Chapter 15: Protists: Algae and Heterotrophic Protists

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Welcome to the Deep Dive, where we break down complex topics into easily digestible insights.

Today we're plunging into the microscopic world that started it all on our amazing water planet.

That's right.

Earth is mostly water, something like 70 percent.

And life's origins?

Well, they trace back at least 3 .5 billion years to simple prokaryotes in those ancient waters.

And we have fossils that old.

Well, eukaryotic fossils anyway.

Phew!

Remarkably ancient ones.

Our source, Raven Biology of Plants, mentions Grypania.

These are these distinctive coiled fossils, maybe 2 .1 billion years old.

Some were surprisingly large, up to half a meter.

Half a meter, wow.

Yeah.

And then there's Bangeomorpha, 1 .2 billion years old.

It's a multicellular red alga that honestly looks almost exactly like modern Bange.

It just shows the incredible staying power of these life forms.

So an ancient lineage indeed.

Okay, so what exactly are we focusing on today from this vast history?

We're zeroing in on the protists.

These are the eukaryotes that don't neatly fit the definition of plants, fungi, or animals.

A really diverse bunch.

It's kind of the miscellaneous category of eukaryotes.

You could sort of think of it that way, yeah.

We'll look mainly at two groups.

The photosynthetic algae think of them as the plants of the aquatic world, the primary producers.

And then certain heterotrophic protists like Umesetes and slime molds.

Ah, okay.

The ones traditionally studied by mycologists, but they aren't actually fungi, right?

Exactly.

We want to unpack their diversity, their ecological importance, which is huge, and some of their unique characteristics.

We're sticking closely to the raven biology of plants chapter here.

Got it.

And the book mentions some cool interactions, too, like algae helping out a giant clam.

Yeah, the zooxanthellae symbiosis.

The algae live inside the clam, providing nutrition, and the clam, well, it provides waste products the algae can use.

A neat little cycle.

Fascinating stuff.

Okay, let's dive into the ecology first.

Algae is the great meadow of the sea.

What does that mean?

It means they form the foundation.

Think about plankton.

The term means wanderers, microscopic organisms drifting in water everywhere.

The photosynthetic part of that plankton, the phytoplankton.

Which includes algae and cyanobacteria.

Right.

They are the primary producers.

They capture sunlight, make food, basically fueling the entire aquatic food web.

They're the base that feeds the zooplankton.

And zooplankton are the tiny animals.

Crustaceans, larvae.

Tiny crustaceans, larvae, other heterotrophic protists, even bacteria.

Yeah, they all rely on that phytoplankton base.

In freshwater, you see lots of crucifites, diatoms, green algae, dinoflagellates.

Marine waters have more haptophytes, dinoflagellates, and diatoms.

Essential stuff.

So fundamentally important ecologically.

But you mentioned human connections, too.

Food industry.

Absolutely.

Red and brown algae think kelps, like kombu or porphyra, which is nori for sushi.

They're dietary staples, particularly in East Asia.

Maybe not super high in carbs for us, but packed with salts, vitamins, trace elements.

And this leads to farming them.

Mariculture.

Precisely.

Mariculture is just cultivating marine organisms.

We farm shrimp, shellfish, and yeah, seaweeds, too.

Industrially, kelps give us alginates.

Alginates.

Those are the thickening agents.

Yeah, used in all sorts of things.

Food, textiles, cosmetics, paints.

They stabilize colloids.

And from red algae, we get agar and carrageenan.

Agar, like for picture dishes.

Exactly, for culture media.

But also in capsules, dental impressions, cosmetics, and again, as food stabilizers.

It's amazing how versatile these algal products are.

They even harvest kelp sustainably in California, just trimming the blades so they regrow.

Okay, so food and industry.

But the climate connection seems even bigger.

How do algae control climate?

It's mainly through the carbon cycle.

Photosynthesis obviously pulls CO2 out of the atmosphere and turns it into carbohydrates.

But some algae also make calcium carbonate shells, or scales.

By locking carbon away.

And it's a massive amount.

Marine phytoplankton alone are estimated to absorb about half of the CO2 humans generate.

Half.

That's staggering.

It is.

And when phytoplankton form those calcium carbonate scales, they pull dissolved CO2 from the water.

That deficit then draws more CO2 down from the atmosphere to replace it.

It's a powerful drawdown mechanism.

And this is what formed things like the White Cliffs of Dover.

Yes.

Over millions of years.

That's solidified phytoplankton.

And even indirectly, North Sea oil deposits have links back to these ancient marine organisms.

Incredible.

Any other climate effects?

There's the sulfur cycle too.

Certain algae like haptophytes and dinoflagellates produce sulfur compounds.

When these get into the atmosphere, they can form sulfur oxides, which act as nuclei for cloud condensation.

