EarthPoem Archives
Site Map
Join MorningEarth
Teacher Resources
Learn Ecology
Kids' Earth Art
Members' Writing
John Caddy
Contact MorningEarth

Protists includes a huge variety of organisms, mostly one-celled, mostly microscopic. Protist means "first," as in first animals, first plants.

The classification of organisms is of enormous interest to biologists, who argue about it interminably. At one time, any organism that was not an animal, a plant, or a fungus, was lumped willy-nilly into the Protista. Later, bacteria were excluded as their cells were recognized to be fundamentally different.

Currently Kingdom Protista is being subdivided into many genetic lineages, which are too arguable to introduce here. The traditional division is two large groups, the Algae and the Protozoans. But there are many kinds of protists that do not neatly fit those categories.


PROTOZOA are extremely variable. They are extraordinarily diverse in body, eating habits, gene exchange, and associations with other organisms.

Along with algae, protozoa began to evolve their variety over 1.6 billion years ago.

Protozoans are still largely unknown, even though a few kinds, such as paramecia and amoebas, have been intensively studied in laboratory settings. Since the time of van Leeuwenhoek, we have looked at common pondwater protozoans taken away from nature, or cultured from cysts. Few studies have yet been done in natural settings. 

Although some 50,000 species have been described, protozoans exist in so many still undescribed kinds that they may well represent the bulk of eucaryote diversity.


Some groups of protozoans are:

smallest, move by whipping flagella
protean pseudopods; some in shells
largest; covered by fine hairlike cilia used to swim
  Sporozoa can't move on own, most are parasites of animals
  Heliozoans freshwater "sun animals" with stiff amoeboid pseudopods, no internal skeleton
  Radiolarians marine, complex skeletons, amoeboid pseudopods


trichonympha, symbiont in termite gut;anaerobe
© Wim van Egmond
another flagellate, flagella clearly evident
© Wim van Egmond
soil flagellate,
photo Wilhelm Foissner, University of Salzbur


amoeba racing toward three algae that smell good.
shelled amoeba, psudopods extending from shell opening
Chaos diffluens, well-named free amoeba


two Paramecia toward
end of binary fission
Ophryoscolex, one of the many rumen symbiotes of grazing mammals, its bacterial symbionts digest cellulose
Carchesium colonial ciliate, fused cilia sheets
ripple into mouth
© Wim van Egmond
Stentor: stalked, sessile, large; cillia pull food into bell mouth
Cothurnia stalked ciliate
photo by Jason Oyadomari
Stylonchia 'walking'
up an alga filament
on stalks of fused cilia


Trypanosoma in blood
causes sleeping sickness
Plasmodium falciparum in blood, causes malaria
toxoplasma gondii, causes toxoplasmosis


Heliozoa, aka "sun animalcules"

photo by G.P. Mathews
heliozoan showing typically dense pseudopods
heliozoan showing many food vacuoles; eating well
Radiolarians by Ernst Haeckel, 1862
The Art of Protozoa

Ernst Haeckel was one of the most visible scientists of the 19th century. He was an ardent champion of Charles Darwin's theories, yet they took him in some directions that are not compatible with modern valuing. In brief, he was a believer in the superiority of the Aryan 'race,' as personified by Germans. His science, however, was influential. He published his book of radiolarian lithographs in 1862, which made a sensation at the time. The power of his art was unmistakeable. He also described over 400 new species of radiolarian. These specimens came to him from Alexander von Humboldt's Challenger expedition.

Haeckel's art was infused with Art Noveau romanticism, but there is no denying the brilliance of his radiolarian lithographs.


top of page

Ecological Roles of Protozoa

Within their ecosystems, protozoans play several roles. Many are predators that graze on procaryotes, bacteria and archaea. Such predators also eat algae and fungi. Planktonic ciliates, such as paramecia, are the most important consumers of algae in many lakes and marine systems.

Protozoa, much larger, feed primarily on bacteria, but also eat other protozoa, organic matter, and sometimes fungi.

Some protozoans are autotrophs, self-feeding, because they carry photosynthetic algae such as Chorella inside them. Several kinds of protozoans show great adaptive flexibility in feeding. They can switch from autotroph to heterotroph at need. Appropriately, they are called mixotrophs, like famous Euglena.

In the community dynamic of eating and being eaten, protozoa are an important food for invertebrates small and large. Filter feeders such as oysters and mussels depend heavily on planktonic protozoa. In a way, protozoans are trophic bridges that transfer the nutrients and energy of bacteria and algae up the food web.

One important role of soil protozoa is to make nitrogen available to algae and plants and other soil organisms. The bacteria they eat contain more nitrogen than the protozoa can use, so they excrete the excess as ammonium. This process is called mineralization. This process is of large importance to soil fertility.

