Chapter 7: Lichens: Dual and Triple Extremophile Organisms

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Welcome back to The Deep Dive.

Today we're unlocking the secrets of one of Earth's most astonishing survivors, often right under our noses.

That's right, things you might walk past every day.

Exactly.

Imagine an organism so incredibly tough it can not only exist but actually flourish on bare rock in like scorching deserts, the frozen arctic.

Even way up high on Mount Everest.

It's wild.

The question is, how do they do it?

What's their secret weapon for surviving where almost nothing else can?

That's the million dollar question, isn't it?

So we're plunging into the world of lichens, these unique composite beings, to answer that very question.

Our mission in this deep dive is to explore what makes them tick, how they thrive in these impossible places, and why they're so crucial to our planet.

And we're drawing this from Bryce Kendricks, the fifth kingdom, trying to paint a picture just with words.

Exactly.

No visuals needed here.

We'll break down the complex bits.

Let's start decoding their incredible resilience.

So let's set the stage.

When we say lichens are ultimate survivors, we really mean it.

You can find them almost anywhere.

Just about, yeah.

Picture the windswept rocky cliffs along Lake Superior, covered in them, or the frozen ground in northern Quebec.

Tough spots.

Totally.

They even cling to tree bark in tropical Queensland, Australia.

These aren't exactly cozy environments, you know.

Far from it.

We're talking intense temperature swings, harsh radiation,

serious drought.

They conquer hot deserts, the sub -zero arctic.

And get this, sometimes they live beneath the surface of desert rocks.

Yeah, those crypto -endolyphic ones, literally inside the stone.

It's amazing.

They truly thrive where pretty much nothing else can survive.

But there's one catch, right?

One critical requirement.

Clean air.

That's their Achilles heel, surprisingly.

Interesting.

And that extreme adaptability, like you were saying, it points directly to their fundamental design.

Oh, okay.

See, the secret to their toughness isn't that they're a single, super resilient organism, not quite.

It's that they're a highly specialized partnership.

Okay, here's where it gets really interesting then.

This incredible toughness comes from a team effort, usually between a fungus.

Which we call the mycobiont.

Right, the mycobiont.

Yeah.

And at least one type of alga or cyanobacteria.

Known as the photobiont.

Yeah.

So can you walk us through who does what in that partnership?

Because it sounds pretty dynamic.

Absolutely.

Think of the fungus, the mycobiont, as the architect and the protector.

It builds the main body, the structure we actually see, called the thallus.

The thallus.

That gives the lichen its shape.

The fungus also grabs water and essential minerals from the environment and it makes the reproductive parts.

And it's the bulk of the organism.

Massively so.

Over 95 % of the lichen's total biomass is fungus.

It's basically a fungus that's learned to farm.

Wow.

Okay.

So where does the photobiont fit in?

The alga or cyanobacteria?

Well, the photobiont lives inside this fungal thallus.

It might only be 5 to 10 % of the total weight.

Tiny fraction.

Yeah.

Usually in a specific zone, just below the upper surface.

But it's the engine.

It's a power plant.

Oh, yeah.

It photosynthesizes, turns sunlight, water, and CO2 into energy -rich sugars.

The food source.

Exactly.

It feeds the entire organism, including the fungus.

So the fungus provides the house and like the utilities and the alga provides the groceries.

That's a pretty good analogy.

Can you help us picture the inside?

If we say sliced a lichen thallus open, what would we see?

Okay.

Imagine a cross section.

On the top and bottom surfaces, you'd see fungal threads or hyphae packed really densely together.

Like a protective skin.

Exactly.

Forming tough outer layers, we call these the cortical layers, like a natural shield.

Okay.

Then just beneath that top protective layer, the upper cortex, you'd find the algal cells.

Usually round little green cells.

Floating around?

Not quite.

They're nestled within a much looser network of fungal hyphae.

It's almost like they're caged or held in place.

Held captive, almost.

It sounds a bit demanding for the alga.

It really is.

The relationship is often described as exploitation or maybe balanced parasitism.

Exploitation.

Wow.

Yeah.

The fungus has essentially, you could say,

captured or domesticated the alga and it pirates.

