- They are eukaryotes because they all have a nucleus.
- Most have mitochondria (although some never did and some have later lost theirs). Mitochondria were probably derived from aerobic bacteria (prokaryotes) that once lived within their cells.
- Many have chloroplasts with which they carry on photosynthesis. Chloroplasts were probably derived from photosynthetic cyanobacteria (also prokaryotes) living within their cells.
- The name Protista means "the very first", and many of the 80-odd groups of organisms that we classify as protists probably have long, independent evolutionary histories stretching as far back as 2 billion years.
- As a result, some of the groups are probably placed together more for our convenience than as a reflection of close kinship.
- However, genome analysis added to other criteria enables us to postulate some natural groupings or clades, which are shown in the figure.
The cladogram also shows the protistan branches that led to the plants, fungi, and animals.
There are several groups of protists that have no mitochondria. For some, this may represent a secondary loss as they evolved from ancestors that had them. For others, it seems to be a primitive condition and, if so, these organisms are close to the base of the tree of eukaryotic life (figure).
Some examples are:
- Microsporidia
- All are obligate intracellular parasites.
- Many are pathogenic in insects (one is even marketed commercially as a biocontrol agent).
- Some contaminate drinking water supplies and can cause gastrointestinal upsets in humans. (Microsporidia are a common cause of diarrhea in AIDS patients.)
- Giardia lamblia
- Frequently encountered in public water supplies contaminated by animal feces. Causes diarrhea in humans.
- Entamoeba histolytica. Causes amebic dysentery.
- Trichomonas vaginalis. Despite its name, it is as common a sexually-transmitted disease (STD) in males as in females. Has some prokaryotic features suggesting that it and a few close relatives are the most primitive eukaryotes alive today.
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. 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.
- Most are unicellular.
- Many swim by means of a single flagellum.
- They are not encased in a cell wall so they are flexible as well as motile.
- Euglena is a typical member of the group (which numbers about 1600 species).
- Because some members of the group (like Euglena) have chloroplasts, these organisms used to be called "Euglenophytes", but in fact they are neither plants ("phytes") nor animals ("zoa").
- Analysis of their genomes suggests, instead, that they are the living descendants of some of the very earliest eukaryotes.
- Trypanosoma brucei, the cause of sleeping sickness in humans, is a member of the group. The electron micrograph (by L. Tetley; courtesy of Keith Vickerman) shows T. brucei as it occurs in the salivary gland of the tsetse fly ready to be injected into the mammalian host when the fly bites. The specimen is 12 µm long.
- Move by the rhythmic beating of their cilia.
- Although single-celled, some are large enough to be seen with the naked eye.
- Examples: Paramecium, Stentor, Vorticella.
- Feed by sweeping a stream of particle-laden water through a "mouth" and "gullet" and into a food vacuole.
- Undigested wastes are discharged at a permanent site.
- Fresh water ciliates cope with the continuous influx of water from their hypotonic surroundings by pumping it out with one or more contractile vacuoles.
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.
All the sporozoans are parasitic. They lack the power of locomotion during most (in some cases, all) of their life cycle.
The genus Plasmodium causes malaria, one of the greatest scourges of humans. 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 life cycle of Plasmodium vivax.
- The mosquito bite injects sporozoites into the human host.
- These invade the liver where they develop into merozoites.
- The merozoites invade red blood cells where they reproduce.
- Periodically, they all break out of the red cells together bringing on the chills and fever characteristic of the disease.
- Eventually some merozoites develop into either male or female gametocytes.
- These will die unless they are sucked up by the bite of an anopheline mosquito.
- Once in the stomach of the mosquito, the gametocytes form gametes: sperm and eggs.
- These fuse to form zygotes.
- The zygote invades the stomach wall of the mosquito forming thousands of sporozoites.
- These migrate to the salivary gland, ready to be injected into a new human host.
Most forms of malaria are chronic. The organisms may coexist with their host for years (but cannot complete their life cycle there).
- About 1000 species.
- Most are unicellular.
- Unlike most eukaryotes, they
- lack histones on their chromosomes and
- have a simpler form of mitosis
- They do have the eukaryotic type ("9 + 2") of flagellum (two of them in fact).
- Occasionally they reproduce explosively, creating poisonous red tides that may cause extensive kills of marine fish and make filter-feeding marine animals like clams unfit for human consumption.
The first three members of this clade share:
- a yellow-brown pigment (which gives them their color). It is a carotenoid called fucoxanthin and
- chlorophylls a and c
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 the main producers in aquatic environments; that is, they are responsible for the majority 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.
- Most are unicellular.
- Found in fresh water.
- Important producers in some aquatic food chains.
- In low light conditions, may lose their chlorophyll and turn heterotrophic feeding on bacteria and/or diatoms.
- Over 1000 species alive today; many more in the fossil record.
- The rockweeds and kelps. Some kelps grow as long as 30 m.
- All are multicellular although without much specialization of cell types.
- Most are found in salt water.
- Used for food in some coastal areas of the world and harvested in the U. S. for fertilizer and as a source of iodine.
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 brown algae).
Some notable water molds:
- Some species (e.g., Saprolegnia, Achyla) are parasites of fishes and can be a serious problem in fish hatcheries.
- Downy mildews damage grapes and other crops.
- Phytophthora infestans, the cause of the "late blight" of potatoes. In 1845 and again in 1846, it was responsible for the almost total destruction of the potato crop in Ireland. This led to the great Irish famine of 1845-1860. During this period, approximately 1 million people starved to death and many more emigrated to the New World. By the end of the period, death and emigration had reduced the population of Ireland from 9 million to 4 million.
- The red algae are almost exclusively marine.
- Some are unicellular but most are multicellular.
- Approximately 4000 species have been identified.
- They are photosynthetic using chlorophyll a and chlorophyll d.
- The structure of the membranes in their chloroplasts is quite different from that of the green plants but resembles that found in the cyanobacteria.
- Like the cyanobacteria, they use
- phycoerythrin (which makes them red) and
- phycocyanin
as antenna pigments.
- They do not have the eukaryotic "9+2" flagellum.
- Some are used as food in coastal regions of Asia.
- Agar, the base for culturing bacteria and other microorganisms, is extracted from a red alga.
Plants, including the green algae (Chlorophyta) are described in a separate page. Link to it.
At one stage in their life cycle, these organisms consist of a spreading, slimy 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.
The fungi are discussed in a page of their own. Link to it.
The metazoa are described in two separate pages:
13 April 2001
