Chapter 16: Lichenized fungi (chiefly Hymenoascomycetes: Lecanorales)

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.

We're here to take complex stuff and, well, make it make sense.

Hopefully with some surprises along the way.

Definitely.

So today, we're diving into something you've probably seen a million times but maybe never really thought about, lichens.

Ah, yes, the ultimate partnership.

Exactly, because the first surprise is a lichen isn't, you know, just one organism, right?

Not at all.

It's this amazing cooperative multiple partners living as one and they live in some pretty tough places.

So our mission today is to unpack chapter 16 of Introduction to Fungi, all about these lichenized fungi.

We want to make the science clear, engaging and do it all just through audio.

No pictures needed.

We'll paint the picture with words.

Okay, so let's start with the basics.

The core idea, which folks started figuring out way back in the 1860s.

1867, it was really nailed there.

Right, is that a lichen is a self -supporting association.

It's a fungus, the mycobiont, living together with a green alga or maybe a cyanobacterium.

That's the photobiont.

And together, they make this stable, unique structure that's completely different from either partner on its own.

And the dynamic is fascinating.

The fungus, the mycobiont, it makes up most of the bulk, right?

The outside structure.

Yeah, pretty much.

It forms the body, the surface you actually see and touch.

We call it the exavidant.

It's building the house.

Okay, so the fungus builds the house and the photobiont, the algae or cyanobacteria.

They live inside.

They're the inhabitant.

They're the ones doing the photosynthesis, making the food.

Which explains why lichen classification is basically fungal classification.

The fungus dictates the form.

Almost entirely, yes.

The fungus gives the lichen its shape, its structure.

It sounds like a really tight team.

But here's where it gets, well, really interesting for me.

What happens if you try to split them up, like in a lab?

Ah, the pure culture problem.

Yeah.

The classic experiment, and it tells you a lot.

So what happens?

Well, you try to grow them separately.

The photobiont, the algae or cyanobacterium, it usually grows okay on its own.

Nothing's spectacular, but it grows.

But the fungus,

the mycobiont, usually grows really slowly.

Often it just forms this shapeless sterile blob.

And critically, it doesn't make that characteristic lichen structure you see in nature.

Wow.

So the fungus really needs its partner to become itself, to reach its full potential.

That's a great way to put it.

It's remarkable.

In that symbiosis, the fungus completes its life cycle, sexually and asexually.

But get this, the fungus actually controls the cell division of the photobiont inside the lichen.

Whoa.

So it's not just roommates, it's more complex.

Much more.

It highlights just how unique and interdependent this relationship is.

They create something entirely new together.

And this strategy has been successful.

You mentioned diversity.

Usually successful.

We're talking around 13 ,500 described of fungi that form lichens.

13 ,500.

That's a lot of fungi.

What about the other side, the photobionts?

That's the surprising part.

There are only about 100 known species of photobionts involved in all those partnerships.

Only 100.

So a few algae and cyanobacteria are really popular partners.

Very popular.

One green alga, Trebupcea, is found in maybe half of all lichens.

Incredible.

Then you have Trentapolia, another green alga, and the cyanobacterium nostoc is other common ones.

So mostly green algae partners.

About 85 % green algae.

Yeah.

Around 10 % use cyanobacteria.

And a few actually manage to partner with both.

And the fungi themselves, are they all related?

Interestingly, almost all of them, like 98%, are Euscomycenes, a huge group within the fungi.

They're definitely the master architects of the lichen world.

Okay.

So specialized fungi, specialized partnerships, and this lets them live, well, pretty much anywhere.

Even really extreme places.

That's one of their superpowers.

Incredible resilience.

They can handle drying out completely, then getting wet again over and over.

Extreme cold, extreme heat, high UV radiation.

Stuff that would kill a normal plant.

Absolutely.

That's why they're often the first things you see growing on bare rock or really poor soil.

True pioneers.

You called them earth sculptors before because they weather rock.

Yeah.

They secrete things like oxalic acid.

It's slow, you know, maybe 0 .5 to 3 millimeters over a century.

Slow, but steady.

But sometimes.

Surprisingly fast.

There's one, Dorina Maciliensis F.

Sorriata, that can eat away landstone monuments much quicker.

