The protists of Kindom Protista are the simplest eukaryotes, yet they represent an incredibly diverse group. Most are unicellular, some are colonial, and other are simple multicellular organisms closely related to single protist cells. One group of protists, the algae, are autotrophic photosynthesizers, while the rest eat bacterial or other protistan cells, or small organic particles suspended or dissolved in water. Due to these feeding habits, and the weakness of non-aquatic locomotion capabilities, protists thrive in aquatic habitats.
As with all eukaryotic cells, protists contain membrane-bound nuclei and endomembrane systems, as well as numerous organelles. Movement is often provided by one or more flagella, and cilia are often present on the plasma membrane as sensory organelles. As is typical of eukaryotes, the protistan flagella consist of a 9+2 pattern of microtubules. Unlike prokaryotes, protistan nuclei contain multiple DNA strands, though the total number of nucleotides is significantly less than in more complex eukaryotes. Protists can reproduce mitotically, and some are capable of meiosis for sexual reproduction. Cellular respiration in the kingdom is primarily an aerobic process, but some protists, including those that live in mud below ponds or in animal digestive tracts, are strict or facultative anaerobes.
Protists represent an important step in early evolution, evolving from prokaryotes and eventually giving rise to the entire line of eukaryotes. The first protists probably evolved 1.7 billion years ago, 2.3 billion years after the origin of life, from simple communities of prokaryotic cells. Membrane infolding was one of the defining processes in this evolution: in some prokaryotic cells, parts of the plama membrane folded into the cell to create the nuclear envelope and the other organelles of the endomembrane system. The second major step in the evolution of protists from bacteria was the process of endosymbiosis, which introduced the mitochondrion and chloroplast as organelles of eukaryotic cells. Small prokaryotic cells capable of cellular respiration or photosynthesis entered eukaryotic cells, either as parasites or indigestible food, and these prokaryotes evolved into mitochondria and chloroplasts as they developed a symbiotic relationship with the host cell. (Because mitochondria are present in all eukaryoptic cells, this process probably happened to mitochondria first.) As a result of these two processes, protists evolved as sucessful organisms. Eventually, colonial protists evolved into plants, fungi, and animals, eukaryotic kingdoms that came to dominate the earth.
The Kingdom Protista is divided into four major groups, not technical divisions but instead grouped by lifestyle: the protozoans, the slime molds, the unicellular algae, and the multicellular algae. Protozoans include all protists that ingest their food, and thus they live primarily in aquatic habitats, such as ponds, drops of water in soil, or the digestive tracts of animals. In the latter capacity, a small number of protozoans function as parasites. The second group, the slime molds, are unique in having both unicellular and multicellular stages. When sufficient bacteria (food) are present, cellular slime molds are single amoeboid cells; however, when food becomes scarce, they aggregate into sluglike colonies, which become large reproductive structures. Plasmodial slime molds also exist as single cells when nutrients are plentiful, but each cell can grow into a large, branching plasmodium, with many nuclei and a continuous nucleus. This differentiates into reproductive structures when food is short. The third and fourth groups of protists, the algae, contain chloroplasts and photosynthesize like plants; these can be unicellular, colonial, or multicellular. Multicellular marine algae, the seaweeds, are similar to marine plants, and many biologists support moving seaweed into the plant kingdom.
Under Linnaeus's original two-kingdom scheme, protists had no clear classification. This was one of the numerous reasons for the reorganization of the classification system, and the introduction of the five-kingdom system. Now, due to the incredible diversity within the kingdom, a growing number of biologists and taxonomists are favoring splitting the Kingdom Protista up into several new kingdoms.
The Kingdom Protista is, as aforementioned, incredibly diverse, as it contains the ancestors of all the other eukaryotic kingdoms. There are hundreds of phyla within Kingdom Protista, which can be divided into several main groups. Descriptions of these groups, with lists of their phyla, are given on the following pages: The Chromobionts contain the brown algae and the golden-brown algae, from the unicellular diatoms to the seaweed of kelp forests. The Cholorobionts are the green algae, including unicellular, colonial, and multicellular types; these are believed to have been the ancestors of modern plants. Euglenozoa are a small group of unicellular algae not closely related to the plants. Red algae, the Rhodophytes, typically live deep under the ocean surface, and thus have a different array of photosynthetic pigments, resulting in their red color. The Cryptomonads are considered both algae and protozoans, and contain several rare photosynthetic pigments. The Dinozoa comprise the dinoflagellates, unicellular algae that move with two oppositely situated flagella to produce the whirling motion they are known for. The sarcodines comprise two major groups of protozoans, and all move by means of pseudopodia: the Rhizopod sarcodines include amoebas, and other types of protozoans that typically have a stage of the life cycle marked by the presence of a protective test, while the Actinopod sarcodines are radially stiffened axopods, usually containing a structured cytoskeleton and a complex central capsule. The Apicomplexans are parasitic protozoans, with specialized organelle complexes, designed to penetrate host cells and tissue, at the apex of the cell. Microsporidians are a group of small parasitic protozoans, one of the oldest protozoan groups known. Myxozoa are a group of parasitic protozoans found mainly in cold-blooded vertebrates such as fish. The ciliates are primarily free-living protozoans who move by means of numerous cilia, and contain two different types of nuclei: a large macronucleus that controls cell metabolism and development, and many smaller micronuclei that function in sexual reproduction. The slime molds, formerly thought to be fungi but now known to be completely unrelated to the fungi, are protists with unique unicellular, colonial, and multicellular (mating structure) forms depending on availability of nutrients. Arasiomycota are the cellular slime molds, with a predominantly amoeboid unicellular stage, while Myxomycota are the plasmodial slime molds, with a unicellular stage, the plasmodium, of vast, streaming cytoplasm with many nuclei.
To see how the protists gave rise to the three other eukaryotic kingdoms, click here.
Chromobionts |Chlorobionts |Euglenozoa |Rhodophytes
Cryptomonads |Dinozoa |Rhizopod Sarcodines |Actinopod Sarcodines
Apicomplexans |Microsporidia |Myxozoa |Ciliates