So more clouds?

Potentially.

Yeah.

More clouds mean more sunlight reflected back into space, which could have a cooling effect.

It's complex, but they're definitely players in the global climate system.

But it's not all good news, right?

We hear about harmful algal blooms, red tides.

Unfortunately, yes.

When there's an excess of nutrients, often from human sources like agricultural runoff or sewage, certain algae populations can explode.

They bloom.

And that's what causes those discolored tides.

Red or brown?

Usually, yeah.

It's the sheer number of cells packed with accessory pigments.

But the color isn't the main issue.

It's the toxins some produce.

Like the Florida red tides.

Right.

Caused by Karenia brevis.

It produces brevetoxins, neurotoxins that can become airborne, causing respiratory issues, and may lead to massive fish kills.

Nasty stuff.

Although the source mentioned a potential upside.

Cystic fibrosis treatment.

That's a surprising twist, yeah.

A compound from Karenia is being looked at for that.

But other dinoflagellates like gonelix or gymnodinium produce potent nerve toxins that accumulate in shellfish.

Leading to things like paralytic or amnesic shellfish poisoning.

Exactly.

Makes shellfish unsafe to eat.

It's important to remember, though, that red tide is a bit misleading.

Not all blooms are red, not all are toxic.

And some toxic blooms don't even discolor the water noticeably.

Okay, a really complex picture.

Let's maybe zoom in on some specific groups now.

Euglenoids.

You called them shapeshifters.

Nah, yeah.

There are about 800 to 1 ,000 species, mostly freshwater.

What's key is they don't have a rigid cell wall.

They have a proteinaceous pellicle under the plasma membrane.

Pellicle.

So it's flexible.

It can be flexible or rigid, but often it allows them to change shape, to sort of squeeze through mud and tight spaces.

Very useful.

About a third are photosynthetic.

Probably got their chloroplasts from green algae via endosymbiosis.

And the other two -thirds?

They're heterotrophs.

They eat things or absorb nutrients.

Super versatile.

They also store energy as paramylon, a unique polysaccharide.

Not starch -like plants, and it's stored in the cytosol, not the plastids.

Interesting.

Okay, next up.

Cryptomonads.

Small,

fast -growing.

What's their claim to fame?

Cryptomonads are really important for understanding secondary endosymbiosis.

Their chloroplasts are weirdly complex.

They have four membranes surrounding them.

Four?

Why so many?

Well, the thinking is an ancestral cryptomonad engulfed a eukaryotic red alga that was already photosynthetic.

So you get the original two chloroplast membranes plus the red alga's plasma membrane, plus the membrane from the cryptomonad's food vacuole.

Whoa, okay.

It has layers upon layers of evolutionary history.

Exactly.

And between membranes two and three, there's often a remnant nucleus called a nucleomorph, the leftover nucleus of that engulfed red alga.

Plus, their pigments include phycobilins, otherwise mostly found in cyanobacteria and red algae, further supporting that red algal origin.

Mind -bending stuff.

All right, moving on to haptophytes, coccoliths, and climate.

Right.

Mostly marine phytoplankton.

They have a unique structure called a haptenema.

It looks a bit like a flagellum, but isn't.

It can coil and capture prey, but they're famous for coccoliths.

The tiny calcium carbonate scales.

Yes, beautiful intricate scales.

The organisms covered in them are coccolithophoreids.

These coccoliths sink when the organism dies, forming massive chalk deposits over geological time, think White Cliffs of Dover again, and playing a huge role in transporting carbon to the deep ocean.

So they're major players in carbon sequestration.

Absolutely.

And some, like Amelia Huxley, form enormous blooms you can see from space.

They also contribute to that sulfur cycle and potential cloud formation we mentioned, though some related genera can cause toxic blooms that kill fish.

Okay, a mixed bag there, too.

Next.

Dinoflagellates, the spinning wonders.

Yeah, they're two flagella, beat in grooves, one around the middle like a belt, one perpendicular, making them spin.

About half are photosynthetic, but many others eat things.

Some even extrude a sort of feeding tube, a peduncle, to suck contents out of prey.

Versatile feeders, too.

But their big ecological role is symbiosis.

Definitely.

Zoxanthellae are dinoflagellates, those golden spheres living inside corals, giant clams, jellyfish.

They're the powerhouses of coral reefs, doing most of the photosynthesis.

The coral gets food, like glycerol, from the algae.

Which brings us to coral bleaching.

Tragically, yes.

Rising water temperatures, ocean acidification from CO2.

It stresses the coral, causing them to expel their zooxanthellae.

Without them, the coral loses its color bleaches and often dies.

It's a massive threat to reefs worldwide.

A direct link between climate change and these tiny symbionts.

It really is.

Dinoflagellates also form resting cysts when conditions are bad, which can survive for years and sediment and seed future blooms.

