Protist Lineages - Online Tutor, Practice Problems & Exam Prep

Protists are a diverse group of eukaryotes, including lineages like Excavata, SAR clade, and Archaeplastida. Excavata features organisms like diplomonads and parabasalids, often lacking mitochondria. The SAR clade encompasses stramenopiles, alveolates, and rhizarians, with diatoms and dinoflagellates playing key roles in photosynthesis. Archaeplastida includes green algae and land plants, showcasing primary endosymbiosis. Apicomplexans, such as Plasmodium, exhibit complex life cycles involving multiple hosts. Ciliates utilize cilia for movement and feeding, while amoebozoans, including slime molds, demonstrate unique reproductive strategies, highlighting the evolutionary significance and ecological roles of protists.

Excavata and Archaeplastida (Plantae)

Hi. In this video, we're going to talk about some of the key lineages of protists. Up first are the Excavata, which are a major group of unicellular eukaryotes. Many of the species in this group actually lack mitochondria, though we'll talk a little bit more about that in a second. Most of them also reproduce asexually. Now, first up are the Diplomonads. An interesting thing about Diplomonads is that they lack mitochondria. They actually contain this reduced mitochondrial structure that lacks electron transport. So, first of all, that means that these guys have an anaerobic metabolism, but additionally, it means that they did undergo symbiogenesis. Right? That did occur, but now their mitochondria have taken on this other form, you know, evolved to be different than what we know of as mitochondria. An interesting thing about Diplomonads: they have 2 nuclei. You can see in this picture of a Diplomonad right here. We have 2 nuclei. This is actually Giardia intestinalis, a nasty little parasite that will make you very sick. Oh, the last thing, you can see that the cell has these flagella, which it uses for motion.

Alright. Next up, the Parabasalids. You can see these guys right here. They also lack mitochondria and use flagella to move. A cool thing about some of these species is that they will undulate their membrane, almost, you know, think of a manta ray or something swimming. That's sort of what it's like. They'll undulate their membrane to move. Many of these species are parasitic as well.

Next up, we have the Glenozoans, which are some of the earliest cells known to contain mitochondria, and among them, we have Kinetoplastids which you can see right here. Kinetoplastids. And this is actually Trypanosoma, a protist that causes sleeping sickness and it's transmitted by the African tsetse fly. It's a big problem in Sub-Saharan Africa, this disease, this sickness. A little interesting thing to note about these cells is that they have large mitochondria, not that they lack mitochondria.

Next up, we have Euglenoids. Right here. And these guys contain chloroplasts. Look at that. See these nice chloroplasts in there? Some of these species are photosynthetic; many are heterotrophic and photosynthetic, so they're mixotrophs. Mixotroph just means that they are photosynthetic and heterotrophic. Another major lineage is the Archaeplastida. Sometimes these are just referred to as Planti. Technically, Archaeplastida is like a slightly broader categorization. Planti technically doesn't include red algae for example, or if we were in England, I'd say algae, but I'm not going to do that to you. Now, this group is another major monophyletic group of eukaryotes, and it is the group that contains green algae and land plants, which we're going to talk about in a later chapter more extensively.

Now these organisms originate from protists that engulfed cyanobacterium by endosymbiosis, right? So we're talking about a primary endosymbiosis, not secondary. In fact, a lot of secondary symbiosis happens with red algae. Speaking of red algae, these are mostly multicellular organisms. Some contain multiple nuclei and many reproduce via alteration of generations. A pretty cool thing, they contain this photosynthetic pigment called phycoerythrin. Erythrin, like erythrocyte, red blood cell. Right? So red kind of pigment. And again, this is a photosynthetic pigment that actually masks the green from chlorophyll. That's why these appear red. And something pretty cool, the red intensity of these cells actually correlates with depth. Just an interesting little fact. Now, green algae are much more similar to land plants, and they have a similar composition of photosynthetic pigments to land plants as well. And they can actually be broken down into 2 groups: Charophytes, which are algae that are most similar to land plants. You can see an example of those there. And Chlorophytes, which are a mix of unicellular and multicellular species that have a haploid-dominant life cycle. Alright. With that, let's flip the page.

