Algae are a diverse group of organisms which are photosynthetic like plants and have unicellular forms, or simpler multicellular forms lacking the organs of land plants. The most complex marine forms are called seaweeds, including kelp. Algae live in water, are at the bottom of the food chain, and thus all life depends on them.
The scientific study of algae is called phycology. Phycologists generally specialize in either freshwater or salt water algae, and either diatoms (with skeletal structures) or soft algae (also called non-diatoms; these include seaweeds).