Chapter 12: The Eukaryotes: Fungi, Algae, Protozoa, and Helminths

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Welcome to the Deep Dive.

Today we're jumping right into a really striking medical scene.

Picture this.

You're in a clinic, West Africa.

Yeah.

A mother comes in with her four -year -old daughter.

Okay.

The little girl's got a swollen stomach, right.

And then the mother shows you this large white worm, cylindrical, maybe 10 centimeters long, deep danes.

The girl coughed it up.

Wow.

That's quite an image.

Really vivid.

Yeah.

So the immediate questions are, you know, what is that?

And how did it even get there?

Well, it throws you straight into the reality of parasitic diseases, doesn't it?

The global scale is just immense.

We're talking over two billion people affected worldwide.

Two billion.

That's staggering.

It really is.

And think about malaria, a million deaths every year, mostly kids, plus, you know, hundreds of millions dealing with intestinal worms and protozoa.

And it's not just a problem over there, is it?

Even in the U .S., the numbers are significant.

Absolutely.

You've got over a million new cases of trichomoniasis annually,

maybe 60 million people infected with trypanosoma cruzivitagas parasite.

It's why the CDC prioritizes certain parasitic diseases.

Because they're common, they're serious, but also

they're preventable and treatable.

Exactly.

And we're seeing new challenges, too, like focal pathogens,

Cryptococcus gati popping up in North America.

These things are constantly evolving.

So that leads us to our mission for this deep dive.

We want to really explore this whole world of eukaryotic microorganisms, the fungi, algae, protozoa, those parasitic worms, even the arthropods that spread them.

Right.

We'll dig into what makes them tick their structures, life cycles, the diseases, obviously, but also some maybe surprising applications.

We're aiming to pull out those key bits of knowledge for you.

And the fundamental challenge, which we have to keep in mind, is that these are eukaryotes.

Their cells are structurally similar to our cells.

Which makes treatment so difficult.

It's hard to target the pathogen without harming the host.

A very different ballgame than treating bacterial infections.

Okay, so let's start unpacking this.

Where should we begin?

Maybe with fungi.

Sounds good.

Fungi.

They're in their own kingdom.

Kingdom fungi.

And they're chemoheterotrophs.

Okay.

Chemoheterotrophs.

Meaning they get energy and carbon from organic stuff.

Precisely.

But they don't eat it like animals do.

They absorb it.

They secrete enzymes outside their bodies, break down complex molecules, and then just up the nutrients.

Absorption.

And we usually think of mushrooms, molds.

Those are multicellular.

But yeasts are fungi too, right?

The single -celled ones?

Correct.

Yeasts are unicellular.

And a really key thing about most fungi is their reproduction.

They often use both sexual and asexual spores.

That's a big part of why they're everywhere.

Spores.

Okay.

So if we look at molds, what are we actually seeing?

Those fuzzy bits.

That fuzzy stuff is usually a mycelium.

It's a visible mass made up of tiny inner -woven filaments called hyphae.

Think of them like threads.

Hyphae.

Got it.

Yeah.

Are they all the same?

Not quite.

Some hyphae, septate hyphae have cross walls, kind of dividing them into cell -like units.

Others, coenocytic hyphae, are basically long, continuous tubes with multiple nuclei floating around.

Huh.

And do they have different jobs?

Yeah.

You can think of vegetative hyphae as the ones doing the nutrient absorption, often unseen within the food source.

And aerial hyphae project upwards.

And their job is usually reproduction, burying the spores.

Okay.

So hyphae for molds.

What about the yeasts?

They're just single cells.

Yep.

Non -filamentous, usually spherical or oval.

And they reproduce differently too.

Many, like Saccharomyces, brewers, yeast, divide unevenly by budding.

A little bud pops off the parent cell.

Budding.

Okay.

But then you have fission yeasts, which divide evenly, like bacteria almost.

And some, like Candida albicans, can do this interesting thing where buds don't fully detach.

They form a chain called a pseudohypha.

Pseudohypha.

Like a fake hypha.

Kind of.

It allows Candida to stretch out and invade tissues more effectively.

It's a neat adaptation.

And that leads to something called dimorphism, right?

Two forms.

Exactly.

Dimorphic fungi.

This is super important, especially for pathogens.