That's the term used about half of the food the alga makes through photosynthesis.

Half.

That sounds like a raw deal for the poor alga.

It does, doesn't it?

And maybe it is to some extent, but there's a trade -off.

Okay.

What's the upside for the alga?

In return, the fungus provides that crucial protection from drying out really fast, which is vital in exposed places, and it supplies essential mineral elements it absorbs.

Ah, so shelter and minerals in exchange for food.

Precisely.

This combination, this partnership across kingdoms makes the whole unit incredibly tough and successful.

It allows them to conquer those harsh environments.

It's such a fungal -dominated setup that taxonomically, lichens are basically classified as fungi now.

That's quite the bargain they've struck.

And speaking of these critical photobionts, the algae or cyanobacteria, what's really striking is how few types there actually are, relatively speaking.

That is surprising, isn't it?

Yeah.

You'd think with thousands of lichen species, you'd have thousands of algal partners, but no.

Nope.

Most lichens, the vast majority, associate with just a handful of specific algal or cyanobacterial genera.

Who are the main players?

Well, the big one is a unicellular green alga called Trebuchsia.

It's found in something like 80 % of all known lichens.

80%, okay.

Then you've got a filamentous green alga, Trentapolia, and maybe 10%,

and a filamentous cyanobacterium no -stock, also in about 10%.

So just three main types cover almost everything.

Pretty much the lion's share, yeah.

And this is why lichen taxonomy, how we name and classify them, is primarily based on the fungus.

The lichen gets its name from its fungal partner.

Because the fungus is calling the shots, structure -wise.

Exactly.

But you know, it's incredible how efficient this partnership is, but you mentioned earlier, triple organisms.

Ah, yes, I did.

What's that about?

Well, things get even more complex and really fascinating.

About 500 or so species of lichens actually take this a step further.

They'll have their main green algal partner doing the bulk of the photosynthesis, but then they also develop these specialized, often like wart -like bumps or structures, either on or sometimes within their main body.

These structures are called cephalodia.

Cephalodia.

And inside these cephalodia, that's where you find a different photobiont specifically.

Blue -green algae, the cyanobacteria, often no -stock.

So two different photobionts in one lichen.

That's right.

You see this in some common genera like loberia or peltigra.

So a green alga for one job and a blue -green alga or cyanobacterium for another.

What's the advantage?

Why the extra partner?

That seems to be the case.

The green algae, like trabucia or trentapolia, provide sugar alcohols, things like ribatol or erythritol to the fungus as their main energy contribution.

Okay, standard food supply.

Right.

But the cyanobacteria, like no -stock, provide glucose, but more importantly, they bring a unique and incredibly valuable skill to the table.

Which is?

They can take nitrogen gas directly from the atmosphere and fix it into a usable form, like ammonia,

nitrogen fixation.

Ah, like legumes do with bacteria in their roots.

Exactly like that.

This fixed nitrogen is then passed to the fungus, often through specialized cells in the cyanobacteria called heterocysts.

So in these three -part lichens, the cyanobacteria are basically fertilizer factories.

That's very likely their main function, yes.

Supplying that vital nitrogen, which is often scarce, enriching the whole partnership.

Pretty clever setup.

It really is.

Okay, so from that microscopic internal complexity, let's zoom out and look at the sheer variety of shapes and forms that lichens take on the outside, which you said is still dictated by the fungus?

Still the fungus, yeah.

It determines the overall morphology.

And just a quick term.

Many of the lichens we'll describe are called discolichins, because their fungal fruiting bodies, if they make them, are usually open disc -like or cup -like structures called apothecia.

Got it.

Discolichins.

So let's paint a picture of these different forms.

What's the first type?

Okay, first up we have crustose lichens.

Crustose, like a crust.

Exactly.

Imagine a thin, flat layer, almost like a splash of paint, that's fused so tightly against a rock or tree bark that you literally can't peel it off without breaking it or the substrate.

Wow.

Really stuck on there.

Totally.

They become one with the surface.

You see them as brilliant orange patches or maybe blue -green films, sometimes even growing into the rock itself.

They're the ultimate clingers.

