You're probably used to organisms being grouped by what they have in common. For example, sharks have cartilaginous skeletons, fish breathe using gills, mammals have hair, etc. So to hear of a group, or taxa, of organisms defined by what they are not probably sounds pretty strange.
Protists are a whole kingdom of mostly single-celled, eukaryotic organisms that can neither be classified as plants, animals, fungi, nor bacteria. Protists, as a group, resist clear classification because they are polyphyletic. Polyphyletic means multiple phylums. In other words, if you were to try and trace the lineage of the group, you would be unable to locate one, shared, common ancestral origin. Crazy, right?
Geneticists have actually found that most protists are more closely genetically linked to organisms of other kingdoms than they are to one another. So why don't we just divvy them up among the kingdoms they are most closely related to? Well, because they also don't possess the defining characteristics that 'define' organisms as belonging to that group. Talk about a predicament!
Other than protists being mostly unicellular and eukaryotic, they share very little else in common. Different species exhibit a wide range of body structures, food acquisition methods, habitats, and modes of motility.
Herein we are going to focus a specific type of protist (although once we get into it you'll see just how dissimilar they are), called 'flagellates', or flagellated protists.
Monkeys have tails, dogs have tails, but did you know that these tiny single-celled protists have a tail sometimes too? It's called a flagellum, a tail-like projection which the organism whips in a circular motion to propel itself forward. Some flagellates have a single flagellum while others have multiple (called 'flagella'). Now, flagellates aren't limited to any one specific environment, all they need is moisture for moving around in and bacteria to eat. Therefore, they can be found in oceans, freshwater, soil, and internal environments - by that we mean inside other animals.
Relationships: Symbiotes & Parasites
Some flagellates are symbiotic, like in the class Parabasalia. Parabasalids live in the guts of termites and cockroaches (now there's a vacation spot!). They aid these insects in digesting the cellulose (like wood) that they consume. In these instances, both the host and the symbiote mutually benefit from the relationship - the host gets to absorb nutrients from wood, while the flagellate has a safe environment and food at the ready.
Other flagellates are parasitic. Species from the genus Giardia make the host suffer greatly and, if untreated, they may die from the diarrheal illnesses that Giardia cause. Giardia lamblia is frequently a concern for household pets who drink from outside water sources as well as campers who don't properly sanitize water from a stream or river.
Heterotrophs, Mixotrophs & Autotrophs
Flagellated protists also run the gamut on feeding strategies. Some are obligate autotrophs that get energy from photosynthesizing or from chemical reactions (chemosynthesis), some are heterotrophs that have to consume prey for energy, and some are mixotrophs, which both consume prey and photosynthesize, getting a much wider banquet of options by combining the two methods.
- Chlamydomonas is a genus of algal species found in still and stagnant waters, soil, and even snow. Alpine regions will commonly get something called Watermelon snow, which is red in coloration from these cold-loving algae.
- Choanoflagellates, such as Sphaeroeca volvox, are heterotrophic and sometimes a colonial class of protists. The choano-flagellate comes from their collar-like funnel that their flagellum protrude from.
- Mixotrophic flagellum, such as in the genus Euglena and certain dinoflagellate species like Karenia brevis get the best of both worlds. In the presence of sunlight they can use their chloroplasts to photosynthesize and, when the sun disappears, they can revert to heterotrophic hunting behavior.
Protists are a kingdom of mostly single-celled, eukaryotic organisms (having membrane bound organelles), which can neither be classified as plants, animals, fungi, or bacteria. Flagellated protists are those protists with a tail-like projection called a flagellum, which the organism whips in a circular motion to propel itself forward and are found in almost all moist environments including saltwater, freshwater, soil, internal environments, and even snow. They come in both symbiotic and parasitic variants and utilize a wide range of feeding strategies, including autotrophic (photo- or chemosynthetic), heterotrophic (energy from prey), and mixotrophic (both!) methods.
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