And, of course, some are bioluminescent, that sparkling light in nighttime waves.

Probably a defense mechanism.

Cool.

Okay, shifting gears to a larger group.

Photosynthetic streminopiles, or hairy flagellates.

What unites them?

It's a structural thing, seen with electron microscopes.

Their swimming cells usually have two flagella, and one of them is covered in distinctive, stiff, straw -like hairs.

Heterocont is another name for them.

And this group includes?

A real mix.

Diatoms, golden algae, yellow -green algae, the massive brown algae, and even the heterotrophic ume seeds we'll get to later.

Let's start with diatoms, then.

Glass houses of the sea.

You said they account for a quarter of global carbon fixation?

Roughly, yeah.

It's an incredible contribution, equal to all the world's rain forests combined.

They are a super important food source, especially in colder waters.

And they're glass houses.

Their cell walls, called frustules, are made of silica, essentially glass.

They fit together in two halves, like a petri dish, often with incredibly intricate patterns.

These silica shells are what accumulate to form diatomaceous earth.

The stuff used in filters and polish.

That's the one.

Diatoms themselves are mostly unicellular, either radially -symmetrical centric or bilaterally -symmetrical penate.

They reproduce asexually mostly, which interestingly causes them to get smaller over generations.

More.

How does that work?

Each new cell gets one half of the parent frustule and builds a new, slightly smaller inner half.

Eventually they get too small, which triggers sexual reproduction to restore the maximum size.

A really neat cycle.

Fascinating adaptation.

Okay, how about the golden algae?

Chrysophytes.

Mostly freshwater, often unicellular or colonial.

Their golden brown color comes from lots of fucoxanthin pigment masking the chlorophylls.

They store energy as chrysaliminarin.

And are they just photosynthetic?

Many are, but quite a few are also phagocytic.

They eat bacteria or other small particles.

Some need to eat certain things to survive, even if they photosynthesize.

They can be dominant in low -nutrient lakes, and sometimes cause taste and odor problems in drinking water if they bloom.

And yellow -green algae.

Santhophytes.

About 600 species, mostly freshwater or soil dwellers.

They're similar to golden algae, but lack fucoxanthin, so they look yellow -green.

They store food reserves mainly as oil.

A common example is valkyria, or waterfelt, a filamentous type.

Now for the big ones.

Brown algae.

Kelps and rockweeds.

Pheofecia.

Almost entirely marine, these are the dominant seaweeds on many rocky shores, especially in cooler waters.

They range from simple filaments to complex bodies that look almost plant -like.

Like kelp, with a holdfast stipe and blade.

Exactly.

Kelps like laminaria, or the giant macrosystis, have that structure.

They grow incredibly fast from a meristem between the stipe and blade.

Macrosystis can be harvested repeatedly because of this.

They also produce algin, that slimy stuff.

For flexibility and protection.

Yeah, helps against wave action, drying out at low tide, and makes it harder for other things to grow on them.

And remarkably, kelps have specialized cells in the stipe that conduct food, functionally similar to phloem in plants.

Wow, convergent evolution.

Seems like it.

Rockweeds like fucus or sargassum, which forms huge floating mats in the Sargasso Sea, are also common.

Their life cycles often involve Alternation of generations, sometimes isomorphic, sometimes heteromorphic like laminaria, and some, like fucus, have a life cycle more like animals, with dominant deployed adults.

Complex life strategies.

Moving to red algae, rotophyta, deep water specialists.

Mostly, yeah.

About 6 ,000 species.

Abundant in tropical waters, but also cooler seas.

Very few are unicellular.

Most are macroscopic seaweeds.

What's really unique is they completely lack flagella and centrioles at any life stage.

No swimming cells at all?

None.

They rely on water currents for fertilization.

Their red color comes from ficabilin pigments, which are excellent at absorbing the blue and green light that penetrates deeper water.

This lets them live deeper than many other algae.

And they give us agar and carrageenan.

Correct.

Their cell walls have cellulose, plus those gel -like sulfated polymers.

Some red algae, the coralline algae, deposit calcium carbonate in their walls, becoming hard and stony.

They're really important reef builders and stabilizers.

Like corals.

They help cement reefs together.

And intriguingly, a recent finding mentioned in the text is small amounts of lignin in some coralline algae.

A lignin was thought to be exclusive to vascular plants.

Another evolutionary surprise.

And their life cycles are complicated.

Very.

Often a three -phase cycle involving a haploid gamophyte, then a diploid carposporophyte that grows on the female game to fight, and finally a free -living diploid tetrasporophyte that produces haploid spores through meiosis.

It's thought to be an adaptation to lacking motile sperm, maximizing the output from each fertilization event.

Collect us to unpack there.

Finally, the green algae.