Okay, crustose.

What's next?

Next, think folios.

These look more like small, flattened leaves or lobes.

Think foliage.

Leaf -like, okay.

They're often somewhat ruffled or lobed at the edges, and they're generally attached more loosely than crustose lichens, often by little root -like threads underneath called rhizenes.

Rhizenes, like tiny anchors.

Sort of, yeah.

Picture a gray -green, roughly circular patch spreading across a rock, looking like a delicate, leathery rosette that's often a folios lichen, like Parmelia.

Okay, crusty ones stuck tight, leafy ones more loosely attached.

What else?

Then we get the really three -dimensional ones, fruticose lichens.

Fruticose.

Yeah.

Like fruit.

Good question, but no.

It actually means bush -like or shrubby.

These are often complex, branched structures that grow away from the substrate, standing upright or hanging down.

Ah, so not flat against the surface.

Right.

Think of a tiny, intricate bush.

Or picture those bright yellow, lacy branched lichens you sometimes see on dead tree branches that could be Lothoria.

Or the delicate, wispy strands like old man's beard, of Nea hanging from pine trees in wet forests.

Those are classic fruticose forms.

Okay, bush -like or dangly.

Got it.

It's three types.

Fourth type is squamulose.

This one's a bit different.

It's made up of many small, overlapping, often upturned scales, squamules.

You often find these growing on soil.

Until scales.

Yeah.

And a great example here are species of Cladonia, which includes things like reindeer moss.

Many Cladonia have what's called a dimorphic thallus.

Dimorphic, meaning two forms.

Exactly.

They have the small, leafy squamules forming a base layer, often on the ground.

And then rising up from that base, they produce upright, sometimes branched, stalk -like structures.

These upright structures are called podisha.

They can look like tiny trumpets or branched antlers or little clubs.

And these podisha are often where the reproductive organs, the apothecia, are formed.

So a base of scales and then these upright stalks.

Podisha.

Interesting.

What else?

Fifth, there's lepros.

This is maybe the simplest form.

The entire lichen body, the thallus, is just made of loose, powdery granules, almost like someone dusted the surface with powder.

No real structure, just powder.

Pretty much, yeah.

Very basic.

And finally, number six, there are the more unusual gelatinous lichens.

As the name suggests, when they're wet, they often have a rather soft, swollen, jelly -like texture, often dark -colored.

Wow.

Quite a range.

Crusty, leafy, bushy, scaly, powdery, and jelly -like.

Covered a lot of ground, doesn't it?

All determined by the fungal partner.

So we have these incredibly diverse forms, masters of survival.

But how do these unique composite organisms actually reproduce and pass on their legacy?

Well, reflecting their complex nature, they have a multi -faceted approach.

It's not just one method.

Many lichens do reproduce sexually using the machinery of their fungal component.

As we mentioned, most are discolichins forming open, cup -like apothecia, but some form flask -shaped structures called perithesia instead.

Interestingly, some lichen fungi, like Lepruria, the powdery one, seem to have lost the ability to reproduce sexually altogether.

They rely entirely on other means.

All eggs in one basket, then?

Sort of.

And while, as we said, over 98 % of lichen fungi are Ascomycetes, there are a few rare Bacidiomycete lichens, too.

Their fungal partner is more like a tiny mushroom.

And the spores produced by the fungus?

Are they just regular fungal spores?

They are fungal spores, yes.

Specifically, ascospores in most cases, but they show a lot of variety, too, reflecting the diversity of the fungi involved.

The ascii, the little sacs holding the spores, come in different types.

Okay.

And unlike many non -lichenized fungi, you don't often see a big puff of spores.

They tend to mature slowly, be released gradually.

The spores themselves also vary single -celled, two -celled, multi -celled.

It highlights how specialized these fungi have become within the lichen partnership.

But what's really fascinating, I think, is that,

beyond the fungus doing its own sexual reproduction,

many lichens also rely heavily on asexual strategies, right?

Like they have backup plans involving both partners.

Absolutely.

It's a very pragmatic approach to ensure the whole partnership continues.

Many lichens produce tiny, dark, flask -shaped structures called pycnidia, which release asexual fungal spores called knidia.

