Protista constitutes a diverse kingdom containing thousands of species of single-celled eukaryotic organisms. Because many questions still persist concerning the ancestry of these organisms, deciphering which organisms should be classified in this kingdom is often a more arbitrary decision than most biologists would like. Protista is presented in the majority of biology texts as one of the six kingdoms; that's how the organisms are presented here. These are Archaea (Archaebacteria), Eubacteria, Fungi, Protists, Plantae, and Animalia.
Because multicellularity evolved many times, many multicellular organisms are more closely related to their ancestral unicellular lineages than they are to other multicellular organisms. This accounts for why some members of the plant kingdom (such as the large multicellular algae) are sometimes considered to be multicellular protists. Because certain members of the fungal kingdom (such as slime molds) are sometimes considered closer to the protistal lineage than to that of the fungi, they therefore are placed in the former. And in some classifications, certain single-celled, heterotrophic protists are grouped with the animal kingdom. There are even respectable classifications in which all the major groups considered to be protists are placed in other kingdoms, and Protista is entirely dispensed with. The kingdom Protista as presented here, however, reflects the most widely accepted classification found in the majority of biology texts.
Discrepancies between different classifications are partially attributable to the way protists are defined. Rather than being grouped together by their shared characteristics, they are grouped by exclusion. That is, in addition to usually being unicellular, all protists are eukaryotes, so they are not included among the phylum Monera; since none develop from an embryo, they are not included among the phylum Plantae; since most do not develop from spores, they are not included among the phylum Fungi; and since none develop from a blastula, they are not included among the phylum Animalia. The organisms that remain tend to be those placed in this kingdom, Protista.
This kingdom includes the most simple, and often the most primitive, eukaryotic microorganisms and all their immediate descendants. Each protist cell has a nucleus and all the other eukaryotic properties. Members of this kingdom vary considerably in structure and physiology, ranging from heterotrophs (usually free-living, although there are parasitic forms) to photosynthetic autotrophs.
Protists appear to have evolved from a moneran type of ancestor. Protists possess specialized features, such as endoplasmic reticulum, Golgi bodies, centrioles, chloroplasts, and mitochondria, as well as different kinds of vacuoles, granuoles, and fibrils. In addition, the average unicellular protist is considerably larger than the average moneran, and its cell division has become distinct from moneran cell division, having evolved mitotic and meiotic cell division.
It is theorized that the primitive protists were both plant-like and animal-like, having the capacity to obtain food by different mechanisms, as well as being able to photosynthesize additional food internally. There is considerable evidence that symbiotic relationships with prokaryotes living inside some early eukaryotes led to the development of chloroplasts and mitochondria. These organelles are contained in protists, as well as in many other more advanced eukaryotes.
Protista consists of several widely divergent phyla. Some of the unicellular, nonphotosynthetic protists are grouped as the Protozoa. These are subdivided into four classes: the Mastigophora (flagellates), the Sarcodina (amoebas), the Ciliophora (ciliates), and the Sporozoa (spore formers). The first three classes are identified according to their locomotor structures; however, sporozoans have no locomotor organelles, and instead they are characterized by their spores. To date, about 50,000 species of protozoans have been described.
Together, there are several other phyla that are often called true algae. They include about 25,000 described species, some of which belong to evolutionary lineages that were already well developed more than 450 million years ago. Nearly all the members of these phyla are photosynthetic. They include forms that occur as single cells, as filaments of cells, as plates or in planes of cells, or as a solid body. They range in size from unicellular microscopic organisms to giant multicellular forms such as the kelps, which often reach lengths exceeding 50 m (150 ft). A brief description of these photosynthetic groups is presented below.
There are three phyla of unicellular algae. Euglenophyta (euglenoids) live in fresh water, move by means of one to three flagella per cell, and have no cell wall. Chrysophyta usually include the yellow-brown algae and yellow-green algae, and the diatoms. They are mostly marine and contain pectic compounds, with siliceous materials providing the cell wall components. Pyrrophyta (dinoflagellates) live in marine environments, in fresh water, and in moist soil. They are characterized by having two flagella that beat in different planes, causing the organisms to spin. They often have distinctive, if not bizarrely shaped, cellulose walls. Like diatoms, the dinoflagellates are major components of the phytoplankton; they are aquatic, free-floating, photosynthetic, and usually microscopic. Some include the unicellular algae, Bacillariophyta (diatoms) with the Chrysophyta. Others put the diatoms in their own phylum.
There is another group of algae, sometimes called the true algae, discussed in Chapter 20. The classification by Margulis and Schwartz groups the true algae with the protists, along with the water molds, slime molds, and slime nets, forming a kingdom they call Protoctista.
| Chrysophyta |
| Ciliophora |
| diatoms |
| dinoflagellates |
| euglenoids |
| Euglenophyta |
| Mastigophora |
| Protista |
| Protoctista |
| Protozoa |
| Pyrrophyta |
| Sarcodina |
| Sporozoa |
| yellow-brown algae |
| yellow-green algae |