What are algae? Compared to other types of plants, the algae seem very simple. They lack such usual plant organs as roots, stems, and leaves. They even lack specialized tissues for carring water and, for the most part, food.
Yet in terms of their reproductive processes and biochemical capabilities, algae are quite complex. Certainly they are among the most successful life forms, having dominated water environments
for more than a billion years. Algae are found in many different forms. Some are one-celled and can be seen only with a microscope. These forms may float in the water, settle to the bottom, or
swim about with flagella. Other algae, made up of great numbers of cells, may be quite large. The giant sea kelps, for example, often grow more than thirty meters tall. 
Through photosynthesis, algae are the main producers of food and oxygen in marine environments. Thus, directly or indirectly, most other water organisms depend on them for life.