That's one way.

Okay.

Asexual fungal spores.

But even more common, and arguably more important for spreading the complete lichen, are specialized somatic propagules.

Somatic propagules, meaning bits of the body.

Exactly.

They're essentially prepackaged, miniature lichens containing both the fungus and the alga, ready to break off and grow somewhere new.

Ah, clever.

What do they look like?

There are two main types.

One is called serratia.

These are tiny, powdery masses consisting of a few algal cells tangled up in fungal hyphae.

They erupt through the lichen's surface, like dust, and get dispersed by wind or water.

Like little lichen dust bunnies.

Kind of, yeah.

The other type is acidia.

These are small, finger -like or sometimes branched or coral -like outgrowths from the lichen surface.

They're covered by the protective cortex layer, making them a bit more robust than serratia, and they simply break off.

So, serratia are powdery bits, acidia are little break -off fingers.

That's the gist.

And lichens that rely heavily on producing lots of sordia or acidia are often less likely to bother forming the sexual fungal fruiting bodies, the apothecia.

It's like they specialize in one strategy or the other.

And those podaea in Cladonia, those upright stalks, they fit in here too.

They do.

Those podaea are amazing structures.

They can bear the sexual apothecia at their tips, yes, but sometimes they can also produce serratia along their sides or even become quite leaf -like and photosynthetic themselves.

They're very versatile.

So, if we connect all this amazing biology to the bigger picture, the way lichens are classified taxonomically, it seems to reveal something pretty profound about evolution itself.

It truly does.

The key point is that the process of a lichenization,

this evolutionary invention of forming a stable fungus -alga partnership, has happened independently multiple times throughout the history of fungi.

Ah, so it wasn't just one ancestor that figured it out.

Not at all.

It evolved again and again in completely different fungal groups.

This means lichens are not a single, unified group with one common ancestor, like mammals or birds.

So what are they then, classification -wise?

They are what we call a polyphaletic group.

It's basically a nutritional group or an ecological group, defined by their symbiotic lifestyle, how they get their food, rather than a strictly taxonomic one based on shared ancestry.

That's fascinating.

Evolution hitting on the same winning idea multiple times.

Exactly.

It's a testament to nature's ingenuity and how successful this partnership strategy is.

And while it happened in several groups, the vast majority, something like 40 % of all known Ascomycetes are lichenized, but very few Basidiomycetes tried it.

So mostly an Ascomycete phenomenon.

Overwhelmingly.

You find most common lichens, like Reindeer moss, Cladonia, or Rock tripe, umbilicaria, within just a few major orders of Ascomycetes.

Given all this complexity, the partnerships, the different forms, the independent evolution,

how do scientists actually identify these things, especially if some look identical but are chemically different?

Ah, that's where their unique chemistry becomes crucial.

Lichens are like little chemical factories.

They produce about 300 unique organic compounds, often crystalline, that you don't typically find elsewhere in nature.

We call them lichen substances.

I get substances.

These are mainly weak phenolic acids, things like Depsides and Depsidones.

Some, like Acinic acid, even have antibiotic properties.

Fun fact.

The litmus dye used in chemistry labs originally came from lichens containing these substances.

No way.

Litmus paper comes from lichens.

Originally, yes.

And these substances are vital for identification.

Lichenologists use simple chemical spot tests right in the field or lab.

What kind of tests?

Usually using drops of potassium hydroxide solution, that's the KOH test, or K -test, sometimes followed up by chlorine bleach, the Sialer C test,

and another chemical called para -phenylenediamine, the PPD or P test.

OK, KCP test.

Right.

You apply these chemicals, sometimes in sequence, to different parts of the lichen thallus, and specific lichen substances will react to produce characteristic color changes, yellow, orange, deep red, et cetera.

Like a chemical fingerprint for that species.

Exactly.

It's often essential for telling apart species that look almost identical visually.

But I imagine for really tricky cases, or those chemical strings you mentioned, where they look the same but have different chemistry,

you need more advanced techniques.

Absolutely.

For more precise identification, especially confirming those chemical variations,

scientists move to lab techniques.

