Kingdom Protista


The Kingdom Protista is a diverse group of eukaryotes with simple levels of organization. Their members are ancestors of the other eukaryotic Kingdoms.

Introduction to Kingdom Protista

The Kingdom Protista is a large, diverse collection of eukaryotic organisms. This heterogeneous Kingdom is referred to as the ‘junk drawer’ Kingdom because organisms that do not belong in the other eukaryotic Kingdoms (Plants, Fungi, Animals) are placed here (similar to objects in in a junk drawer that have no home). Most Protista are microscopic and unicellular, though other members are colonial and multicellular. Members of this Kingdom may be heterotrophic, autotrophic, or both. They do not have specialized tissues.


Members in this diverse kingdom can be as small as a Paramecium (left) and, in the case of kelp, large enough to swim through (right)

Additional characteristics of Protista include:

  • Their habitat always includes liquid water: freshwater, marine, and moist terrestrial habitats.
  • All reproduce asexually but a few can also reproduce sexually, exchanging genetic information
  • Were the first eukaryotic organisms; thought to have evolved about 1.5 billion years ago

The groups within Protista are loosely related based on molecular evidence; there has been debate and ongoing research as to whether this Kingdom should be split and reclassified.

Look at the phylogeny below. It does not show all the groups of protista, but the groups it does show red stars) demonstrate how this group is not a clade (they are not composed of one ancestor and all of it’s descendants).  Therefore, Protista are considered paraphyletic.


There are dozens of Protist phyla that are currently accepted by scientists. This course will focus on some common, representative phyla.

Before beginning to study the notes on Protists, take a look at the following short video from Teachers Domain titled Single Celled Organisms.

Protist Origins: Endosymbiont Theory

The name “protist” in Greek, means “the very first,” and refers to the idea that the protists were the first eukaryotes. How did the very first eukaryotes arise on Earth? The endosymbiont theory explains this. It states that free living bacteria (that could photosynthesize or use cellular respiration) were taken in by another cell and remained there permanently, in a symbiotic relationship. In an earlier module we discussed evidence for this theory.

Protistan Groups

There are 3 groups that we can use to organize Kingdom Protista: (these are not taxa but informal names)

  1. Animal-like (Protozoa)
  2. Plant-like (Algae, or Protophyta)
  3. Fungus-like (Slime Molds and Water Molds) protists

Each group is composed of multiple phyla based on their nutrition and motility.

Beneficial Protists

Protists play a crucial role in helping humans to survive on Earth. Here are some reasons to value these “junk drawer” organisms:

  1. Phytoplankton (algae) serves as the basis for many food chains and plays a significant role in the cycling of carbon and oxygen. They produce about half of the oxygen for the planet and maintain a sink of carbon dioxide in the ocean (rather than in our atmosphere, which would warm it).
  2. Some protists serve as major decomposers in ecosystems.
  3. Byproducts from some protists are used in industry. Carrageenan (from red algae) is used as a thickener or smoothing agent in items such as toothpaste or ice cream.
  4. Chemicals isolated from protists are used in medicines to treat blood pressure or even ulcers.

Harmful Protists

Protists can be harmful to humans as well. This is most notable when they cause disease, but they can cause ecological damage that affects our lives, too.

  1. Protists are parasites for humans (Giardia, Malaria, Amoebic dysentery, African Sleeping Sickness)
  2. Dinoflagellates can cause red tides, or harmful algal blooms, that poison fish and humans as well as deplete oxygen.
  3. Eutrophication occurs when an excess of fertilizer in a body of water causes an algal bloom that depletes the water of oxygen, therefore killing other life.