They can switch between being yeast -like and mold -like, depending on the environment.

So like mold in the soil, yeast in the body.

Often, yes.

Yeast -like at body temperature, 37 Celsius.

And mold -like at cooler temps, say 25 Celsius.

This switch helps them survive in different places and cause disease.

Some even switch based on CO2 levels.

Wow.

Okay.

Reproduction, again.

You mentioned spores.

Asexual and sexual.

Right.

Asexual reproduction can happen just by fragments of hyphae breaking off and growing.

Yeah.

But they also produce true asexual spores through mitosis.

These are genetically identical to the parent.

So different from bacterial endospores, which are just for survival.

Totally different.

Fungal spores are for reproduction.

And there are different kinds, canidiospores in chains, like in penicillium, arthrocanidia from fragmented hyphae, blastocanidia from buds, clemetocanidia, which are thick -walled survival spores within a hypha, and sporangiospores formed inside a sac.

Lots of variety.

Okay.

That's asexual.

What about sexual reproduction?

Sexual reproduction involves the fusion of nuclei from two different mating strains.

It creates genetic diversity.

There are three phases.

Plasmogamy, the cytoplasm fuses, karyogamy, the nuclei fused to form a diploid zygote, and then meiosis, where the diploid nucleus divides to form haploid sexual spores.

More complex.

So fungi seem pretty adaptable.

Where do they thrive?

Almost anywhere, really.

They often prefer slightly acidic conditions around pH 5, which inhibits many bacteria.

Most molds need oxygen, but yeasts are often facultative anaerobes they can manage without it.

And they handle tough conditions, too, like dryness or high salt.

Yeah, they're generally more resistant to high osmotic pressure than bacteria.

That's why they can grow on things like dry fruit or salty meats.

They also need less nitrogen and can digest complex carbs like lignin, the tough stuff, and wood.

Explains why you see mold on bathroom walls, old shoes, newspapers.

Exactly.

They're decomposition masters.

Let's talk medical importance.

What are the main groups we should know?

Okay, briefly.

You've got the zygomycota, like black bread mold.

Rhizopus.

Microsporidia are weird tiny intracellular parasites, no mitochondria, often seen in AIDS patients.

Ascomycota, or sac fungi, is a huge group, including aspergillus, candida, penicillium.

Basidiomycota are the club fungi, including mushrooms and cryptococcus.

And some fungi produce both sexual and asexual spores, while others only seem to produce asexual ones.

Right.

We call the ones that do both teleomorphs.

If we've only ever seen asexual spores, historically they were dumped into a group called deuteromycota.

But now with RNA sequencing, we can usually place them correctly.

Those previously only known by their asexual state are sometimes called anamorphs.

Okay.

So fungal disease is mycosis.

You said they tend to be chronic.

Yes, because fungi grow slowly.

And remember that treatment difficulty.

We usually classify mycosis by how deep they go in the body.

How does that work?

Systemic mycosis are the deepest affecting multiple organs, usually inhaled from soil fungi like histoplasmosis, not typically contagious person to person.

Subcutaneous are just beneath the skin, often from puncture wounds.

Sporotrichosis is an example.

And the ones on the surface.

Those are cutaneous mycosis or

dermatomycosis.

Think ringworm, athlete's foot.

They affect the epidermis, hair, nails, anything with keratin.

The fungi involved, dermatophytes, actually secrete keratinase to digest it, transmitted by contact.

Okay.

And superficial ones?

Superficial mycosis are very localized, like on hair shafts, more common in the tropics.

But you also mentioned opportunistic ones.

Those sound particularly worrying.

They are.

These are fungi that are normally harmless, but cause disease in compromised hosts, someone who's debilitated on broad spectrum antibiotics, immunosuppressed, maybe has lung disease.

And they can cause serious problems.

Definitely.

They can infect almost any tissue and often become systemic.

Pneumocystis pneumonia in AIDS patients is a classic example.

Snakibocres, the black mold on damp walls, can cause issues.

Mucormycosis in diabetics, aspergillosis in cancer patients, candidiasis.

Candid again, thrush, vaginal yeast infections.

Exactly.

Often happens after antibiotics kill off protective bacteria or in newborns or AIDS patients.

It's a major opportunistic pathogen.

This actually loops back to a clinical case mentioned earlier about a man named Ethan and his dog Waldo.

