4.3 Protists

Classification of Protists on the Phylogenetic Tree of Life

Protists are a diverse group of mostly single-celled organisms that don’t fit neatly into the other kingdoms of life. They are eukaryotes, meaning their cells have a nucleus and other membrane-bound organelles, but they are not animals, plants, or fungi. Because of their diversity, protists are often considered a “catch-all” category in biological classification.

Characteristics of Protists

There are currently over 100,000 described living species of protists, though scientists believe many more remain undiscovered. Given this diversity, it is not surprising that few characteristics are common to all protists.

Habitat

Nearly all protists exist in some type of aquatic environment, including freshwater and marine environments, damp soil, and even snow. Several protist species are parasites that infect animals or plants. A parasite is an organism that lives on or in another organism and feeds on it, often without killing it. A few protist species live on dead organisms or their wastes, and contribute to their decay.

Structure

Protist cells are among the most complex of all eukaryotic cells. While most are microscopic and unicellular, some form colonies or even true multicellular structures. Others, like certain seaweeds, are massive single cells with multiple nuclei (sometimes reaching 3 meters in length!). Most protists are motile and have evolved various ways to move: some use flagella or cilia, while others extend pseudopodia to crawl. Some can even sense and move toward light.

Human Pathogens

Many protists are pathogenic parasites that must infect other organisms to survive and reproduce. These parasitic protists are responsible for several serious human diseases, including malaria, African sleeping sickness, and giardiasis (“Beaver Fever”, a waterborne gastroenteritis). In addition to affecting humans, some protist pathogens target plants, causing widespread crop damage.

Malaria is a life-threatening disease caused by parasites of the genus Plasmodium. These parasites must infect both a mosquito and a vertebrate host to complete their life cycle. In humans, the parasite first develops in liver cells and then invades red blood cells, where it multiplies asexually. It eventually bursts out, destroying the blood cells and releasing waste products that trigger intense immune responses, including high fever and delirium. The most deadly species, Plasmodium falciparum, can destroy over half of a person’s circulating red blood cells, leading to severe anemia. It is transmitted to humans by the Anopheles gambiae mosquito, a highly aggressive vector found primarily in Africa. Controlling this mosquito through insecticides, bed nets, and environmental management is essential to malaria prevention. According to the World Health Organization, malaria causes hundreds of thousands of deaths each year, with the vast majority occurring in sub-Saharan Africa. Over the course of human history, malaria is estimated to have caused up to 5 billion deaths, making it one of the deadliest diseases ever known.

Protist Diversity

Protists are incredibly diverse, so scientists often group them into three major categories based on how they obtain energy:

Protozoa (Animal-like Protists)

Protozoans are animal-like protists that are unicellular, heterotrophic, and often motile, meaning they can move to find food.

They are typically grouped into four main sub-categories based on their movement and structure:

Amoebas

Amoebas move using pseudopodia (temporary extensions of their cell membrane).

 

Flagellates

Flagellates use one or more flagella to swim. Some are free-living, while others are parasitic.

Cillates

Ciliates are covered in cilia to help them move and feed.

Apicomplexans

Apicomplexans are non-motile and often parasitic.

 

Fungus-like Protists

Fungus-like protists are heterotrophic organisms that absorb nutrients from decaying organic matter, similar to true fungi. They are often found in moist environments and play an important role as decomposers. This group includes slime molds, which exhibit a unique life cycle. In their feeding stage, slime molds exist as unicellular, amoeba-like organisms that engulf food particles. However, under certain conditions, many of these cells can unite to form large, multicellular structures that move and reproduce collectively.

 

Algae (Plant-like Protists)

Algae are autotrophic protists that perform photosynthesis, much like plants. They are incredibly diverse and can be unicellular, colonial, or multicellular. Unicellular algae are found in various aquatic environments and play a crucial role as primary producers, commonly referred to as phytoplankton.

Multicellular algae, also known as seaweeds, are typically classified into sub-groups based on their pigments:

• Green algae are closely related to land plants.
• Brown algae include large seaweeds such as kelp.
• Red algae are often found in deeper ocean waters due to their ability to absorb blue light.

 

 

Green algae

Brown algae

Red algae

“13.3 Protists” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.