Stramenopila and Alveolata

The SAR clade is a monophyletic supergroup made up of the clades Stramenopiles, Alveolata, and Rhizaria. Many of the organisms contained in this supergroup are photosynthetic, and this is also the group that contains the water molds, which you hopefully realize are not actually molds, because these are not fungi; they are protists. So again, just another case where there's a bit of a misnomer in the biological terminology, which hopefully you're starting to see, happens all the time. It's just part of science, unfortunately. Anyhow, Stramenopiles and Alveolates contain chloroplasts, and those chloroplasts likely arose from secondary endosymbiosis, which you might remember was when a eukaryotic cell engulfed another eukaryotic cell that had already undergone endosymbiosis and already had chloroplasts inside of it. Most likely this was a red algae that was being engulfed. So the Stramenopiles are mostly made up of photosynthetic algae, and they are both unicellular and multicellular organisms, and the feature that defines this group are these flagella that have hair-like projections. And you can see in this image of a generic Stramenopile cell, right here, that we have this flagella coming off the cell, and it's got all these little lines coming off of it. Those are supposed to be the hair-like projections. Now, many species in this group exhibit diploid dominant life cycles, and diatoms are no exception. These are unicellular photosynthetic organisms whose key feature is that they're encased in these protective shells. They're actually made of silicon dioxide. So, kind of different from like a seashell, for example. But, different chemically, but kind of the same idea. It's a protective casing for the organism. Now, as you can see in this image here, diatoms come in a wide variety of morphologies. These are all different, you know, shapes and sizes of the diatom's shells, and fun fact, these have actually been arranged by people on slides as an art form. Yeah. Believe it or not, people will take brushes that have a single bristle and work under a microscope, will actually poke around little diatoms and arrange them into a collage like you see here. Pretty pretty funky stuff. And this is not actually a modern thing. People have been doing this kind of art for a really surprisingly long time. Super cool thing about diatoms, even though they're these tiny little microorganisms, they're actually responsible for a lot of Earth's photosynthesis, and they can actually have a noticeable impact on atmospheric carbon. So when they spring up and, you know, there's a large amount of them concentrated in a certain area, you can actually notice a significant dip in the amount of carbon in the air in that area, because they're using up so much of it for photosynthesis. Super cool. Now, gold algae, like we see here, have a distinctive kind of yellowish-brown color. They get that from these yellow and brown carotenoids, which you might remember, carotenoids are a type of photosynthetic pigment in plants. They're usually like an accessory pigment. Here, they're the distinctive pigment that is coloring the organism that we see. Most gold algae are unicellular organisms and even though they contain these chloroplasts, which, just a quick reminder, came from secondary endosymbiosis. Right? So probably a red algae being engulfed. Even though they contain these chloroplasts, many of these organisms are actually mixotrophs. Meaning that they do photosynthesis, but then they also obtain nutrients like heterotrophs as well. So they mix it up. Right? That's where the term comes from. Now lastly, we have brown algae, which you can see in this image here in a multicellular form as a big kelp stalk. But brown algae can also be unicellular. And, it gets its distinctive color from brown carotenoids, similar to gold algae, and a cool thing about these big kelp stalks, right, these are large structures and they'll actually be rooted often at the seafloor. But they want to get close to the surface because that's where the sunlight penetrates best. Right? They're going to get the most sunlight at higher depths, if that makes sense. Higher up, closer to the surface, they're gonna get more sunlight there. So they actually have these little gas-filled chambers in their photosynthetic regions that cause these kelp stalks to float up to the surface, so that they can obtain sunlight more efficiently. And that's why, when you see like big clumps of seaweed or kelp or whatever on the beach, and they have all those little bubbles that are or pods or whatever that are kind of fun to pop open, that's what those are actually for. They're via alteration of generations, and they can be both hetero- via alteration of generations, and they can be both hetero- and isomorphic. We saw examples of that in the lesson on protist life cycles. Now moving on, we have Alveolata, and these protists are, sort of the defining feature of this clade are these membrane-enclosed sacs called alveoli that are right under the plasma membrane. And we're actually going to break up our discussion of different types of Alveolata over a few videos, because I want to get into some specifics about some of these subgroups. So first off, let's talk about dinoflagellates. I have always sort of thought of dinoflagellates as being kind of similar to diatoms, and that's mostly because they both have protective shells and they're both basically plankton. I'm not saying that they're exactly the same. There are many differences, they're in different clades. I'm merely trying to say that there are some parallels between them. Now, dinoflagellates are mostly unicellular, again, mostly aquatic. And they're actually not enclosed by a silicon dioxide shell, but by these two cellulose plates. And you can see in this image of a dinoflagellate here, we have this plate and then this one too. So those are the two plates coming together. And you can see that over in this image, this dinoflagellate looks pretty different. Totally normal, that is the way they're supposed to look, or that's a way they can look, I should say. They come in many shapes and sizes. Now, dinoflagellates rather, have two flagella. Yes. This is sort of where their name is coming from. But these flagella are pretty different. One projects outward and another actually runs around the groove between the two plates. So on this dinoflagellate here, you can probably see this flagella coming off of it. And it might be a little harder to make out, but there's actually flagella running around the side of this organism, kind of doing a loop around it. Right? So those are the two flagella of the organism. Now, roughly half of dinoflagellates are actually heterotrophic and about half are phototrophic. Though many of these phototrophic dinoflagellates are actually mixotrophs. Right? So again, they perform photosynthesis, but they also are going to obtain nutrients in a heterotrophic fashion. Now, most dinoflagellates have chromosomes that lack histones and actually attach directly to the nuclear envelope in a structure called a dinokaryon. And last thing about dinoflagellates, you can see here an example of their life cycle, which is a haploid dominant life cycle. Right? So these cells, These or some of these cells, I should say, are haploid, let me get a new color here, right. So we've got these nice haploid cells, right, these two are haploid and they're gonna fuse together and form this diploid cell, right, n and 2n, trying to show haploid diploid. Right? And then this is eventually going to, go through meiosis and form those haploid cells again. So with that, let's flip the page.

