Protista Characteristics, Animal Sake

What are protistas? Protista is a kingdom containing eukaryotic microorganisms called protists. They are fascinating tiny organisms that don't quite fit into any other kingdom, not quite animals, not quite plants, and not quite fungi.

What Are Protists?

Protista was first observed by John Hogg in the 1860s as primitive unicellular forms of both plants and animals. At that time, this kingdom was known as "Protoctista," which literally means first established beings. Later, in the year 1866, the term "protista" was coined by Ernst Haeckel. This kingdom belongs to the domain Eukarya.

Scientists have been making recent classifications, listing organisms on a hierarchical basis. Protists have different characteristics that set them apart from other kingdoms. The most common examples of protista include amoeba, plasmodium, paramecium, slime mold, red algae, brown algae, golden algae, fungi, and diatoms.

General Characteristics of Protista

Protists share a set of key features that make them unique among living things:

They are mostly unicellular but some are multicellular and colonial organisms.
They are either free-living or parasitic.
They have an aerobic mode of respiration and have mitochondria for cellular respiration.
They are true eukaryotes and are nucleated.
They have a (9+2) arrangement of flagella and have membranous organelles.
They reproduce both sexually (syngamy) and asexually.
They are grouped into 3 categories: animal-like (protozoa), fungus-like, and plant-like.
According to the category, they have different modes of nutrition, heterotrophy or autotrophy.
Plant-like protists (algae) have chlorophyll and accessory pigments named xanthophylls, phycobilins, and carotene.

Classification of Protists, Based on Sub-Groups

Plant-Like Characteristics: e.g. Algae

Plant-like protists are further classified as Euglenoids, Diatoms, and Dinoflagellates.

Euglenoids, e.g. Euglena

They are unicellular
Animal-like movement
Flagellum present
Presence of nucleus, chloroplasts, flagellum, and cell membrane
Asexual reproduction

Diatoms, e.g. Chrysophyta (golden algae)

Golden-brown diatoms under a microscope, showing their intricate two-part silicon glass shells and geometric patterns. — Diatoms have beautiful two-part shells made of silicon glass. Their golden-brown color gives golden algae its name.

Two-part shell of silicon glass
Golden-brown in color
Stores food in the form of oil

Dinoflagellates, e.g. Pyrophyta (red algae)

They are di-flagellated
Phosphorescent algae
Covered with plates
Color renders "red tides"
Has toxins

Animal-Like Characteristics: e.g. Protozoans

Animal-like protists, or protozoans, share several features:

Presence of pseudopodia (false feet used to move)
Shell composed of calcium
They are heterotrophs (they eat other organisms)
Free-living, saprophytic, mutualistic, or parasitic

The animal-like protists are further classified as the following. You can read more about how marine animals interact with protists in ocean ecosystems.

Sarcodina: amoeba-like movement (pseudopodia)
Flagellates: moves with flagella
Paramecium: moves with cilia
Sporozoans: produce spores

Take the Protista Quiz!

5 quick questions. Test what you know about the weird and wonderful world of protists.

Fungus-Like Characteristics: e.g. Slime Molds

Bright orange-yellow slime mold creeping across a damp mossy log in a forest, showing its branching network of tiny spore nodes. — Slime molds look like something from a sci-fi film! They are multicellular, live in damp places, and change shape throughout their life cycle.

They are multicellular
Change forms during their life cycle
Tend to live in a damp climate
Plasmodial (acellular) slime molds (Myxomycetes)

Phylogeny of Protista

Kingdoms: Chromista, Archaezoa, Protista/Protozoa
Candidate Kingdoms: Archaezoa, Euglenozoa, Alveolata, Stramenopila, Rhodophyta

Uses of the Protista Cell Wall

The cell wall of red algae is made up of agar and carrageenan. Both are used as additives in food. Agar is used as a gelling agent and thickener. Shells made up of calcium carbonate protect protists from grazing animals. In brown algae, the cell wall of algin and cellulose protects them against waves and exposure to air. This component also ensures uniform freezing and melting of food items. Fungi have cell walls made up of cellulose or chitin.

These remarkable organisms remind us that life comes in far more forms than we might expect, and that even the smallest creatures, like the simple sponge, can play a huge role in how ecosystems work. The information about protists shows just how varied and surprising the living world can be.

Term	What it means	Example
Eukaryote	An organism whose cells have a true nucleus enclosed in a membrane	Euglena, amoeba, humans
Flagellum	A whip-like tail that some cells use to swim or move	Euglena (1 flagellum), dinoflagellates (2 flagella)
Pseudopodia	Temporary "false feet" that amoeba-like protists extend to move and engulf food	Amoeba, Sarcodina
Syngamy	A form of sexual reproduction where two cells fuse completely together	Found in many protists including algae
Xanthophylls	Yellow-brown pigments found alongside chlorophyll in plant-like protists	Diatoms, dinoflagellates
Heterotroph	An organism that cannot make its own food and must eat other organisms	Amoeba, Paramecium, Sporozoans

Protista Characteristics

What Are Protists?

General Characteristics of Protista