Like up to two millimeters in just 12 years.

Wow.

So these little crusts are actually changing the landscape or even historical buildings.

They are.

Slow motion geological and sometimes architectural engineers.

And it's not just rocks, right?

They're on trees.

Oh, yeah.

Tree bark leaves, we call those corticolous lichens, even underwater, freshwater and marine ones from the Arctic tundra, where they can dominate right down to the humid tropics.

They're everywhere.

And they live a long time.

Astonishingly long.

Some individual lichens called thali can live for over a thousand years.

A thousand years.

That's incredible.

And because some grow so slowly and predictably, we can use them to date surfaces.

It's a whole field called lichenometry.

Like tree reams, but for rocks.

Kind of, yeah.

They've used it to help date the standing stones on Easter Island.

Or figure out when a rock face was exposed by an avalanche or earthquake.

Especially using the slow growing crusty ones like the map lichen, rhizocarpon geographicum.

Okay, since we don't have pictures, let's try to visualize these.

You mentioned different forms.

Right.

Three main types, just based on what they look like.

First is crustos.

Crusty.

Like, it sounds.

Exactly.

Imagine a thin crust, like dried paint, fused really tightly to the surface.

You can't peel it off easily.

Some are just powdery, called lepros, no real layers.

Others are more structured, stratified with a protective top layer, the algal layer underneath, maybe some air pockets.

Okay, crustos.

Got it.

Flat and tight.

Then there's squamilos, which is sort of a variation.

These small overlapping scales, maybe slightly lifted at the edges.

Gives it a kind of scurfy look.

They're like little flakes.

What's next?

Folios.

Think leafy.

These look more like leaves or lobes.

You can usually lift the edges off the surface.

They're attached by little root -like bundles of fungal threads, called rhizenae, and they have a distinct underside, a lower cortex.

Leafy.

Okay.

And the third type.

Fruiticos.

These are the most three -dimensional.

Think miniature shrubs, or hanging threads.

They could be upright and bushy, or hang down like beards.

Like old man's beard lichen.

Exactly like that.

Internally, they're often round or tubular in cross -section, with the layers arranged concentrically.

Cortex outside, then algae, then a central core or medulla.

So crusty, leafy, and shrubby.

Different strategies for different places.

You got it.

Ingenious designs for survival.

Okay, let's talk about how they, you know, make more lichens.

Reproduction.

It sounds like a bit of a challenge, keeping the team together.

It really is a delicate dance.

They have a couple of main strategies.

First, there's sexual reproduction, but that's mostly just the fungus doing its thing.

So the fungus makes spores, like other fungi.

Yep.

Often in structures like apothecia, little cups or disks you might see on the surface.

Sometimes other structures too.

But here's the catch.

Oh, there's always a catch.

The fungal spores are released alone, without the photobiont partner.

Oh,

so that spore has to land somewhere, germinate, and then find the right kind of free -living alga or cyanobacterium to team up with all over again.

Exactly.

It has to re -lichenize.

And frankly, we're not entirely sure how often that successfully happens in nature.

Seems like a bit of a long shot sometimes.

Yeah, sounds tough.

So how do they usually manage it?

How does the partnership itself reproduce?

That's where vegetative reproduction comes in.

It's much more common and reliable for most lichens.

They produce these amazing little packages called vegetative propagules.

And these packages contain?

Both partners.

The fungus and the photobiont bundled together, ready to go.

Smart.

Like sending out a fully equipped starter kit.

Pretty much.

There are two main types you hear about.

First,

serratia.

Serratia?

What are they like?

Imagine microscopic dust bunnies.

Tiny powdery clumps where a few algal cells are wrapped up in fungal threads.

They're water repellent, super light, perfect for catching the wind.

Raindrop splashing can help launch them too.

Little windblown lichen starters.

Cool.

What's the other type?

Ecedia.

These are bigger, more like tiny outgrowths from the lichen surface, often cylindrical or branched.

They also contain both partners.

They increase the surface area, but they're also designed to break off easily and start a new lichen.

So serratia like dust, Ecedia are like little branches breaking off.

Good way to think about it.

And sometimes even bits of squamules, those little scales, can break off and establish a new lichen.

Especially in things like Cladonia.

And animals can help spread them too.

