Chapter 24: Protists – Eukaryotic Microbes & Their Roles

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Welcome to the Deep Dive, where we try to make sense of complex research.

Today we're tackling a huge,

sometimes bewildering group of organisms, the protists.

Yeah, it's a big one.

A whole universe of microscopic eukaryotes.

And they're tricky because, well, they're designed more by what they aren't than what they are.

Right.

They're not plants, animals, or fungi, so they get lumped together.

Exactly.

Over 64 ,000 known forms.

It's this massive, incredibly diverse group.

Essential biology, really.

So our goal today is to cut through that complexity.

We'll look at their structure, how they survived, the major groups, basically give you a solid handle on this crucial chapter.

And we're going to start with something I found genuinely amazing, microbial farmers.

Farmers?

You mean like tiny amoebas tending crops?

Sort of.

We're talking about Dictiocelium discoidium.

Usually it's just a single cell hunting bacteria in the soil.

Okay, a standard amoeba predator.

Right.

But when the food runs out, thousands of them get together,

aggregate.

They team up.

Yeah, they form this multicellular slug.

Crawls around looking for a better spot.

It's wild, a kind of desperate cooperation.

But the farming part, where does that come in?

That's the really clever bit.

About a third of the Dicti types, when they're farming their spore structure, the sores.

Which holds the spores for dispersal.

Exactly.

They actually pack living bacteria in there, along with the spores.

No way.

So they're packing a lunch.

That's a great way to put it.

They ensure that wherever those spores land, even if it's totally barren, they have an instant food source to get started.

That's incredible forward planning for a microbe.

But there's a catch.

An evolutionary trade -off.

The non -farmers.

They travel farther as slugs.

They're lighter, leaner.

Ah.

So if they land somewhere with plenty of food already, the farmers wasted energy carrying their lunch.

Precisely.

And the non -farmers got a head start in the race.

So it's a gamble.

Which probably explains why only about a third of them do it.

Depends on the environment.

OK.

That really sets the scene for the diversity we're dealing with.

This huge kind of artificial group protista.

Let's lock down some terms.

Good idea.

Functionally, we often call the chemorganotrophs, the ones eating organic stuff, protozoa.

And the photosynthetic ones.

Those are typically called algae, especially if they have cell walls.

And the whole field is protostology.

The key thing is they all need moisture, right?

And they're incredibly specialized within that single cell.

Absolutely.

Everything, eating, moving, reproducing, balancing water, it's all managed internally with sophisticated structures.

All right.

Let's unpack that core biology.

Nutrition first.

You said some act like animals, some like fungi.

Pretty much.

You have saprophytes.

They secrete enzymes outside the cell, break down food, and then absorb the nutrients.

That's osmotrophy.

External digestion, basically.

Then you have the hunters, the holozoic feeders.

They engulf solid food particles.

Phagocytosis.

Like a classic amoeba eating.

Exactly.

And then there are the mixotrophs.

These guys are flexible.

They can use organic molecules and fix CO2.

Best of both worlds sometimes.

Structurally, what are we looking at inside?

Well, the outer membrane is the plasma lemma.

Inside, the cytoplasm is often divided a firmer outer layer, the ectoplasm, and a more fluid inner part, the endoplasm.

And some have extra support.

You mentioned euglena earlier.

Right.

Many have a pellicle.

It's this layer just under the plasma lemma.

In euglena, it's made of protein strips that interlock.

Like tongue and groove.

Like floorboards.

Exactly like that.

It gives them shape, but also allows for that characteristic flexible movement.

Really neat design.

That structure must be crucial, especially for dealing with water, right?

In freshwater, water's always rushing in.

Huge problem for them.

Osmotic balance is key.

They solve it with contractile vacuoles.

Little pumps.

Essentially, yes.

They constantly collect incoming water and pump it back out.

Non -stop bailing to keep from bursting in those hypotonic environments.

And when they eat via phagocytosis, that whole process...

It's very organized.

