The traditional classifications of the Protista included plant-like algae, fungus-like slime molds and animal-like protozoans. Today, the study of Protista indicates that while these classifications possess superficial similarities, there is no relation by common descent. Currently, the kingdom Protista consists of unicellular and multicellular organisms that are not a part of any other kingdom.
Red and green algae are both a part of the Protista kingdom. Red algae, including coralline red algae and most seaweed, is abundant in tropical seas. Some red algae also produce viscous polysaccharides, or soluble plant fibers, which are useful as thickening agents in various products including cosmetics, ice cream, paints and salad dressings. Some green algae, such as chlorella, are responsible for the green color of many organisms such as hydras, flatworms and sponges. Chlorella also live within the cells of other animals.
Basal Protista are the most primitive form of protists. Some basal Protista possess mitochondria, tubular-shaped organelles found in eukaryotic cell cytoplasm, and some do not. Some basal Protista lacking in mitochondria are parasites including Trichomonas, or vaginal parasites, Giardia, or intestinal parasites and the Entamoeba, parasites that cause amoebic dysentery. While most basal Protista containing mitochondria closely resemble bacteria, some are also parasites including Trypanosoma, blood parasites that cause diseases such as African sleeping sickness. Other basal Protista containing mitochondria are Physarum, terrestrial slime mold and Euglena, a green freshwater flagellated protozoan.
Stramenopiles are a type of Protista that includes diatoms, brown and golden algae and water molds. Diatoms are a form of microscopic unicellular algae surrounded by delicate and ornate glasslike walls and live in most aquatic environments. Brown and golden algae include various forms of seaweed such as kelp, which grows in dense underwater forests of up to 100 feet in high. Water molds are similar to fungus, live in wet environments and feed on decaying or living organisms.