Protozoa and bacteria-eating nematode roundworms compete for their common food resource: bacteria. Some soils have high numbers of either nematodes or protozoa, but not both. The significance of this difference to plants is not known. Both competitors consume bacteria and release nitrogen, a critical plant nutrient.

top of page

Habitat and Lifestyle

Protozoa live almost everywhere, in all waters, fresh and salt. Wetlands, lakes, rivers, and of course oceans, support many protozoa. Soils and wet sediments are the choice habitat for a great many kinds as well. Although protozoans are much larger than procaryotes, they are microscopic,so there are an enormous number of potential ecological niches for protoazoa to exploit, which enables great diversity and speciation.

Two popular protozoan lifestyles are planktonic and sessile. Planktonic protozoa float in the sunlit surface layers of ocean, ponds and stream backwaters. They graze primarily on bacteria, but some contain photosynthetic symbiont algae and feed themselves.

Sessile means attached, so sessile protozoans glue themselves to submerged surfaces where there may be flow enough to carry good food into their waiting mouths. Leaves and stems of submersed plants, pebbles, stones and sunken wood are choice locations for the sessile kind. Some attach to fish, turtles and whoever else moves around underwater. Sessile protozoans attach by special adhesive organelles. These organisms often swim as larva, then attach as adults.

Many sessile protozoans, such as stentor and vorticella (pictured), have a stalk with the main body and mouth on the upper end. Theses stalks are stretchy and can elongate to get food and when danger threatens, violently contract so quickly it can’t be seen. Some protozoans are colonial, with some division of labor and some physical differentiation, analogous to animal tissues.

Moist soil is another Earthwide habitat.  As in the sunlit ocean, the surface layers of soil are busy with organisms, many of them decomposers and detritus feeders. Topsoils are busy with fungi and algae too, but bacteria are the food of choice for soil protozoans. The root zone of plants, or rhizosphere, is a hotspot for all microscopic life.

Living in soil requires the ability to survive drying out, so soil protozoa can form cysts which hold their lives suspended until moisture returns.

Protozoa need bacteria to eat and water in which to move, so moisture plays a big role in determining which types of protozoa will be present and active. Like bacteria, protozoa are particularly active in the rhizosphere next to roots.

Typical numbers of protozoa in soil vary widely – from a thousand per teaspoon in low fertility soils to a million per teaspoon in some highly fertile soils. Fungal-dominated soils (such as forests) tend to have more testate amoebae and ciliates than other types. In bacterial-dominated soils, flagellates and naked amoebae predominate. In general, high clay-content soils contain a higher number of smaller protozoa (flagellates and naked amoebae), while coarser textured soils contain more large flagellates, amoebae of both varieties, and ciliates.

Protozoan Extremophiles

Where environmental conditions are so extreme that soils lack higher organisms, such as earthworms, protozoa are especially abundant.

High mountain habitats above the timberline and in Antarctica are typical extreme environments where protozoa often account for one third or more of the total heterotrophic biomass. For instance, moss carpets of Wilkes Land, East Antarctica, are populated with many active ciliates and thousands of shelled amoebae.

Compared to multicellular organisms, diversity is high, that is, about 100 protozoan species in lower Antarctica and more than 200 species in high mountain areas.

High acidity creates another kind of extreme habitat, in conifer litter and soil. High acidity commonly excludes earthworms, but is preferred by shelled amoebae, which occur there in over 100 species with an average of about 10,000 individuals per gram, dry mass. Thus, half of the heterotrophic soil biomass may consist of protozoa.

Deserts and highly saline (salty) soils provide other examples for extreme habitats and were studied in Namibia, Southwest Africa. Both deserts and salt pans had a surprisingly high ciliate diversity composed of many undescribed species. When dune sand of the Namib Desert or soil from the Etosha Pan was moistened, masses of ciliates appeared within 48 hours, showing the presence of high numbers of viable cysts.

Central Australian deserts have been studied as well. There too, cilliates thrived in the dry soils, although many were in cyst state. Greatest abundance was in shrub shade and in cryptobiotic crusts, which supported some cyanobacteria-grazing protozoans that can’t encyst.

Parasite Protozoans

Some protozoans are parasitic. Those we know most about, of course, parasitize ourselves and our livestock. Members of phylum Sporozoa cause sleeping sickness (Trypanosoma) and malaria (Plasmodium falciparum). Both attack red blood cells. Cryptosporidium damages intestinal cells.In Phylum Flagella, Giardia parasitizes the intestines. Entamoeba histolytica attacks intestinal cells and liver cells. Many other protozoans parasitize other mammals, marsupials, birds, reptiles and insects. However, beneficial symbioses are a much more common relationship for protozoans in general.

top of page


Protozoans engage in many symbiotic relationships that are mutually beneficial. Bacteria and archaea are symbiotes inside many, probably most, protozoans, just as they are with most macroscopic organisms. In many symbioses, these procaryotes release nutrients to their hosts by digesting substances such as cellulose and lignin, that the host cannot digest. In many cases, these  bacteria and archaea also create and make available some of the amino acids and vitamins that their hosts require.