Food comes in through a specific spot, the cytostome, the cell mouth.

Okay.

Then it's enclosed in a phagocytic vacuole.

That vacuole becomes ascetic, digests the food, nutrients get absorbed...

And the waste.

That gets ejected at another specific site, the cytoproct.

So a whole little digestive tract within one cell.

Amazing.

What about energy?

Mitochondria?

For aerobic ones, yes.

Photosynthetic ones have chloroplasts, sometimes with a pyrenoid inside for starch synthesis.

And anaerobic ones.

They often have hydrogenisms instead of mitochondria.

Different pathway.

But what about when things get tough, like drought or starvation?

They form cysts, right?

Exactly.

The active feeding stage is the trophozoite.

When conditions worsen, many can undergo incisment, forming a dormant, protective cyst.

So the cyst wall protects them.

Yes, from drying out, lack of food, even some chemical insults.

But crucially, for parasites.

Ah, this is the infectious stage.

Often, yes.

The cyst is tough, can survive outside a host, allows transmission.

Then when it gets into a favorable environment, like inside a new host...

It hatches out.

That's excisment.

The organism becomes active again, a critical part of many parasitic life cycles.

And reproduction.

I bet it's not simple.

Definitely not always simple binary fission, though that happens.

Nucleus divides.

Then the cell.

Mitosis, then cytokinesis.

But there's more complex stuff.

Oh yeah.

Multiple fission, budding,

and sexually you have gamonts producing gametes.

Fusion is syngamy.

And they can swap genetic material?

Right.

Sometimes between two individuals, that's conjugation, like you see in ciliates, or even within one individual autogamy.

Okay, you mentioned ciliates and conjugation.

That brings up their dual nucleus system.

That's wild.

Isn't it?

They have this huge macronucleus.

Polyploid, lots of gene copies, runs the cell day to day.

All the housekeeping.

And the little one?

The micronucleus, diploid.

Genetically pristine, it's basically silent most of the time, only used for sexual reproduction.

So it's like keeping a safe backup copy of the genome, just for passing on genes?

That's the idea.

Protects the core genetic information from the wear and tear of daily operations in the macronucleus.

Really clever.

Okay, let's move into the supergroups.

Big categories.

Starting with excavata.

Sounds primitive.

Yeah, they include some really early diverging eukaryotes.

Take the fornicata,

like Geordie intestinalis.

The waterborne illness, beaver fever.

That's the one.

Very common cause of diarrhea in the U .S.

And what's interesting is it lacks mitochondria.

No mitochondria.

How does it make energy?

It has these remnant organelles called mitosomes, related to mitochondria, but they don't do respiration, no ATP synthesis that way.

It relies on anaerobic pathways.

And it's potent, just 10 cysts can make you sick.

Wow.

And others in excavata also lack mitochondria.

Some use hydrogenosomes instead, like the parabasalia.

This includes things like trichomonas vaginalis, the STI.

Does trichomonas make cysts?

Interestingly, no.

Because it's transmitted directly person to person, it hasn't needed to evolve that resistance stage, lost the ability.

But parabasalia also includes beneficial ones.

Absolutely.

Like trichin infids in termite guts, they digest cellulose for the termite.

A classic mutualism.

And euglinozor in this group too, like euglena itself.

Yep.

Famous for its flexible pellicle, that eye spot or stigma for sensing light.

Photoautotrophic.

But this group also has some really serious pathogens, right?

Extremely serious.

The trypanosomes.

Trypanosoma cruzi causes Chagas disease, spread by kissing bugs.

Nasty.

And trypanosoma bruchii causes African sleeping sickness.

Transmitted by setse flies.

Huge global health burden.

Tens of thousands of deaths a year.

Okay, switching gears to the next supergroup.

Amoebizoa.

These are the ones moving with pseudopods.

That's their defining feature.

Those false feet and they come in different shapes.

You have blunt finger -like lobocodia.

Okay.

Thin, pointy filopodia.