Ah, yes, the Cryptococcus gaddi case.

Both developed respiratory symptoms, difficulty walking for the dog.

And they hiked a lot in Douglas fir forests.

So environmental exposure.

Almost certainly.

C.

gaddi is a dimorphic fungus found in soil associated with certain trees.

It grows yeast -like at body temp, which is what they found in Ethan's lung biopsy.

But it exists with hyphae in the environment.

It's a perfect example of an environmental fungus causing serious opportunistic or sometimes even primary infection.

Luckily, they were treated successfully.

It really highlights the connection.

Now, fungi aren't all bad.

Right, right.

They have huge economic importance, too.

Oh, absolutely essential.

They are the primary decomposers in many ecosystems, recycling nutrients.

Mycorrhizae, those symbiotic fungi on plant roots, are vital for plant growth, helping them get water and minerals.

And we use them directly.

All the time.

Food, obviously mushrooms.

Flementation saccharomyces for bread, beer, wine.

Industrial production like citric acid from aspergillus niger.

Even producing medicines like the hepatitis B vaccine using genetically modified yeast or the anti -cancer drug taxol from taxomyces.

Biological control, too.

Yes.

Some fungi produce enzymes like cellulase, trichoderma.

Others kill insect pests or even other harmful fungi.

But there's a downside, too.

Spoilage.

Plant diseases.

Big time.

Mold spoilage of food is a huge economic loss, partly because they tolerate conditions that stop bacteria.

And plant diseases like chestnut blight or Dutch elm disease have wiped out entire tree populations.

They're powerful forces.

And just like bacteria, we have our own fungal community, a mycobiome.

We do indeed.

Candida is a common resident in the mouth, gut, vagina, malicacea on the skin.

Usually harmless, part of the normal balance.

But opportunistic, like you said, if the balance shifts.

Exactly.

Antibiotics knock out bacteria.

Candida takes over.

Immune system weakens.

Fungi can cause trouble.

But there are also interesting interactions.

Some fungi might inhibit others, like Pichia inhibiting Candida.

Or saccharomyces boulardii, a probiotic yeast, can help digest C.

diff toxins.

It's a complex ecosystem.

Okay, fascinating.

Let's shift gears slightly.

What about lichens?

They involve fungi, right?

They do.

Lichens are amazing.

They're not a single organism, but a mutualistic team up between a fungus, usually an ascomycete, and an alga, or sometimes a cyanobacterium.

Mutualistic, so both benefit.

Totally.

The fungus forms the main structure, weaving hyphae around the algal cells.

The alga does photosynthesis and provides sugars, sometimes giving up to 60 % to the fungus.

In return, the fungus gives structure, attachment, and protection from drying out.

A perfect partnership.

Allows them to live where neither could alone.

Precisely.

They're often pioneer organisms on bare rock, or soil.

They slowly break down rock with acids, build up organic matter, paving the way for plants.

Any other uses?

Historically used for dyes, like litmus paper dye.

Some produce antimicrobial compounds, like istuanic acid.

And they're fantastic bioindicators of air quality.

They absorb airborne substances, so their health reflects pollution levels, especially things like sulfur

Studies near Chernobyl even showed reindeer eating lichens that had absorbed radioactive cesium.

Wow.

Okay, from lichens to algae themselves.

What defines algae?

Algae are basically simple eukaryotic photoautotrophs.

They use light for energy, but they lack the true roots, stems, and leaves you see in plants.

It's more of a functional description than a strict taxonomic group.

Most are aquatic.

And the seaweeds we see.

Those are multicellular algae.

Their body, the phallus, often has holdfast to anchor, steaks like stems, and leaf -like blades.

But no complex transport tissues like plants.

They absorb nutrients over their whole surface.

Some have gas splatters, pneumaticists to float.

How do they reproduce?

All can reproduce asexually, just fragmentation or cell division.

Many also reproduce sexually, sometimes with complex alternations of generations.

And they come in different colors.

Brown, red, green.

Yes.

Those colors come from accessory photosynthetic pigments along with chlorophyll.

Brown algae, like kelp, can be huge, grow incredibly fast.

They're a source of algin, a thickener.

Red algae can live deeper because their pigments capture blue light.

They give us agar and carrageenan.