You mentioned mites.

Yeah, it's pretty cool.

Studies on lichens like the bright yellow Xanthoria peritina found that mites munching on them actually spread both the fungus and the alga in their droppings.

Okay, nature finds a way.

So once these propagols, or a lonely spore, land somewhere, how does the partnership actually form or reform?

Well, if it's a spore, the initial contact can be a bit random.

The fungal hyphae might bump into various algae.

A meat cute for microbes?

Sort of.

Yeah.

If it's a compatible match, they form what's called a pre -thallus.

It's like a little non -layered crust, just the cells wrapped by fungus.

So redia or icedia can also germinate into this pre -thallus stage.

Okay, the first step.

Then, if the conditions are right, the light, the moisture, who knows what else this pre -thallus grows,

starts to differentiate, secrete some mucilage, and develops into that mature layered lichen structure.

What decides if they're compatible?

Is it purely genetic?

That's the million dollar question.

Specificity is complex.

It's definitely got genetic components, but environmental factors play a huge role too.

We're still figuring out the exact triggers.

And I read something about lichens being mosaics, like multiple individuals fused together.

Yes.

A single mature lichen thallus can actually be a jigsaw of genetically different fungi and algae.

They all started as separate pre -thallus and then merged.

Talk about cooperation.

Oh, and sometimes they even have two different photobiants.

Some do.

Usually, a green alga is the main partner, but they might also incorporate a nanobacterium in specific distinct zones called cephalodia.

These often look different.

Maybe darker bumps or patches on the main lichen body.

Okay, this whole finding the right partner thing sounds crucial, but difficult.

And some fungi have found a sneaky shortcut,

something called kleptobiosis.

Ah, yes, klepto like stealing.

It's a wild idea.

Finding the right photobiant, especially one like Trebuchia that might not be common just floating around, is hard for a fungal spore.

So some fungi become like anechylous lichens.

They basically attack an existing lichen and poach or steal its photobiants to start their own partnership.

They steal the algae.

That's amazing.

Resourceful, I guess.

Or just parasitic.

Some fungi just feed off the lichen without forming their own stable phallus.

But kleptobiosis forming a new lichen with stolen goods, that's really something else.

Okay, let's switch gears to the chemistry.

How does this partnership actually work nutritionally?

It's a pretty elegant system.

The photobiant does the photosynthesis, makes sugars.

The fungus gets most of those sugars.

How did scientists figure that out?

Using a clever method called the inhibition technique.

Basically, they figured out which sugars move from the photobiant to the fungus.

And what are they sharing?

If the partner is a cyanobacterium, it exports glucose.

If it's a sorbitol or ribital.

Different partners, different sugars.

Yep.

And once the fungus takes these up, it quickly converts them into mannitol.

Maybe to keep the concentration gradient flowing, you know.

And it's an efficient transfer.

The fungus gets a good deal.

A very good deal.

In some cases, over 70 % of the carbon fixed by the photobiant gets transferred to the fungus.

70%.

Wow.

Does the fungus control that flow?

How that export is regulated by the photobiant?

That's still a bit of a mystery.

Okay, so how do they physically connect inside for this transfer?

Do fungal threads poke into the algal cells?

They make very close contact.

The fungal hyphae might push into the gelatinous sheath around cyanobacteria or press right up against the cell wall of green algae.

They form structures, kind of like apresoria for holding on and hostoria for nutrient exchange.

Hostoria?

Do they actually break through the algal cell wall?

Sometimes, in simpler lichens, they might form intracellular hostoria that go inside the cell.

But in most complex lichens, they use intraparietal hostoria.

Intraparietal meaning?

Meaning within the wall.

These are more like pads that press against the algal cell wall, maybe indenting it, but they don't break through the algal cell membrane.

Why is that distinction important?

Why not just punch through?

It's thought to be a key adaptation for survival.

The main nutrient transfer happens apoplastically through the cell wall space between the fungus and the alga, helped by special proteins.

By not puncturing the delicate cell membrane, they protect it.

Remember, these lichens are constantly drying out and getting wet again.

Membranes are really vulnerable to damage during those cycles.

This hands -off approach helps keep the algal cells alive and functioning through those stresses.

Ah, that makes sense.

Protecting the power source.