Apicomplexans

Apicomplexans are a type of alveolata that are all parasitic, and mostly they're parasitic to animals. Their cells lack cilia and flagella, and contain these modified plastids, which are called apicoplastids. Most likely, these arose from red algae, which remember, are acquired, so they would have been acquired, that is, by secondary endosymbiosis. Now these cells, names, basically, or the name apical complex comes from the fact that these cells have what are called apical, complex structures. So, here we actually, in this image, we actually have 2 cells. Hopefully, you can see them. I'll try to outline them right here. And hopefully, you can notice that these cells have these tips. Those are the apical complex structures. And essentially, these apical complex structures are there to help these cells penetrate into their host cell, which, kind of reaffirms the fact that they're all parasites. Right? Their defining feature is something that enables their parasitism. Now, another cool thing about these organisms is they have kind of interesting life cycles. They reproduce, both sexually and asexually, and they actually sometimes move between species to do so.

So, an apicomplexin that is near and dear to my heart is this organism called Toxoplasma. It's actually a group of organisms. It's a genus. So, Toxoplasma, are these cool apicomplexans and they reproduce sexually in cats. So, they have sexual reproduction in cats, and then they actually will be excreted, so the cat poops them out, they live in the intestines of the cat. They reproduce sexually there and then the cat poops them out and they'll get picked up by other animals and they'll reproduce asexually in these other warm-blooded animals. And, important to note, people are able to be infected by these organisms. And what's so cool about Toxoplasmosis is they actually go into animal's brains, and they change their brain wiring basically. Specifically, they make rodents, like we see here, they make rodents stop being afraid of the smell of cats. So that, the organisms, like these rodents, get infected with the Toxoplasma, stop being afraid of cats, which are their predators, right? And then they'll be eaten by the cats again, and that is how the Toxoplasmosis will get back into the cat's intestines where they can reproduce sexually again. So, kind of this really wild life cycle. Right? And it all revolves around this very strange kind of crazy feature of these organisms, where they can rewire a rodent's brain to make them stop being afraid of cats.