Green algae are thought to be the ancestors of land plants, similar chlorophyll, cellulose walls, store starch.

What about the microscopic ones, diatoms, dinoflagellates?

Diatoms are unique unicellular, with intricate silica cell walls like little petri dishes.

They store energy as oil.

Dinoflagellates are mostly unicellular plankton.

Many have cellulose plates for structure.

And some of these microscopic ones are trouble, right?

Toxins.

Definitely.

Some diatoms produce domoic acid, a neurotoxin that can accumulate in shellfish and poison humans or marine animals.

And many dinoflagellates produce neurotoxins.

Red tides.

Exactly.

Caused by blooms of dinoflagellates like alexandrium, their saxitoxins cause paralytic shellfish poisoning, PSP.

Others like carania kill fish.

Gamber discus causes ciguatera fish poisoning, common in tropical reefs.

So despite the toxins, what's the big role of algae in nature?

Absolutely fundamental.

They are the primary producers in most aquatic ecosystems, fixing vast amounts of carbon dioxide into organic matter.

Planktonic algae generate maybe 80 % of Earth's oxygen?

80 %?

That's incredible.

It is.

And ancient algae are the source of petroleum deposits.

There are also symbionts in some animals, like giant clams.

Of course, blooms can be bad.

Decomposing algae can deplete oxygen in the water, creating dead zones.

Okay, let's move to another group of single -celled eukaryotes, protozoa.

Right, protozoa.

Unicellular eukaryotes, mostly found in water and soil.

Their active feeding growing stage is called the trophozoite.

Many are harmless, even part of our normal microbiota.

How do they survive harsh conditions or move between hosts?

Many form a cyst.

It's a protective, dormant structure that resists drying, lack of food, harsh chemicals.

For parasites, the cyst is often the infectious stage that gets transmitted.

Like GRD cysts in water?

Exactly.

Or entomoeba histolytica cysts passed in feces.

Some protozoa, the apicomplexa, form a special reproductive structure called an oot cyst.

How do they reproduce?

Asexualy, often by simple fission or budding.

Some do schizogony, multiple nuclear divisions followed by cell division, making many daughter cells quickly.

Sexual reproduction happens too, like conjugation in paramecium, or the formation and fusion of gametes.

And nutrition.

Are they all hunters?

Mostly aerobic heterotrophs.

Some intestinal ones are anaerobic.

They absorb food across their membrane or use specialized structures.

Ciliates have a mouth called a cytostome and use cilia to sweep food in.

Amoebas use pseudopods for phagocytosis engulfing food particles.

Let's hit some medically important grooves.

The excavata.

Yeah, these often have a feeding groove, typically spindle -shaped with flagella, includes diplomynads like giardia which lacks mitochondria, perbacillids like trichomonas vaginalis causes STIs, also lacks mitochondria, no cyst stage, needs rapid transfer,

and euglinozoa.

Like euglena, the photosynthetic ones.

Some are, like euglena.

But this group also includes the hemoflagellates blood parasites, very important.

Think trypanosoma.

Sleeping sickness and chagas disease.

That's them.

Trypanosoma bruce for African sleeping sickness, spread by tsetse flies.

Trypanosoma cruzi for chagas, spread by kissing bugs.

Both have complex life cycles involving insect vectors.

What about the amoeba, moving with pseudopods?

Right.

The big pathogen here is entamoeba histolytica, causing amoebic dysentery.

It actually lysis host cells, important to distinguish it from non -pathogenic amoeba.

There are also free -living ones, like acanthamoeba that can cause nasty corneal infections, or balimuthia causing brain infections, especially in the immunocompromised.

And the apicomplexa.

You said non -modals specialize for invasion.

Yes, a really significant group of parasites.

They have apical organelles to penetrate host tissues.

Complex life cycles, often multiple hosts.

Malaria fits here, right?

Plasmodium.

Absolutely.

Huge global impact.

The life cycle is a classic example.

Sexual reproduction in the Anopheles mosquito, definitive host.

Asexual reproduction in humans, intermediate host.

Sporzoites from the mosquito bite infect liver cells, then release merozoites that infect red blood cells.

The bursting of red cells causes the fever and chills.

What else is in this group?

Babesia, similar to malaria, spread by ticks.

Toxoplasma gondicats are the definitive host.

