteristic thick -walled resting spore, the zygospore.

Great survivors.

Bread mold group.

Tough spores.

Fourth, glomeromycetes.

Not a huge group in terms of name species, maybe only 200 or so, but ecologically massive.

Why?

Because all known members form our buscular mycorrhizae at AM with plants.

The essential plant partners.

Exactly.

They can't even be grown in labs without their plant hosts.

They reproduce asexually with large spores produced underground.

Deeply symbiotic.

Very.

Fifth, ascomycetes, or sac fungi.

This is the biggest group, over 32 ,000 species.

Huge diversity includes yeasts, morels, truffles, cup fungi, but also many plant pathogens like powdery mildews and Dutch elm disease fungus.

Sac fungi?

What's the sac?

Their defining feature is the ascus, a microscopic sac -like structure, where sexual spores, the ascospores, are produced after meiosis.

Often these assy are packed together in a larger structure, an ascocarp, which might be cup -shaped, flask -shaped, or totally enclosed.

Think morels, or the cup shape of some cup fungi.

They also reproduce asexually via spores called canadia.

Okay, largest group, defined by the ascus, includes goodies and baddies.

And many molds like penicillium and aspergillus, famous for antibiotics and food production but also toxins, belong here, even if we mostly see their asexual stage.

Sixth, basidiomycetes, or club fungi.

These are often the most conspicuous mushrooms, buff balls, fill fungi, stinkhorns.

Also include major plant pathogens, the rusts, and smuts.

Kind of.

They produce their sexual spores, basidiospores, externally on a typically club -shaped cell called a basidium.

Think of the gills under a mushroom cap.

They're lined with millions of these basidia -releasing spores.

This group is huge for decomposition, especially breaking down wood lignin.

Their hyphae are always septate with cross walls, often with complex pores.

Their life cycle prominently features that dicariotic N plus N stage, forming the familiar mushroom structure, the basidioma, which is entirely dicariotic.

So mushrooms are mostly N plus N tissue.

That's right.

And this group includes everything from edible field mushrooms and cultivated button mushrooms to deadly poisonous ameta mushrooms, hallucinogenic psilocybe, and those cool fairy rings you sometimes see.

Fairy rings are fungi.

Yep.

The visible ring of mushrooms is just the edge of a large circular underground mycelium expanding outwards year after year.

Wow.

And the rusts and smuts.

Also basidiomycetes, major plant parasites.

Rusts often have incredibly complex life cycles, meeting two different host plants to complete their cycle.

Smuts are a bit simpler, often just one host, causing black city masses of spores like corn smud.

Okay.

That's a helpful tour of the diversity.

Let's circle back now and dig a bit deeper into those key symbiotic relationships.

Lichens and mycorrhizas.

You said they were critical.

Absolutely critical.

Lichens first.

They are a composite organism, a tight partnership between a fungus, the mycorbient, usually an ascomycete, and a photosynthetic partner, the photobiont, either green algae or cyanobacteria.

A dual organism living together as one.

Pretty much.

And they are masters of harsh environments.

You find them everywhere.

Deserts, arctic tundra, bear rock, tree bark.

Often the very first things to colonize bear in places.

They look quite different, don't they?

They do.

We classify them by growth form.

Crustose lichens look like a crust painted onto a rock.

Follios are leafy, fruticose are shrubby or hair -like.

Inside, the fungus usually makes up the bulk of the structure, sandwiching a layer of the photosynthetic cells.

And they can survive drying out completely.

That's their superpower.

They enter a dormant state when dry, which protects them from damage by intense light or heat.

They just wait for moisture to revive.

Ecologically, what's their main role?

Several key roles.

They slowly weather rock, helping create soil.

Those with cyanobacteria partners fix atmospheric nitrogen, adding crucial nutrients to ecosystems.

And they're super sensitive to air pollution, especially sulfur dioxide, making them great bioindicators.

If you see lots of diverse lichens, the air is probably pretty clean.

So pioneers, nutrient cyclers, and air quality monitors?

Impressive.

Definitely.

It's a fascinating mutualism.

Even if at the cellular level, the fungus sometimes looks like it's parasitizing the algae a bit.

Yeah.

But together, they thrive where neither could alone.

Okay.

And the other big one,

mycorrhizas, fungus roots.

Fungus roots, yeah.

This is maybe the most widespread and important plant symbiosis.

Most vascular plants have them.

And the benefit for the plant is?

Huge increase in nutrient and water uptake.

The fungal hyphae act, like extensions of the root system, exploring the soil much more effectively, especially for things like phosphorus.

They also offer protection against root pathogens.

And the fungus gets?

Sugars.

Carbohydrates produced by the plant through photosynthesis.

It's a classic trade.

Nutrients for sugars.

Plus, these fungal networks can connect different plants underground.

An underground network trading resources between trees.

You got it.

Sharing information and nutrients.

You mentioned two main types, endo and ecto.

Right.

Endomycorrhizas, the most common type, also called arbuscular mycorrhizas, AM.

Here, the fungal hyphae actually penetrate into the root cortical cells.

They don't break the cell membrane, but push into it, forming these highly branched structures called arbuscules.

Think of a tiny, intricate tree inside the cell, massive surface area for exchange.

These are formed by those Blomro mycete fungi we talked about.

Penetrating inside the cells.

And ectomycorrhizas.

Ectomycorrhizas are different.

Common on trees like pines, oaks, birches.

Here, the fungus forms a dense sheath, or mantle, around the outside of the root tip.

It doesn't penetrate the cells.

Instead, hyphae grow between the root cells, forming a network called the heartignet.

That's where the exchange happens.

Many familiar mushrooms are the fruiting bodies of ectomycorrhizal fungi.

So AM fungi go inside the cells, ecto -fungi stay outside in between.

That's the key difference in structure, yes.

Both incredibly important mutualisms.

And the evolutionary punchline here.

The punchline is that we find fossil evidence of these mycorrhizal relationships, specifically the AM type, associated with the very earliest land plant fossils, over 400 million years old.

It strongly suggests that plants couldn't have successfully colonized land, especially the nutrient -poor soils back then, without their fungal partners.

This symbiosis likely made terrestrial ecosystems possible.

Incredible.

So wrapping this all up, our deep dive into the kingdom fungi reveals this incredibly diverse, unique group, actually closer to us animals than plants.

They're the planet's essential decomposers, the great recyclers.

They have huge economic impacts, food, medicine, industry, but also major downsides as pests and pathogens.

And their symbiotic relationships, especially lichens and mycorrhizas, weren't just incidental.

They were absolutely fundamental to the evolution of life on land as we know it.

From microscopic spores to yeast cells to those humongous underground networks, fungi show just amazing adaptability in how they live, reproduce, and interact.

It really makes you think, doesn't it?

This intricate web of decomposition, disease, mutualism, all happening mostly unseen.

It prompts a question, perhaps.

What other hidden symbiosis, these intricate partnerships, are shaping our world right now, right under our noses, that we haven't even begun to understand?

That's a great thought to leave with.

Thank you for joining us on this deep dive into the kingdom of fungi.

We really hope you enjoyed exploring their complex and absolutely vital world.

And remember, this was a warm thank you from the Last Minute Lecture Team.