Index to this page

The Protists

What are protists?

The Euglenozoa

Ciliates, Sporozoans, and Dinoflagellates: the Alveolates

These three phyla are grouped in a clade — the alveolates — because


All of this rightly suggests that although they are unicellular, there is nothing rudimentary about the ciliates. Their single cell is far more elaborate in its organization than any cell out of which multicellular organisms are made.

Link to discussion of reproduction in the ciliates.

Sporozoans (Apicomplexa)

The members of this group share an "apical complex" of microtubules at one end of the cell (hence the name that many prefer to the old name of sporozoans). All the members of the phylum are parasites.

The genus Plasmodium causes malaria, one of the greatest scourges of humans. There are 4 species that infect humans of which Plasmodium falciparum is the most dangerous. Malaria has probably caused more human deaths than any other infectious disease; even today it is estimated to kill a million people a year in the sub-Saharan Africa.

The organism is transmitted from human to human through the bite of mosquitoes of the genus Anopheles.

The diagram shows the Plasmodium life cycle.

Most forms of malaria are chronic. The organisms may coexist with their host for years (but cannot complete their life cycle there).
How they evade the immune response of their host.

Toxoplasma gondii is another parasitic member of this group.

Plasmodium, Toxoplasma, and some of the other members of this group contain a membrane-enclosed organelle called the apicoplast. They seem to have inherited it from a common ancestor that acquired it by engulfing a chloroplast. Link to a discussion of this example of secondary endosymbiosis.


Diatoms, Golden Algae, Brown Algae, and Water Molds: The Stramenopiles

These organisms belong to a single clade, the stramenopiles (a/k/a heterokonts). The first three members share:

All four of them (plus a number of other groups not listed) share genes closely-homologous to those in both green and red algae. This suggests that they are all descended from a heterotrophic eukaryotic ancestor that acquired both a green alga and a red alga by a secondary endosymbiosis. (While the water molds no longer are photosynthetic, they still retain both green and red alga genes.)


Diatoms are unicellular. Their cell wall or shell is made of two overlapping halves. These are impregnated with silica and often beautifully ornamented. The photo (courtesy of Turtox) is of Arachnoidiscus ehrenbergi magnified some 400 times.

Diatoms are major producers in aquatic environments; that is, they are responsible for as much as 40% of the photosynthesis that occurs in fresh water and in the oceans. They serve as the main base of the food chains in these habitats, supplying calories to heterotrophic protists and small animals. These, in turn, feed larger animals.

Golden Algae (Chrysophyta)

Brown Algae (Phaeophyta)

Water Molds (Oomycetes)

As their name suggests, water molds were once considered to be fungi. But unlike fungi, the cell wall of water molds is made of cellulose, not chitin. Furthermore, their gene sequences are very different from those of fungi (and most closely related to those of diatoms, golden and brown algae).

Some notable water molds:

Red Algae (Rhodophyta)

Slime Molds (Mycetozoa)

Cellular Slime Molds

The organisms in this group have a complex life cycle during the course of which they go through unicellular, multicellular, funguslike (form spores) and protozoanlike (amoeboid) stages.

Thousands of individual amoebalike cells aggregate into a slimy mass — each cell retaining its identity (unlike plasmodial slime molds). The aggregating cells are attracted to each other by the cyclic AMP (cAMP) that they release.

With the exception of one species that causes powdery scab on potatoes, these organisms are of little economic importance. However, their combination of traits makes them of great scientific interest. Molecular phylogenies place them in the same clade as animals (metazoa) and fungi. The link below will introduce you to one of the most popular members of the group.

External Link
Dictyostelium discoideum: Model System in Motion
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Plasmodial (Acellular) Slime Molds (Myxomycetes)

At one stage in their life cycle, these organisms consist of a spreading, slimy, multinucleate mass called a plasmodium that moves slowly over its substrate (e.g., a rotting log) engulfing food and growing as it does so.

Eventually, the plasmodium develops stalks that produce and release spores. If the spores land in a suitable location, they germinate forming single cells that move by both flagella and pseudopodia. These fuse in pairs and start forming a new plasmodium.

The left photo (courtesy of Prof. I. K. Ross) shows the plasmodial stage of Stemonitis just before it formed sporangia. The right photo (courtesy of Turtox) shows the fully developed sporangia of Stemonitis.

Physarum polycephalum, another member of this group, is the subject of many laboratory studies.

Protists without typical mitochondria

There are several groups of protists that were long thought to have no mitochondria. However, most (perhaps all) had them in the past. Today, only remnants of their ancestor's mitochondria — called mitosomes — remain.

Some examples are:


These are single-celled (e.g., Monosiga), aquatic (both fresh water and marine) protists that have a single flagellum surrounded by a collar ("choano" = collar) of microvilli. Some (e.g., Proterospongia) form simple colonies during part of their life. The flagellum is used for swimming and also beats bacteria-containing water through the collar for feeding.

Sponges also use collar cells to filter food from the water.

Not only does this suggest a close relationship between the two groups, but other evidence indicates that choanoflagellates are the closest protistan relatives of all animals (metazoa). Although single cells, they express genes for several proteins that are essential to cell-cell interactions in metazoans, such as What functions these proteins have in the single-celled choanoflagellates remain to be discovered.

Other Groups of Eukaryotes

External Link
A portal into the various groups of protists (and other eukaryotes) with many illustrations in color.
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30 May 2024