They might extract the lichen substances with a solvent -like acetone, let it evaporate, and look at the tiny crystals formed under a microscope, sometimes using UV light.

Wow.

Or, more commonly now, they use methods like thin -layer chromatography, TLC, or high -performance liquid chromatography, HPLC, to separate out all the different chemicals and create a detailed profile.

And of course, increasingly, DNA analysis is used too.

So it's a real mix of field observation, chemistry, and modern molecular biology.

It really is.

All needed to unravel the identity of these complex organisms.

Now, what's really fascinating here is this long -standing puzzle you hinted at earlier, lichen synthesis.

How do new lichens actually form in nature?

Yeah, that was a real head -scratcher for a long time.

See, the fungal spores, the ascospores, are usually released without any algal cells attached.

Right, just the fungus part.

So for a new lichen to form, that tiny fungal spore, after landing somewhere, has to find the right kind of free -living alde nearby and somehow initiate the partnership all over again.

This must happen constantly in nature.

Makes sense.

But for decades, scientists tried to replicate this in the lab, mixing the right fungus and the right alga together, and they just couldn't get them to form a proper lichen.

It always failed.

So what was the missing piece?

What was the trick?

The breakthrough finally came when researchers realized the secret.

Both prospective partners, the fungus and the alga, needed to be in a, quote, thoroughly debilitated condition, basically stressed out and weakened.

Debilitated, you mean like starved?

Pretty much.

Grown under nutrient -poor conditions, struggling a bit.

Only then, it seems, would the fungus successfully embrace the alga, and only then would the alga kind of allow itself to be incorporated without putting up too much resistance.

Huh.

So the fungus needs to catch the alga when its guard is down.

It still sounds a bit like a forced partnership.

It does have that element, doesn't it?

And once contact is made, the fungal hyphae grow around the algal cell and form these tiny specialized attachment structures, almost like little suction cups or pads, right on the algal cell wall.

What are those called?

Those are called apresoria.

They seem to be key for establishing that intimate physical connection.

Okay, so the fungus grips the weakened alga with apresoria.

Then what?

How does the food transfer start?

Well, once the alga is integrated into the thallus, something subtle changes in its physiology.

The fungus seems to induce the algal cell wall to become leaky.

Leaky?

Yeah.

The alga keeps photosynthesizing more or less normally, but it starts losing large quantities of the soluble carbohydrates.

It produces those sugar alcohols like ribidol from tribuxia, erythritol from trentapolia, or glucose from nostoc.

And the fungus just soaks it all up.

Like a sponge.

The fungus quickly absorbs these leach sugars and immediately converts them into its own fungal carbohydrates, like mannitol or trepholose.

This conversion is thought to be really important for the lichens' incredible ability to survive drying out desiccation tolerance.

Wow.

A perfectly optimized, if slightly coerced, energy transfer system.

Seems to be, yes.

Now, given that the alga, the food producer, makes up such a tiny percentage of the whole lichen body.

Yeah, about 10%.

And that lichens in these exposed environments are probably dried out and inactive for a lot of the year,

you might expect them to grow incredibly slowly.

You'd be absolutely right.

Growth rates are typically very, very slow.

For many common crustose or folios lichens, a respectable growth rate is only maybe 1 to 4 millimeters per year.

Millimeters per year.

Tiny.

Incredibly slow.

However, counterbalancing this slow growth is their remarkable longevity, their tough resistant thalli, and that ability to just shut down when dry or cold and then quickly resume metabolism when conditions improve.

It allows them to persist for centuries, even millennia.

How long are we talking?

Some individual lichen colonies, particularly crustose ones in arctic or alpine regions, are estimated to be 4 ,500 years old.

4 ,500 years.

That's mind boggling.

That's older than the pyramids.

It puts them right up there with ancient bristlecone pines as potentially the oldest living things on earth.

That is absolutely incredible.

And this super slow but measurable growth,

it actually has a practical application, doesn't it?

In dating things.

It does, yeah.

It's a field called lichenometry.

Lichenometry.

Measuring lichens.

Basically, yes.

