Differences between groups are due to diversity in photosynthetic pigments, carbohydrate food reserves, number and position of flagella, life cycle patterns and cell wall characteristics. One group contains diatoms , which create silicate shells found in diatomaceous earth. Green algae are diverse, and are presumed to be the ancestors of green plants. Brown algae, include forms which are very important both ecologically and economically. Extracts are used in food products, cosmetics, animal feeds, fertilizer, et al.
Protozoans are heterotrophic protists.
This assemblage includes a variety of taxa. Fungus-like protists include slime molds and water molds. One type of water mold is the causative agent of the Irish potato famine of 1845, during which about 400,000 people died. In the late 19th Century, the Germ Theory of Disease was formulated. At that time, the German physician, Robert Koch , established a set of rules which could be used to test if a disease could be attributed to a microorganism.