Some protists are autotrophs, a photosynthetic group of phyla referred to as the algae. Autotrophs manufacture their own energy by photosynthesis or chemosynthesis. Algae use various combinations of the major chlorophyll pigments, chlorophyll a, b, and c, mixed with a wide array of other pigments that give some of them very distinctive colors.  

Some protists are heterotrophs, a group of phyla called the protozoa. Heterotrophs get their energy by consuming other organisms. Protists reproduce asexually by binary fission, and a few species are capable of sexual reproduction. Many have very complex life cycles.  

Protists are so small that they do not need any special organs to exchange gases or excrete wastes. They rely on simple diffusion, the passive movement of materials from an area of high concentration to an area of low concentration, to move gases and waste materials in and out of the cell. Diffusion results from the random motion of molecules (black and white marble analogy). This is a two-edged sword. They don’t need to invest energy in complex respiratory or excretory tissue. On the other hand, diffusion only works if you’re really small, so most protists are limited to being small single cells. Their small size is also due to the inability of cilia or flagella to provide enough energy to move a large cell through the water.  

Protists eat by phagocytosis - they engulf their food in their cell membrane, and pinch off a section of membrane to form a hollow space inside the cell. This hollow space, now enclosed by membranes, is called a vacuole. Vacuoles are handy little structures. Protists also use them to store water, enzymes, and waste products. Paramecium and many other protists have a complex type called a contractile vacuole, which drains the cell of waste products and squirts them outside the cell.  

All protists are aquatic. Many protists can move through the water by means of flagella, or cilia, or pseudopodia (= false feet). Cilia and flagella are tiny movable hairs. Motile cells usually have one or two long flagella, or numerous shorter cilia. The internal structure of cilia and flagella is basically the same. All of the characteristics that this group shares are primitive traits, a perilous thing to base any classification on, because convergent evolution may be responsible for these superficial similarities. So the concept of the Kingdom Protista has been justly criticized as a “taxonomic grab bag” for a whole bunch of primitive organisms only distantly related to one another.  

Protists are mainly defined by what they are not - they are not bacteria or fungi, they are not plants or animals. Protists gave rise to all other plants and animals. But where did protists themselves come from? The earliest protists we can recognize in the fossil record date back to about 1 billion, 200 million years ago. We do not know how the various groups of protists are related to one another. We assume they arose from certain groups of bacteria, but which groups and when are still investigating. Different phyla of protists are so unlike one another, many probably evolved independently from completely different groups of bacteria. Lynn Margulis recognizes nearly 50 different phyla of protists, or Protoctista, as this kingdom is sometimes called. We will take a more conservative approach, and focus on nine important phyla of protists.  
 
 

Protozoa = heterotrophic protists:  

Phylum Ciliophora - (Paramecium, Blepharisma) 

Phylum Sarcodina - (Amoeba, radiolarians) 

Phylum Sporozoa - (Plasmodium - malaria) 

Phylum Phaeophyta - brown algae (Fucus)  

Phylum Rhodophyta - red algae (Polysiphonia)  

Phylum Chrysophyta  -  diatoms  

Phylum Euglenophyta - (Euglena)  

Phylum Pyrrophyta - dinoflagellates (Ceratium)  

Phylum Chlorophyta - green algae (Spirogyra, Volvox, Chlamydomonas) 

Phylum Ciliophora (8,000 sp.,) Blepharisma, Paramecium  

These ciliates move by means of numerous small cilia. They are complex little critters, with lots of organelles and specialized structures. Many of them, like Paramecium, even have little toxic threads or darts that they can discharge to defend themselves. Typical ciliates you may see in lab include Paramecium and Blepharisma. 

These ciliates have a most unusual way of getting about. They extend part their body in a certain direction, forming a pseudopod or false foot, and then flow into that extension (cytoplasmic streaming). Many forms have a tiny shell made from organic or inorganic material. They eat other protozoans, algae, and even tiny critters like rotifers. Amoeba is a typical member of this phylum. Many sarcodines are parasites, such as the species Entamoeba histolytica, which causes amoebic dysentery. 10 million Americans are infected at any one time with  some form of parasitic amoeba, and up to half of the population in tropical countries. Somewhat more unusual sarcodines are the Foraminiferans. These “forams” can have fantastically sculptured shells, with prominent spines. They extend cytoplasmic “podia” out along these spines, which function in feeding and in swimming. Forams  are so abundant in the fossil record, and have such distinctive shapes, that they are widely used by geologists as markers to identify different layers of rock. The famous white cliffs of Dover are made up of billions of foraminiferan shells. 

This last group of protozoans is non-motile, and parasitic. They have very complex life cycles, involving intermediate hosts such as the mosquito. They form small resistant spores, small infective bodies that are passed from one host to the next. Plasmodium, the parasite that causes malaria, is typical of this group. In more general terms, spores are haploid reproductive cells that can develop directly into adults. 