Brilliant.

What about other nutrients, like nitrogen?

Good question.

If the partner is a cyanobacterium, the fungus gets a nitrogen bonus.

Cyanobacteria can fix atmospheric nitrogen gas into ammonia.

Which the fungus can then use.

Exactly.

The fungus takes up the ammonium and uses it to build amino acids and other nitrogen compounds.

It's a huge advantage in nitrogen -poor environments.

Okay, so the fungus gets carbs, sometimes nitrogen, and a structure to live in.

What's in it for the photobiote?

Protection seems key.

Massive protection.

Think about UV light.

Many lichens, especially ones living in bright sun or high altitudes, have these brightly colored pigments in their upper cortex.

Things like osinic acid, vulpinic acid, parietin.

The yellow and orange colors you often see.

Exactly.

These pigments can screen out up to 50 % of the incoming UV radiation.

This is vital for partners like buxia, which actually prefer lower light levels.

It's like built -in sunscreen.

And these pigments can be potent, too.

You mentioned vulpinic acid.

Oh, yeah.

Usic acid is only found in lichens.

It's antibacterial, antifungal.

And vulpinic acid from the wolf's lichen, the tharia vulpina.

That stuff is famously toxic.

Historically used in bait to poison wolves and foxes.

Yikes.

So animals probably learn to leave them alone.

Grazing animals generally avoid the really toxic ones, yes.

These compounds,

often called lichen acids, are secondary metabolites.

And the really weird part is, the fungus often only makes these chemicals when it's in the lichen partnership, right?

Not when it's grown alone.

That's right.

It suggests the photobiont partner somehow influences the fungus' metabolism, triggering the production of these unique compounds that you don't find in free -living fungi.

The symbiosis creates novel chemistry.

So the fungus provides a safe, UV -shielded, physically stable cultivation chamber, and the photobiont pays rent with sugar and sometimes nitrogen.

It really is an alternative way to conquer land, isn't it?

A totally different strategy compared to plants.

It works incredibly well.

Which brings us to their role as, well, environmental indicators.

The canary in the coal mine idea for pollution.

Absolutely.

They're super sensitive to air pollutants, especially sulfur dioxide, SO2.

The photobiont seems to be the more sensitive partner.

So if you see lots of sensitive lichens, the air is probably clean, and if they disappear.

It's a strong sign of pollution.

You can map pollution levels just by looking at which lichen species are present or absent.

They're fantastic bio -indicators.

And wasn't there that weird case with Lichonora canisioides, the one that liked pollution?

Ha!

Yes, the exception that proves the rule.

Yeah.

By the 1950s, it was the most common lichen in northern Europe because it tolerated SO2 so well.

But then, when air quality improved in some places.

It declined.

It actually declined.

It seemed like it had adapted to need the higher SO2 levels, almost addicted to the pollution it tolerated.

That is wild.

And the Munich example really shows the recovery is possible.

A classic study.

The lichen desert in the city center grew massively between the 1890s and 1950s as SO2 pollution rose.

Then, as emissions were controlled, that lichen -free zone shrank and eventually disappeared by the 1980s.

But then different problems arose.

Sometimes, yeah.

Different species moved in.

Sometimes ones that thrive on nitrogen pollution or eutrophication.

And occasionally, they can cause issues.

Like that Dorina species decaying limestone we mentioned earlier.

So they reflect the type of pollution too.

And it's not just air quality, right?

They accumulate radioactive stuff.

They do.

Because they absorb nutrients directly from rainwater and air, even tiny amounts, they tend to concentrate substances dissolved in the atmosphere.

Including radioactive isotopes.

Like from Chernobyl.

Exactly.

Lichens in affected areas concentrated radionuclides like cesium -137.

This thing gets into the food chain.

The lichens are eaten by reindeer or caribou.

And then potentially by humans who eat those animals.

So they become part of the pathway for radioactive material.

Yes.

Sadly.

But it also means they are now actively used to monitor radioactive fallout and contamination levels.

Such complex roles.

Given all this, how old are lichens, evolutionarily speaking?

They go way back.

There's a fossil Cynelican one with a cyanobacteria partner dated to 400 million years ago.

400 million.

So they were around when life was really just starting to get a foothold on land.

