All protists require a water-based environment for survival, including fresh or marine water, snow or damp forest areas. Many types of protists, including plankton and algae, are vital for their ecosystems, while some can cause sickness and disease.
There are three major categories of protists: algae, fungus-like protists and protozoa. Protozoa are animal-like protists that ingest their food through mouth-like structures. They eat bacteria and other one-celled organisms. Amoeba are protozoa that live in a freshwater environment, and when ingested they can often cause abdominal distress.
Algae produce their food in a plant-like manner through photosynthesis, and often can become so abundant in the water it can give the water a red-like appearance, as with red tide. The dinoflagellates that are found in red tide are toxic to humans, and can affect the shellfish that live in the area. Chlorophyta are best known as green algae. These are the only group of algae that contain chlorophyll B, so it often through to be an evolutionary ancestor of plants.
Fungi-like protists are called slime molds. They absorb their nutrients from the environment, and they tend to live in decayed wood. The body of the slime mold is simply one giant mass of cytoplasm with an amoeboid movement. They are also bright red or orange in color.