Phylum Phaeophyta (1,500 species, fr. Greek phaios = brown) - Fucus  
 

This phylum contains the brown algae, Sargassum, and the various species of kelp. Brown algae are the largest protists, and are nearly all marine. Kelp blades can stretch up to 100 meters long. Brown algae have thin blades with a central midrib or stipe. Like all algae, their blades are thin because they lack the complex conductive tissues of green plants (phloem), and must rely on simple diffusion, though some kelp have phloem-like conducting cells in the midrib. Kelp form the basis of entire ecosystems off the coast of California and in other cool waters. In the “Sargasso Sea”, the Atlantic Ocean northeast of the Caribbean Islands, the brown algae Sargassum forms huge floating mats, said in older days to trap entire ships, holding them tight until the crew met a watery grave. 

Like brown algae, the red algae also contain complex forms, mostly marine, with elaborate life cycles.  Chloroplasts in this group show pigments very similar to those found in cyanobacteria, and ancient red algae may have engulfed these cyanobacteria as endosymbionts. Red algae have many important commercial applications, such as the agar used for culture plates. Its cell walls contain carrageenan, a polysaccharide used in the manufacture of ice cream, paint, and cosmetics. 

Diatoms have a golden-brown pigment. Some books still place them with the Chrysophyta, the golden-brown algae, but they are now recognized as an entirely separate group. Diatoms have odd little shells made of organic compounds impregnated with silica. The shells fit over the top of one another like a little box. Diatoms usually reform the lower shell after they divide This means they become smaller and smaller, and when they become too small they leave their shells and fuse through sexual reproduction into a larger size and start over again. They are one of the most important organisms in both freshwater and marine food chains. Diatoms are so abundant that the photosynthesis of diatoms accounts for a large percentage of the oxygen added to the atmosphere each year from natural sources. Their dead shells form huge deposits, that are mined for commercial uses. Diatom shells are sold as diatomacious earth, and used in abrasives, talcs, and chalk. Diatoms are so numerous that their shells form thick deposits all over the world. A single quarry in Lompoc, California, yields over 270,000 metric tons per year. One bed in the Santa Monica Ca. oil fields is over 900 meters thick! Various species of diatoms are also widely used as indicator species of clean or polluted water. 

Is it a plant, or is it an animal? It moves around like an animal, and sometimes eats particles of food, but a third of them are also photosynthetic, a nice bright green pigment like a green algae (which it used to be called). This organism may actually have resulted from endosymbiosis, in which an ancestral form engulfed a green algal cell. 

Dinoflagellates are named after their two flagella, which lie along grooves, one like a belt and one like a tail. Many species have a heavy armor of cellulose plates, often encrusted with silica. This species is very important both ecologically and economically. Some species form zooxanthellae, dinoflagellates which have lost their flagella and armor, and live as symbionts in the tissues of mollusks, sea anemones, jellyfish, and corals. These dinoflagellates are responsible for the enormous productivity of coral reefs. They also limit coral reefs to surviving in shallow waters, where sunlight can reach the dinoflagellates. Some dinoflagellate species often form algal blooms in coastal waters, building up enormous populations visible from a great distance. The amazingly potent toxins, that about 20 species produce, poison shellfish, fish, and marine mammals, causing the deadly red tide. This is the organism that can make Louisiana oysters your last meal on Earth!! One outbreak in 1987 killed half of the entire bottlnose dolphin population in the Western Atlantic. 

Several multicellular organisms have arisen from this very diverse group of algae, including the unknown ancestor of all green plants. Like higher plants, they: use chlorophyll a and b for photosynthesis; have cell walls of cellulose and pectin; and store food as starch. There are several colonial forms, such as Volvox. Groups of cells unite to form a colonial organism, in which certain groups of cells perform certain tasks. It is one of the simplest organisms to show a  true division of labor, true multicellularity. Volvox colonies can contain 500-60,000 vegetative cells. The colony has polarity, a head and tail end. It even has special reproductive cells concentrated at its tail end. The flagella that stick out from its surface cells moves the colony forward by causing it to spin clockwise. Volvox crosses a major evolutionary boundary. When Volvox reproduces, the new daughter colonies form inside the parent colony. The only way they can be released is for the parent colony to burst open and die. It is this final act of sacrifice that tells us an invisible line has been crossed. Single celled bacteria and protists are immortal. They can go on dividing in two forever, and so never truly die. But in the Kingdom Protista, we see the beginnings of specialization among groups of cells, specialization which entails the death of certain cells so that other cells can survive. As Volvox reminds us, the price of complex multicellularity is death.  

Protists eat by phagocytosis - they engulf their food in their cell membrane, and pinch off a section of membrane to form a hollow space inside the cell. This hollow space, now enclosed by membranes, is called a vacuole.  
 
 

Economic, Ecological, and Evolutionary Importance  
• Algae and protozoa are important prey in food chains. Even humans eat algae.  
• Many protozoans are important disease causing organisms (malaria, toxoplasmoisis,  amoebic dysentery)  
• Dinoflagellates cause billions of dollars in damage to the seafood industry, and are important symbionts in corals and other marine animals.  
• An extract of red algae is used to make paint, cosmetics, and ice cream. 
• Protozoans gave rise to all higher forms of animal life.  
• Green algae gave rise to all higher plant life.  
• Bacteria first mastered the fine art of photosynthesis. Cyanobacteria established the oxygen atmosphere we breathe today. But diatoms are mainly responsible for current oxygen input from photosynthesis.