Totally wild. Also, this is kind of a side note, but this is sort of, people have theorized that this is sort of where the idea of the "cat lady" comes from, right? She has all these, you know, a person who has a lot of cats and is like kind of weirdly obsessed with cats, so it’s because the Toxoplasmas have gotten into their brain. They've been converted. You know, I'm just kidding. But just a fun little, weird idea that relates back to this class of organisms. Now I also want to talk about the parasite that causes malaria, Plasmodium. And these organisms, also apicomplexans, also have an interesting life cycle where they reproduce, asexually and form their gametocytes in humans. And then in mosquitoes, they form zygotes and produce what are called sporozoites, which, you know, if you think back to, alteration of generations, these are kind of like spores. So the similar idea. They're asexual, they're units of the organism that will reproduce asexually, and, eventually form gametocytes.

So let's take a look at this life cycle. So, you don't really need to worry too much about the terminology here or all steps. I just want you to have, like, a general sense of how this works. So the gametocytes, will get together inside the mosquito. They'll form, what is essentially sort of like the zygote. Again, the terminology is different here. Oocyst is ultimately what they're forming, which is basically just full of those sporozoites, and those sporozoites will, when a mosquito bites a human, the sporozoites will be passed into the human's bloodstream. They'll make their way to the liver, where they'll infect liver cells, and they're going to ultimately form these structures, again, don't worry about the terminology too much, they're going to form these structures in liver cells that will then infect blood cells, and those infected blood cells are where the gametocytes will be produced. Right? And then, of course, a mosquito drinks your blood. Right? The mosquito will slurp up your blood, and those gametocytes will get back into the mosquito, where they can form the gametes, and the osis, and eventually more sporozoites to infect more humans. So pretty crazy life cycle here. Right? Not only are we switching between organisms, but within humans, first something has to happen in liver cells, and then something has to happen in blood cells. There's like, complex stages to this life cycle and totally wild that this organism, right, this Plasmodium, depends on moving between, mosquitoes and, it's not just humans, animals in general, but still, moving between mosquitoes and animals like humans, or cows, or whatever.

Totally wild, wild life cycle that, you know, requires the movement between species. So it's kind of a weird and cool thing about some of these apical complex and species. Alright. Let's turn the page.

Ciliates

Ciliates are the last type of alveoliota that I want to talk about, and they get their name from the fact that they are covered in cilia. Hopefully, that makes sense. They use these cilia for movement and feeding, and mostly they are feeding by preying on bacteria. These are predatory organisms. A cool thing about cilia, they have this diploid micronucleus and macronucleus that is used for transcription purposes. So, copies of the chromosomes, whereas the micronucleus just has 2 copies of chromosomes and is only used for reproductive purposes. Now, they can reproduce asexually by binary fission and also do this unique form of sexual reproduction. So I kind of want to talk about that right now. And just FYI, here we have 2 ciliates. These are actually asexually reproducing, right? They are about to separate right there, and hopefully, you can see it might be a little too faint to see, but there are some cilia coming off these cells.