Humans get it from oocysts and cat feces or undercooked meat.

Very risky for pregnant women.

Cryptosporidium, a major cause of waterborne diarrhea.

It's oocysts resist chlorine.

Cyclospora also causes diarrhea, sometimes linked to contaminated produce.

Okay, one more protozoan group.

Ciliates.

They use cilia for moving and feeding.

Only one is a significant human parasite.

Bellentidium coli causes a severe dysenterylase in the large intestine, transmitted by cysts in feces.

Got it.

What about slime molds?

Are they fungi or protozoa?

Uh, neither, really.

They're actually classified in the phylum amoebizoa, more closely related to amoebas.

There are two main types.

Cellular slime molds live as solitary amoeba -like cells eating bacteria.

But when food runs out, they aggregate, signal each other with campy, form a multicellular slug that crawls, and then differentiate into a stalk and spores.

Pretty amazing coordination.

And the other type.

Plasmodial slime molds.

These exist as a plasmodium, a giant multinucleated mass of protoplasm.

It streams around like one huge amoeba, engulfing debris.

When conditions get tough, it forms stalked structures with spores, similar to the cellular ones.

Strange life cycles.

Okay, let's move to multicellular parasites.

The helminths, or worms.

Right.

Multicellular, eukaryotic animals.

But highly adapted for parasitism.

How so?

What adaptations?

Often, they have reduced systems, may lack a digestive system entirely, just absorbing nutrients from the host.

Reduced nervous system, reduced locomotion, they don't need to hunt or move much inside a host.

But they have very complex reproductive systems, producing enormous numbers of eggs.

To ensure transmission, I guess.

And complex life cycles, too.

Often very complex.

Frequently involve an intermediate host where larval stages develop, and a definitive host where the adult worm lives and reproduces sexually.

Some are dialysious, separate males and females.

Others are minutious or mafriditic, with both sets of organs in one worm.

Let's break them down.

Platyhelminths, the flatworms.

Dorsaventrally flattened, two main groups we care about, trimatodes, flukes, and cystodes, tapeworms.

Flukes, like liver flukes, lung flukes.

Exactly.

Often leaf -shaped, with suckers to attach.

They absorb food through their outer cuticle, named for where the adults live.

The lung fluke, Paragonomus, has a really complex cycle.

Snail is the first intermediate host, then a crayfish or crab is the second, and humans get it from eating undercooked crayfish.

Schistosoma, the blood fluke, is different.

Its circaria can burrow right through your skin from contaminated water.

Oof.

Okay, what about cystodes?

Tapeworms.

Intestinal parasites.

They have a scolix, a head with suckers or hooks to anchor in the gut, and then a chain of segments called proglottids.

Each proglottid matures, develops reproductive organs, gets fertilized, and eventually becomes just a sack of eggs that breaks off and passes out in feces.

Do they have a gut?

Nope.

No digestive system.

They just absorb digested food from the host's intestine directly through their cuticle.

How do we get them?

Usually by eating undercooked meat containing larval cysts.

With the beef tapeworm, Tasenia saginata, humans are the definitive host.

Cattle are intermediate.

We eat measly beef with cysts.

Worm grows in us.

And the pork tapeworm, Taneosolium.

Similar cycle with pigs as intermediate hosts, usually.

But here's the danger.

Humans can also become the intermediate host if they accidentally ingest the eggs from fecal contamination.

Then the larvae and cysts in our tissues, muscles, eyes, even brain.

That's cysticercosis and it's very serious.

Human to human egg transmission is possible.

That sounds much worse.

Any others?

Acunococcus granulosus is important.

Dogs are the definitive hosts.

Humans can accidentally ingest eggs from dog feces and become intermediate hosts.

The larvae form large, hydrated cysts, usually in the liver or lungs.

Often found, incidentally, or even postmortem.

Humans are a dead end for this parasite.

Okay, that's flatworms.

What about nematodes, the roundworms?

Cylindrical bodies tapered at ends.

They have a complete digestive system, mouth to anus, mostly dioecious.

Males, usually smaller than females.

And transmission.

How do we get infected?

Different ways, depending on the species.

Some are transmitted by ingesting the eggs.

Like the worm from our opening scenario, Aqueous.

Exactly, Ascaris lumbricoids.

Huge roundworm affects maybe a billion people.

