In within the kingdoms with which they are

In this lesson, we will learn about protists and the different types of these microorganisms, categorized by either their mode of nutrient acquisition or life-stage characteristic. Afterward, test your knowledge with a brief quiz.

What is a Protist?

So what is a protist? That’s actually a lot more of a difficult question to answer than one might think.

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Protists are defined as eukaryotic (having membrane-bound organelles within their cell membrane) microorganisms that can live in either solitary or colonies. They comprise the kingdom Protista, and a sort-of catch-all kingdom that includes organisms that can’t be classified under Animalia, Plantae, or Fungi. That may sound pretty vague and, truth be told, biologists agree.

The reason why protists are so difficult to define is because they resist universal classification.This resistance stems from the fact that protists are polyphyletic, meaning that if you were to trace their lineages, you would find that protists as a group don’t share a common ancestor. In fact, these ancestors actually overlap other kingdoms. In other words, geneticists have found that protist species are more closely genetically related to organisms within other kingdoms than they are to one another.So why aren’t protists just divvied up and classified within the kingdoms with which they are most closely genetically related? Well, that’s simply because they don’t posses the ‘defining characteristics’ (or the unique features, such as all mammals possessing hair, for example) that all organisms within that particular group universally posses. In other words, the presence of these characteristics ‘define’ organisms as belonging to one group and not another.

Kind of a bit a of a predicament, huh? So what you’ll find as we talk more about these little critters is that they’re quite different from one another and yet, they’re all joined by their eukaryotic quality as well as their incompatibility with other kingdoms.

How Are Protists Grouped?

There are actually a few different ways that biologists talk about protists. Some talk about them in terms of their mode of locomotion (cilia, flagellum, or pseudopodia), while others talk about them in terms of where they live (soil, freshwater, marine, or parasitic). But here we are going to discuss them in terms of their feeding behaviors because we so readily categorize animals based on whether they feed on other organisms (animals), photosynthesize (plants), or break down organic matter (fungi).

Animal-like Protists

When we use the term ‘animal-like protists’ we mean that these protists are motile and heterotrophic (feed by consuming bacteria or other protists).

These protists are essentially hunters of prey, which is a more ‘animal-like’ trait.

Animal-like protists
Animal-like Protists

Protists belonging to this group include a vast array of organisms, like ciliated paramecium, amorphous Amoeba species, zooplankton, such as marine radiolarians and fresh/marine heliozoans, as well as some flagellated organisms like symbiotic parabasalids, which live in the guts of cockroaches. Often, this group of protists is referred to as protozoans, meaning ‘first animals’, which is not to say that all protozoans are related to the animal kingdom, but only that organisms within this group share the ‘animal-like’ behavior of hunting.

Animal/Plant-like Protists

Alright, so if animal-like protists are mobile hunters, then what defines a protists as both animal- and plant-like? Animal/plant-like protists are mixotrophic protists, meaning that they have two modes of food acquisition: hunting prey producing food via photosynthesis.

These protists are, like the animal-like protists from above, also referred to as protozoans, as they too are motile and share the animal-like trait of hunting prey. One common example of these heterotrophic protists are flagellated species from the genus Euglena, which have chloroplasts within their cell body for photosynthesizing light can also hunt and consume other protists as well as bacteria.

Mixotrophic protists
Mixotroph Protists

Plant-Like Protists

Plant-like protists are organisms that, like plants, solely rely on photosynthesis for food production. Phytoplankton, such as diatoms, can be found in both freshwater and marine environments. Another example of plant-like protists are unicellular algae, which are also found in both freshwater and marine environments.

Plant-like Protists
Plant-like Protists

Fungus-like Protists

Fungus-like protists are those protists that form spores when their environment becomes unsuitable or even hazardous to their survival, much like a fungus does. These protists are known as slime molds, and their spores can remain dormant for up to 75 years and still germinate. What’s also interesting to know about these protists is that they tend to be colonial. The slime molds pictured below are all colonies of single-celled protists that have fused their membranes to essentially become one giant, multi-nucleated cell. Pretty cool, huh?

Fungus-like Protists
Fungus-like Protists

Lesson Summary

Protists are eukaryotic organisms that can not be classified under kingdoms Animalia, Plantae, Fungi.

They are polyphyletic. which means their lineages don’t share a common ancestor and actually overlap other kingdoms. Animal-like protists are motile and heterotrophic, meaning they feed by consuming bacteria or other protists.

Animal/plant-like protists are mixotrophic protists, meaning that they have two modes of food acquisition (hunting prey and photosynthesis). Plant-like protists are organisms that, like plants, rely solely on photosynthesis, while fungus-like protists are spore-forming protists.


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