Lichenologists carefully measure the growth rates of certain circular lichen species, often on surfaces where the start date is known, like old gravestones or buildings.

Okay.

Once you have a reliable growth rate for a species in a particular region, you can then measure the size of the largest lichens on a surface of unknown age, say, a rock face exposed by a retreating glacier or an ancient stone structure.

By comparing the size to the growth rate, you can estimate how long that surface has been stable and exposed for the lichen to grow on it.

So, glaciologists can use lichens to figure out how long ago the ice melted back from a particular area.

Exactly.

It's a really useful tool for dating landscape changes over hundreds or even thousands of years.

Amazing.

So, what does all this mean for us then?

Beyond their incredible biology and survival skills, do lichens play a bigger role?

You mentioned clean air earlier.

Yes.

They are critical environmental sentinels, often called the canaries in the coalmine for air quality.

Canaries in the coalmine.

Why?

Because, as we discussed, lichens generally lack roots or specialized absorptive organs like plants have.

They get most of their nutrients, especially minerals,

directly from rainwater and atmospheric dust landing on them.

Right.

They just absorb whatever falls on them.

Precisely.

This makes them extremely sensitive, extremely susceptible to air pollutants dissolved in that rain or settling as dust, especially things like sulfur dioxide, SO2, which was a major component of acid rain.

Ah, so polluted air hits them harder than plants with roots.

Much harder, generally.

SO2, for example, dissolves in water to form sulfurous acid, which directly damages the algal partner and disrupts photosynthesis.

Different lichen species have different tolerance levels, but many are wiped out by even moderate air pollution.

So, if you go into a big city… You often find what's called a lichen desert, an area usually starting in the city center and extending outwards, where only the most pollution -tolerant species can survive, or sometimes no lichens at all.

The diversity increases as you move further out into cleaner air.

So their presence, or absence, and the types present can tell us a lot about the air quality.

Exactly.

They are incredibly sensitive bio -indicators of environmental health.

Monitoring lichen communities can give us early warnings about air pollution problems that could eventually affect other parts of the ecosystem, including forests, crops, and ultimately us.

Their sensitivity reflects dangers to the entire biosphere.

Wow.

So these tiny, slow -growing, ancient organisms are actually holding up a mirror to our own impact on the environment.

In a very real way, yes.

Okay, so let's try and pull this all together.

What's the big takeaway from this incredible deep dive into the world of lichens?

Well, we've seen there are these remarkably resilient composite organisms, right?

True masters of adaptation.

Definitely.

Through that unique fungal -algal partnership, sometimes even a triple partnership.

Yeah, displaying this huge diversity of forms, using all sorts of reproductive strategies.

And crucially, serving as these silent, vital environmental indicators, telling us about the health of our planet just by their presence or absence.

Absolutely.

And I think it also highlights that evolutionary point, again, lichens as a nutritional group, not a single lineage.

It's a beautiful example of how nature can arrive at the same brilliant solution partnership independently, multiple times, to conquer tough conditions.

It really is amazing.

But here's a final thought, maybe a bit provocative.

Despite being such successful extremophiles here on Earth, the author of our source text, Kendrick, suggests that finding lichen -like life on other truly extreme planets might actually be unlikely.

Really?

Why is that?

They seem so tough.

The composite lichen is tough, yes.

But the individual components, the fungus, the alga, they aren't nearly as robust on their own.

The partnership makes them stronger than the sum of their parts, but maybe only up to a point.

If conditions on Mars or Europa or wherever are truly at the absolute edge of what life can tolerate.

Then maybe you need something intrinsically tougher to begin with.

Exactly.

Perhaps we should be looking more towards organisms like the archaea, those single -celled microbes known for thriving in boiling hot springs or deep sea vents, as the kind of life that might make it on truly alien extreme worlds, rather than these complex, evolved partnerships like lichens.

It's just a thought.

That is a fascinating thought to end on.

Well, we certainly hope this Deep Dive has given you, our listeners, a whole new appreciation for these often overlooked, incredibly complex and vital organisms.

I hope so too.

They deserve more attention.

On behalf of the Deep Dive team, thank you so much for joining us for this in -depth look at the world of lichens.