Simple, green, aquatic plants without stems, roots or leaves. They are among the microscopic organisms that form the start of the food chain. Algae are found floating in the sea and fresh water, but they also grow on the surface of damp walls, rocks, the bark of trees and on soil. They contain chlorophyll and other pigments that let them grow by photosynthesis. On land, algae can be useful in improving the fertility of soil by nitrogen fixation.