Eggs are in contaminated soil, get ingested.

Larvae hatch, migrate through the lungs, which is why the girl coughed it up, gets swallowed, and mature in the intestine.

Other egg -transmitted ones.

Triturus, the whipworm, common in tropics.

Enterobius, the pinworm, very common, especially in kids.

The whole life cycle is in humans.

Females lay eggs around the anus at night, causing itching, easy person -to -person spread.

Also, Toxicara from dog -scats, and Dalisascares from raccoons.

These can cause Larvae migrans, where Larvae wander through tissues, causing damage.

Okay, so that's egg transmission.

What about Larvae being infective?

Yes, several important ones here.

Hookworms, Nicator, and Ancylostoma.

Larvae develop in soil from eggs passed in feces, then penetrate bare skin.

They travel to the lungs, get coughed up, swallowed, and mature in the intestine, feeding on blood.

Penetrate skin, wow.

Others.

Strongaloids is similar.

Larvae in soil penetrate skin.

Trichinella spiralis larvae are insisted in muscle tissue.

You get it from undercooked pork or bear meat.

Then there are filarial worms like Dyrifilaria imidus heartworm in dogs and cats, transmitted by mosquitoes.

Humans can occasionally get infected.

And Anesokine worms from raw fish.

So many worms.

Okay, finally, let's talk about arthropods as vectors.

Crucial topic.

Arthropods insects.

Ticks, mites, animals with the segmented bodies, exoskeletons, jointed legs.

Many act as vectors, transmitting pathogens.

Though sometimes the arthropod itself causes the disease, like scabies or lice.

Good point, yes.

But mostly we're focused on them as carriers.

Think arachnids, eight legs like ticks.

Ixodes for Lyme and Babesiosis.

Dermacenter for Rocky Mountain spotted fever.

And mites.

Then insectus, six legs like lice.

Pediculus for typhus.

Fleas.

Xenopsila for plague.

Flies.

Setse fly for sleeping sickness.

Deer fly for tularemia.

And mosquitoes.

Mosquitoes are huge vectors, right?

Absolutely massive.

Anopheles for malaria.

Aedes for dengue, zika, yellow fever, even heartworm.

Culex for various types of viral encephalitis.

And don't forget bugs like the triatoma or kissing bug that transmits Chagas disease.

How exactly do they transmit the disease?

Just by carrying it?

Two main ways.

Mechanical transmission is passive, like a housefly landing on feces, picking up bacteria on its feet, then landing on your food.

Simple transfer.

Okay.

And the other way.

Biological transmission.

This is more complex and often more efficient.

The pathogen actually multiplies inside the vector.

It might undergo a part of its life cycle there.

Think plasmodium reproducing sexually inside the anopheles mosquito.

The mosquito isn't just a taxi, it's a required host, the definitive host in that case.

Ah, so the vector is part of the pathogen's life cycle.

Often, yes.

The pathogen might accumulate in the vector's saliva, like malaria, Lyme disease, or feces, like Chagas disease, to be transmitted during feeding.

This is why controlling the vector population is such a key strategy for controlling these diseases.

If you eliminate the vector, you break the transmission cycle.

Right.

It makes perfect sense.

So what a journey through this eukaryotic world.

It really is incredible diversity.

From fungi acting as nature's recyclers and opportunistic pathogens, to algae generating oxygen and causing toxic blooms, and the protozoa with their intricate life cycles causing devastating diseases like malaria, to the highly specialized parasitic worms living within us, like that Ascaris worm we started with.

And we can't forget the arthropods.

Not just carrying diseases, but acting as essential biological steps for many pathogens.

Their adaptations, their life cycles, it's a constant evolutionary dance.

It truly is.

Which leaves us with a final thought, perhaps.

Looking at this constant interplay, this co -evolution, between eukaryotic microbes and their hosts, humans included,

what's the next big challenge?

What's the next frontier in understanding and managing their impact on us and the planet?

That's a great question to ponder.

The landscape is always shifting with emerging pathogens, resistance,

environmental change.

There's always more to learn.

Indeed.

We hope this deep dive has given you some valuable insights and maybe sparked some curiosity to explore further.

Thanks for joining us today.

Thank you for being part of our deep dive into the microscopic world of eukaryotes.

We hope you found it useful.