Ancestors of land plants, right?

That's the key connection.

They share fundamental traits with land plants.

Chlorophylls, A and B.

Starch storage inside plastids.

Cellulose cell walls, in many.

Molecular data confirms they form a monophyletic group, virideplante, with land plants.

So what links them so strongly?

Within green algae, the Charophyce class is closest.

They share things like asymmetrical flagellated cells, similar to plant sperm,

breakdown of the nuclear envelope during mitosis, and the formation of a phragmoplast during cell division, all key plant features.

Phragmoplast.

That's the structure -guiding cell plate formation.

Exactly.

Groups like holiocete even show zygotes retained and nourished by the parent, hinting at the origins of the plant embryo.

And the stonewarts, Chara, have complex reproductive structures analogous to those in seedless plants, and their zygotes have sporapollin in walls, just like plant spores.

Really strong evidence for that evolutionary link.

Okay, let's switch to the heterotrophic protists mentioned earlier, when we see these water molds and plant destroyers.

Right.

These look like fungi, often filamentous, but they're stromatopiles, related to diatoms and brown algae.

Key differences.

Cellulose walls, not chitin, and they produce those characteristic biflagellate zoospores.

And they cause some infamous plant diseases.

Oh, definitely.

Aquatic ones are often decomposers, but terrestrial ones.

Plasmaparavitigula causes gnawny mildew of grapes.

Its arrival in France nearly wiped out the wine industry before the bordeaux mixture, copper, sulfate, and lime, was developed as the first real fungicide.

And phytothora, the plant destroyer.

A notorious genus.

P.

infestans causes potato late blight, the direct cause of the Irish potato famine.

Still a huge agricultural problem today.

P.

remorum causes sudden oak death.

Devastating oaks in California and Oregon.

P.

cinnamomi kills avocados, eucalyptus.

The list goes on.

Devastating impacts.

Okay, last group.

Slime molds.

First, plasmodial slime molds.

Mixa macota.

These exist, under good conditions, as a plasmodium, a single huge multinucleate mass of protoplasm without cell walls.

Just this creeping amoeboid sheet that engulfs bacteria, yeast, spores.

A giant amoeba, essentially.

Sort of, yeah, but diploid and multinucleate.

When conditions worsen, it stops moving and forms sporangia, often quite beautiful stalked structures.

Meiosis happens inside, producing haploid spores.

These spores are incredibly tough.

And they can form a resistance stage, too.

Yes.

The plasmodium can form a hardened dormant structure, called a sclerotium, to survive drying.

The spores themselves can last for decades.

When a spore germinates, it releases an amoeba or a flagellated cell, which eventually fuse to form a new diploid plasmodium.

Okay.

Distinct from the cellular slime molds, then.

The social amoebas.

Very different.

Dictyostelio micota.

These live as individual, separate, haploid amoeba -like cells called mixed amoebas, feeding on bacteria.

So they're usually solitary.

Why social?

Because when they starve, they do something amazing.

They aggregate.

Tens of thousands of individual cells come together in response to a chemical signal.

Cyclic AMP -CMP.

They signal each other.

Yeah.

They follow the campy gradient and form a multicellular slug, or pseudoplasmodium.

The cells remain individual, but act as a unit.

This slug crawls off to find a better spot.

And then what?

Then they differentiate.

Some cells, usually at the front of the slug, sacrifice themselves.

They form a stalk, undergo programmed cell death, apoptosis, and die.

The other cells crawl up the stalk and become dormant spores, which get dispersed.

Sacrifice themselves.

That's cellular altruism.

It really is.

Though the source mentions there can be cheaters cells that manage to avoid becoming stalk cells and preferentially become spores, it's a fascinating system of cooperation and conflict.

They also have a sexual cycle involving fusion to form a zygote within a protective macrosyst.

Incredible complexity in these seemingly simple organisms.

So to wrap up, we've seen this immense diversity of protists, from the algae forming the base of aquatic food webs.

And influencing global climate through carbon and sulfur cycling.

To their unique adaptations like silica shells, haptanemas,

complex endosymbiotic origins.

And in their direct impact on us through food, industry,

diseases like potato blight, and potentially even climate regulation.

Plus those crucial evolutionary links, especially green algae, showing the path to land plants and the bizarre life cycles of slime molds and oomacids.

It's a world full of surprises and fundamental importance.

So the final thought for you, our listeners.

Consider these tiny, often overlooked life forms.

They've developed incredibly intricate strategies for survival, symbiosis, even communication.

They're not just ancient relics, they actively shape our planet daily.

What other secrets do they hold?

And could understanding them better help us tackle our own global challenges?

We hope this deep dive has illuminated the vital world of protests for you.

Thank you for joining us on the deep dive.

We'll explore another fascinating topic next time.