Many anaerobe protozoans that contain bacterial symbionts live inside larger creatures’ stomachs and intestines. For example, many ciliates make their homes inside large mammals. With the help of their own bacterial symbionts, these protozoans make it possible for cows and other grazing and browsing  mammals to digest cellulose.  Without such arrangements, no cows, no deer, no antelope, no horses.

Other anaerobe ciliates make termites possible. In tropical grasslands, termites eat wood, which is made of cellulose. Termites can’t digest cellulose, so the bacteria inside several species of protozoa do it for them. The numbers of symbiont protozoa in a termite’s gut convince us how small protozoans really are. In two recent studies, termite workers’ guts contained on average between 60, 000 and 90,000 protozoans.

Some ciliates, including Paramecium bursaria, and many radiolarians, host symbiotic algae that provide them with energy but are not integrated into the cell. When light fails, such protozoans can also become heterotrophs and graze bacteria, thus earning the name mixotroph.

When marine protozoa possess photosynthesizing symbionts, they become important global primary producers as well as being heterotroph consumers.

Planktonic radiolarians in warm seas "farm" photosynthetic dinoflagellates as symbionts in their cytoplasm while also feeding on other planktonic organisms. In turn, the dinoflagellates receive nutrient nitrogen from the protozoans.

top of page

Defense and Communication

We often think of self-defense as physical, and some protozoans do use such means. Sessile stalk protozoans can withdraw from the touch of a predator in 1/25 second, which also roils the water, leading to predator confusion. Some ciliates, on the other hand, do act aggressively. Paramecia have elaborated some cilia into trichocysts, tiny pointed filaments that can be fired at a predator. Each trichocyst point has barbs like a harpoon.

The most common forms of defense, however, is by chemical signals released into water. This is an emerging area of study. Little is known about it yet.

As studies of procaryotes and protists increasingly move out of the lab and into natural settings, more such contextual processes come to scientists’ attention. The chemical interactions of the microcosm are many. The only ones studied much to date are those that take place in the rhizosphere, the area immediately surrounding plant roots. Plants are full participants in such interactions. They give off (exude) chemicals which inhibit organisms from eating roots, and also exude other chemicals which invite others, such as mycorrhizal fungi, and nitrogen-fixing bacteria, to join them in symbiosis.

Many algae and protozoans apparently exude chemicals that generally inhibit others from eating them. Many questions remain.

For example, the alga chlorela is a symbiote of many protozoans. Does the alga exude a chemical that says “Eat me—but don’t digest.”? Does the protozoan exude a signal in this process?

A non-defense kind of chemical signalling is aimed at conspecifics, one’s own kind. These common signals are invitations to conjugate, get together to exchange genes and re-invigorate. Those conspecifics that receive the signal receive information about the health of the inviter.

top of page

Reproduction and Gene Exchange

Most protozoa reproduce by cell division. This is non-sexual, or asexual, reproduction.But protozoans of the same kind do get together to exchange DNA. Like procaryotes, they conjugate.
Conjugation rejuvenates both partners. This gene exchange is not a form of reproduction.

Unlike procaryotes, most protozoans do have gender, and some do manage sexual reproduction. But they accomplish this in diverse and complex ways.

For example, paramecium mating types form 16 distinct mating groups. Autogamy (self-fertilization) is a similar process that occurs in one organism, which clones itself. In cytogamy, another type of self-fertilization, two animals join together but do not undergo nuclear exchange.

The amoeba groups have no means of sexual reproduction; they only use cell division.

top of page

Explore Further in Biosphere

Biosphere: Introduction
Biosphere as Place: Introduction
Biosphere as Ocean: Life Zones
Biosphere as Ocean Floor: Benthic Biomes One
Biosphere as Ocean Floor: Benthic Biomes Two
Biosphere on Land: Terrestrial Biomes
Biosphere on Land: Anthropogenic Biomes
Biosphere as Process: Introduction
Biosphere Process: Floating Continents, Tectonic Plates
Biosphere Process: Photosynthesis
Biosphere Process: Life Helps Make Earth's Crust
Biosphere Process:
Rock Cycle--Marriage of Water and Rock
Biosphere Process: Marriage of Wind and Water
Biosphere Process: Gas Exchange
Biosphere as An Expression of Spirit
The Ecological Function of Art
The Earth Goddess
The Tree of Life
The Green Man
Earth Art
Biosphere as Community
Biosphere Microcosm: Bacteria and Archaea
The Procaryote Domain
Biosphere Microcosm: Germs
Biosphere Community: The Eucaryote Domain
Biosphere Community: Protists 1: Algae
  Biosphere Community: Protists 2: Protozoa
Biosphere Community: Plants: What's New?
Biosphere Community: Plant Diversity--Major Groups
Biosphere Community: Plant Defense
Biosphere Community: Plant Pollination
Biosphere Community: Plant Seed Dispersal
Biosphere Community: Kingdom Animals
Biosphere Community: Kingdom Fungi
Biosphere Community: Six Great Extinctions
Return to Ecology Index