Protists are a diverse kingdom of organisms that have generated a lot of debate on how to classify them. This lesson will discuss characteristics shared by all the protists and then break them down into individual phyla.
Biology and Taxonomy
The diversity of life on Earth is astounding. Scientists try to make sense of it by grouping organisms into different categories.
When considering an organism, we start with the broadest categories and work down until we reach a species name which is unique to that particular organism.This process of categorization is called taxonomy, and the major groupings (from broadest to most specific) of all living things are: domain, kingdom, phylum, class, order, family, genus, and species. This means that domains are split into kingdoms, kingdoms are split into phyla, and so on until the chain ends with species.
It’s generally accepted that all organisms can be classified into one of three domains: Bacteria, Archaea, or Eukarya. For the purpose of this lesson, we are interested in the Domain Eukarya. These organisms are eukaryotes, meaning the cells have a defined nucleus that is usually surrounded by some type of membrane.
There are multiple kingdoms found within the Domain Eukarya, and the protists make up one of those kingdoms.The Kingdom Protista comprises a diverse group of organisms, and there have been some arguments over how to classify the kingdom into appropriate phyla. Protists are so diverse, they are usually grouped based on how similar they are to ‘other’ types of organisms. For example, there are plant-like protists, animal-like protists, and fungal-like protists. However, despite these differences, there are a few characteristics shared by them all.
- All protists have eukaryotic cells (which makes sense since they’re found in the Eukarya domain).
- With the exception of some algae groups, all protists are unicellular, meaning the entire organism is made up of one cell. (Some unicellular protists live in colonies and form chains, making them look multicellular, but those chains are really made up of many individual single-celled organisms.)
- Most are small – even microscopic – in size.
- All protists live in aquatic environments, including lakes, rivers, oceans, streams, or even damp soils.
Apart from these traits, protists can be pretty different from one another. Some are capable of making their own food through photosynthesis (like plants); an organism that can make its own food is called an autotroph. Other protists have to get their food by eating; these organisms are called heterotrophs.
Some protists are free-floating in water and can move around through the use of hair-like structures that extend off their bodies (called cilia or flagella). Other protists spend their lives attached to another object, like a rock at the bottom of the water.
Phyla Within the Kingdom Protista
Because there are so many different types of protists, it’s hard to figure out how to group them into phyla.
Some scientists argue there are as many as 45 different phyla, while others simplify it down to two. For simplicity of this lesson, we will stick to three groups of 15 phyla that are grouped based on shared characteristics. Just keep in mind there is no ‘finalized’ agreement on how to categorize protists into phyla.
The following animal-like protists are heterotrophs with hair-like movement structures:
- Sarcomastigophora (protozoans)
- Ciliophora (ciliates)
The following plant-like protists are autotrophs:
The following fungal-like protists are non-mobile heterotrophs:
The following fungal-like protists are non-mobile with spores:
Who knew there was so much to learn about protists?! As you’ve probably figured out, the Kingdom Protista is diverse and filled with all sorts of organisms. Protist taxonomy (categorization) is heavily debated, and the group has been split into phyla in different ways – this lesson presents just one of those ways.
However, we did learn that all protists are eukaryotes and therefore in the Domain Eukarya. All those organisms have a defined nucleus bound in a membrane. Most protists are unicellular organisms. The plant-like protists make their own food through photosynthesis and are called autotrophs, while the fungal-like and animal-like protists need to get food to survive (making them heterotrophs). Some protists use cilia or flagella, hair-like extensions to help with locomotion and feeding, while others are non-mobile. This kingdom is worth exploring further because the protists are fascinating!