And these amazing net -like structures called reticulopodia.

And some are naked, some build shells.

Right.

Make it amoebae just to have their plasma membrane.

Testate amoebae make a protective covering.

A test, sometimes out of collected material.

And this group also includes major pathogens.

Sadly, yes.

Antamoeba histolytica.

Causes amoebic dysentery.

It's the third biggest killer among parasitic diseases worldwide.

Geez.

And the slime molds are here too.

Always found those confusing.

You and everyone else.

Taxonomically, they've bounced around.

They're Yumi setazoa within amoebizoa.

Two main types.

First, the acellular slime molds mix a gastria.

They form a plasmodium.

Not the malaria parasite.

This is different.

It's one enormous multinucleated blob of cytoplasm.

Like one giant cell membrane enclosing thousands of nuclei.

Exactly.

Up to 10 ,000 nuclei, all dividing at the same time.

Streams across surfaces.

It's quite something to see.

Bizarre.

And the second type.

The cellular slime molds.

Like our farmer friend, Dickeostelium.

They live as single cells until starved.

Then they farm that slug, the pseudoplasmodium.

Right.

And the signal for them to aggregate is cyclic AMP, ZMMP.

They crawl towards the source.

And even cooler, they have sentinel cells inside the slug.

Like little guards.

Yeah.

Patrolling for bacteria, engulfing pathogens.

It's like a primitive immune system for the collective.

Yeah.

Shows incredible coordination.

Mind -blowing complexity from such simple beginnings.

Next massive super group.

SAR, straminopiles, alveolates, rosaria.

Huge global impact here.

Absolutely.

Let's start with the rosaria.

These are often amoeboid, but tend to use those thin filopodia, sometimes supported by microtubules, axopodia.

And the big players here are?

The foraminifera.

Forums.

Critically important.

They make these beautiful multi -chambered shells or tests, usually from calcium carbonate.

And they catch food with those net -like pseudopods.

Particulopodia, yeah.

But their main impact is geological.

When they die, those tests sink.

Accumulate on the ocean floor.

Massively.

Over millions of years, this forms chalk, limestone, marble.

Think the White Cliffs of Dover.

That's forum tests.

Wow.

They literally built landscapes.

And they help environmentally, too.

Down in the sediment, they perform denitrification, turning nitrate back into nitrogen gas.

Huge for nutrient cycles.

Also used in oil exploration, their fossils tell geologists about rock layers.

Next in SAR, alveolata, defined by alveoli.

Yeah, small sacs or cavities just under the plasma membrane.

What they do isn't always clear, but they define the group.

And this includes the dinoflagellates.

Whirling ones.

Exactly.

From Greek dinon to whorl.

They have two flagellae set in grooves, one around the middle, girdle, one trailing, sulcus, makes them spin as they swim.

Some are armored?

Yeah, the feldicate ones have cellulose plates, but the sathecate ones don't.

Ecologically, they're huge.

They cause bioluminescence, that sparkling light in the sea.

Beautiful, but also dangerous.

Red tides.

Exactly.

Some produce potent neurotoxins, causing harmful algal blooms or red tides.

But they're also essential symbionts.

Zucsanthellae.

In corals.

Yes.

The photosynthetic dinoflagellates living inside coral tissue.

Absolutely vital for reef health.

Corals couldn't survive without them.

Alveolata also includes the ciliates, right?

Fast movers?

Yeah.

Using cilia for swimming and feeding.

Paramecium is the classic example.

We already mentioned their amazing dual nuclei in conjugation.

And the food path, cytostome to cytoprox.

Yep.

That organized ingestion -digestion -egestion pathway.

Okay.

Last group in alveolata.

Abacomplexins.

All parasites.

Every single one.

Defined by the apical complex, a set of specialized organelles at one end.

For getting into host cells.

Precisely.

Structures like rock trees and micronemes help them penetrate host tissues.

And this group includes plasmodium malaria.

The big one.

And they have that weird remnant organelle.

The apical complex.