Seems like it.

And the genetic evidence suggests that forming lichens isn't a one -off event.

This symbiotic lifestyle has evolved independently multiple times in different fungal groups.

And interestingly, it seems to have been lost multiple times, too.

Some non -lichen fungi might have actually evolved from lichen ancestors.

Wow.

Evolution is messy.

It's not just a straight line.

This partnership strategy is so powerful, it keeps popping up.

It really is a successful evolutionary pathway.

And most of today's lichens, over 75 percent, belong to just one huge order.

The leccanoralis.

Leccanoralis.

Okay.

It's one of the biggest orders in the Azkamecota frangii.

Contained many of the lichens you'd commonly see, they have some distinct microscopic features in how their spores are produced and released.

Can we quickly paint a picture of a few key examples from this group?

Maybe start with leccanora.

Sure.

Leccanora are typically crustose lichens, very common on rocks, walls, monuments, bark.

We already mentioned elkanosides, the pollution -tolerant one.

Many are yellow or orange due to those light -screening pigments.

And there's the famous manna lichen, leccanora escalenta.

Man, like from the Bible.

Well, that's the legend.

It can apparently detach, roll up, and blow around in the wind in arid regions.

Supposedly edible, though maybe not very tasty.

Okay, cool.

What about xanthoria, the bright yellow leafy one?

That's xanthoria peritina.

Very common.

Especially near coasts or farms.

Places with more nutrients.

Bright orange -yellow, folios, lobed.

Doesn't make stordia, but it gets around, effectively, maybe partly through that kleptobiosis trick, stealing algae and getting spread by mites.

Got it.

And peltigera, they look different.

Yeah, peltigera are larger, leafy lichens, often found on the ground among mosses.

They frequently partner with the cyanobacterium nostoc.

And they're known for sometimes forming those morphotype pairs or lichen chimeras we talked about.

Right, where the same fungus with a different algal partner can look totally different, like green versus dark gray or black.

Exactly.

Depends on whether the main partner is green algae or nostoc.

Fascinating.

And cladonia.

Everyone recognizes reindeer moss, right, or those little cup lichens.

Cladonia is a huge and varied genus.

They have that cool two -part thallus, a base of little scales, the primary thallus, and then these upright structures called plodaceae.

Plodaceae.

These plodaceae can be shaped like little trumpets or golf keys.

Cladonia picustata, pixie cups.

Or they can be intricately branched and shrubby, like cladonia rangiferina and its relatives, the reindeer lichens.

Or some are just simple stocks, maybe with bright red tips, like cladonia floriciana, the British soldiers lichen.

British soldiers.

I love that name.

And the cup shape helps spread.

The serratia, yeah.

Rain can splash into the cups and help disperse the powdery serratia.

Very clever design.

And the reindeer lichens, cladonia species, they're actually food.

Absolutely vital winter food for reindeer and caribou in boreal forests and tundra.

Laplanders traditionally harvest it as winter fodder for their animals.

Wow.

And people use them for decorations.

Yep.

Especially in Germany, they're used in wreaths and floral arrangements.

And model railway enthusiasts and architects love them because they look like perfect miniature trees.

From feeding reindeer to decorating train sets, lichens do it all.

All right, let's try and wrap this up.

We've gone from the basic definition, this incredible fungus photobiote partnership.

To their amazing adaptability, surviving extreme conditions, shaping rocks, living for centuries.

Their complex reproduction, the chemistry of their nutrient exchange, their potent defensive compounds.

Their role as pollution indicators,

radionuclide accumulators, and their ancient evolutionary history.

It really is a hidden world hiding in plain sight.

So if there's one big takeaway, what is it?

For me, it's the power of cooperation.

In a world we often view through the lens of competition, lichens are this stunning example of how teamwork, bringing together wildly different life forms,

can create something incredibly resilient,

successful,

and, frankly, beautiful.

A profound lesson from a seemingly simple crust on a rock.

It makes you wonder, doesn't it, what other unexpected partnerships, what other hitter collaborations are thriving all around us, just waiting for us to notice.

A great thought to end on.

There's always more to discover.

Indeed there is.

That's all the time we have for this deep dive into the secret lives of lichens.

Thank you so much for joining us.

Thanks for listening.