Now, how does sexual reproduction work? Well, it involves the micronucleus, as you might recall. So basically, there are, you know, the specifics of this are much more complicated than I want to get into. I really just want you to walk away with a general sense of how it all works. You don't really need to worry about terminology and specifics. Just get an idea of another type of life cycle experienced by eukaryotes. So, there are, well, let's put it this way — 2 cells will get together, and they'll form a pair, and you can see they've the 2 cells are being distinguished here and here by their different colored micro and macronuclei. What's going to happen is these micronuclei, remember these are diploid. They're going to undergo meiosis, and we're going to end up with these haploid, right, so now, these micronuclei are haploid. These have X's in them because they're not going to be used, just these are going to partake in the reproduction. So, some of these, you know, some of these micronuclei that are generated from meiosis are unused. The cells will actually just exchange, you know, one micronucleus or micronuclei, and that's what we see happening. Here, the cells are exchanging micronuclei, and then we're still haploid here. Then those micronuclei are going to fuse together. So that's what we're seeing here. Now, these micronuclei here and here, these are diploid again. Then what's going to happen after we have a fusion of the micronuclei, we're actually going to have a bunch of mitosis occur, on both the micronuclei and the macronuclei, the unused, or sorry, the original macronucleus is going to disintegrate. So here you see the original macronuclei. Those are going to disintegrate. These are our new macronuclei and these again are micronuclei that we formed, back here. Oops, back here in this stage and basically, these are going to split up into a bunch of new cells. So, we are going to end up with it. We're actually only being shown 2 cells. We're actually going to end up with 4 cells from this because, well, I'm just going to put like times 2 right here because in each of these cells, right? Let me just clear some space. In each of these cells, we have 2 macronuclei, right? Here, here, and we also have 2 macronuclei, right? Here, here, and we also have 2 micronuclei here, here, and here and we're going to end up with 4 cells at this point that each have 1 micronucleus and 1 macronucleus, and again, these micronuclei are going to be diploid at this phase again. And then of course the cycle can repeat. So, this is how the ciliates reproduce sexually by exchanging these, by first having their micronuclei undergo meiosis then exchanging haploid micronuclei, having the new micronucleus fuse with the original micronucleus and then undergoing a bunch of mitosis to generate new macronuclei and new micronuclei and then all of those splitting up into new cells. So that is one way to introduce, or that is one way that eukaryotes will perform a type of sexual reproduction. There are many ways that that actually happens and like we saw with the apicomplexans where it gets kind of crazy. You're going between different organisms. Remember, all of this is really just to introduce more genetic variation into the descendants of these organisms. And with that, let's flip the page.

Rhizarians and Amoebozoa

The radiolarians are the last subgroup of the SAR clade, and these are mostly unicellular amoebas that use pseudopodia for feeding. Radiolarians contain an internal silica skeleton-like structure, and they have pseudopodia projections that they use for feeding on microorganisms. You can see an example of a radiolarian here; this is the main part of the organism with an internal silica skeleton-like structure, and these are the pseudopodia that are projecting outward to feed on microorganisms. For foraminifera, they have calcium carbonate shells with little holes through which pseudopodia can emerge. Some of these organisms have cells that contain multiple nuclei. The theme here is pseudopodia used for feeding. Next, we have the Cercozoans, flagellated protists that also use pseudopodia for feeding. The major theme of this group is feeding with pseudopodia.

Lastly, we are going to talk about Opisthokonta, a group that includes fungi and animals, hence not exclusively protists. However, it does include a notable group of protists known as Amoebozoa. To this point, we have used the term "amoeba" somewhat loosely; however, Amoebozoa refers specifically to amoebas that have lobe- and tube-shaped pseudopodia. Here are a few examples of Amoebozoa; these organisms depicted are slime molds. One type, sometimes called the dog vomit slime mold, is a plasmodial slime mold. This yellow mass is one massive continuous cell containing thousands of nuclei. The cell is compartmentalized into smaller sections rather than being one vast cytoplasmic section. It reproduces through fruiting bodies, which I will discuss more when we talk about fungi in a later video.

The other type is the cellular slime mold, which forms fruiting bodies by aggregating smaller individual cells. You can see a fruiting body forming here; all these tiny dots represent individual cells aggregating to form this large structure. This aggregation results in the formation of fruiting bodies, the mechanism through which these slime molds reproduce. While plasmodial slime molds form fruiting bodies as one continuous cell, cellular slime molds do so through the aggregation of many individual cells.

The diverse and varied nature of protists is a key point I want to convey. Protists are an assorted group in terms of morphology and life cycles because they represent a sort of 'catch-all' category for eukaryotes that aren't categorized as plants, animals, or fungi. This diversity underscores the lack of unifying themes within this group. I hope this overview gives you a broad sense of the diversity inherent in the group of organisms known as protists. See you in the next video.