Right.

Plasted like a chloroclast but non -photosynthetic.

Left over from an ancient endosymbiosis with algae or cyanobacteria.

And it's vital.

Absolutely essential.

They need it for making fatty acids, isoprenoids, heme precursors via acetyl -CoA.

Things the parasite can't get easily from the host.

So it's a drug target.

Because we don't have one.

Exactly.

A key vulnerability.

And the metabolism of plasmodium falciparum, the deadliest malaria parasite, is just bizarre.

It's fermentative, mostly.

Even though it has genes for normal respiration.

Yeah.

It relies heavily on glycolysis.

Yeah.

But its TCA cycle, the Krebs cycle, is still active but runs in two directions at once.

How does that even work?

It's wild.

Part of it runs reductively, making citrate and malate to export.

The other part runs oxidatively to make seximal CoA, which it needs for heme synthesis.

Highly adapted, very weird biochemistry for a parasite.

Okay, one more major branch of SAR, stromonopila.

Means straw hair.

Refers to their flagella, usually two.

And one of them has these fine, hair -like projections.

Heteroconflagella.

And this includes?

The diatoms.

Bacillariophyta.

Hugely important primary producers.

They're 50 % of the ocean's carbon fixation.

They're called the rainforests of the sea.

And they have those amazing silica shells?

Frustules?

Yes, like little glass boxes.

Two halves, an epithetica and a hypotheca, fitting together like a petri dish.

Intricate, beautiful patterns.

Then they shrink when they divide.

Because each daughter's cell gets one half of the parent frustule and makes a new, smaller inner half.

So they get progressively smaller over generations.

And until?

Until they get down to about 30 % of their original size.

That triggers sexual reproduction.

They form an osteospore, which goes back to the maximum size, restoring the population's dimensions.

Genius mechanism.

And straminopila also includes things that look like fungi, but aren't.

Right, the perinospora mycetes.

Used to be called umisates, or water molds.

We now know they're protocists.

Cellulose walls.

Deployed life cycle, different from true fungi.

And includes infamous ones.

Phytothora infestans.

The potato blight pathogen.

Caused the Irish famine.

Still a major agricultural threat, often naturally resistant to fungicides.

Shows the power of these microbes.

Okay, quickly then, the last supergroup.

Archaplastida.

This is the group that includes red algae, green algae, and eventually land plants.

So the fluoroplastida, or green algae, are in here.

Think chlamydomonas, a model organism.

Shared features with plants.

Yep.

Chlorophylls A and B.

Store starch.

Cellulose walls.

You see the evolutionary connection.

Huge diversity from single cells, to colonies, to filaments.

That was a whirlwind tour through an incredible amount of diversity.

It really is monumental.

From those farming amoebas, to the forums building cliffs, to parasites like Plasmodium that shape human history.

It might be an artificial grouping, protista, but studying them reviews fundamental mechanisms.

Like Osmo regulation, motility, those complex life cycles, the nuclear tricks.

Incredible evolutionary innovation.

Absolutely.

They are masters of the microbial eukaryotic world.

So the big takeaway for you listening.

Protists aren't just some evolutionary leftovers.

They are active, powerful forces shaping our world geology,

environment, health, in really complex, sometimes devastating ways.

Yeah.

And think about that malaria parasite again, Plasmodium.

Its metabolism is so pared down, so specialized, relies on that weird epicoplast, runs its Krebs cycle backwards and forwards.

It seems almost fragile in its specialization, right?

Exactly.

So here's the thought.

If these incredibly successful world -altering parasites often rely on such unique, perhaps even incomplete biochemical toolkits, what does that tell us about the tightrope walk between adaptation and vulnerability?

Why does parasitism seem to drive these extreme, sometimes fragile metabolic strategies?

Something to ponder about the flexibility and the potential weaknesses required for that lifestyle.

It's a fascinating question about evolution under pressure.

Indeed.

Well, that's all the time we have for this deep dive.

Thank you for joining us.