Algae are primitive, primarily aquatic, one-celled or multicellular plant-like organisms that lack true stems, roots, and leaves but usually contain chlorophyll. There are both marine and freshwater algae, and algae are found almost everywhere on earth. The Washington Department of Ecology's algae program focuses on algae problems in freshwater. Pond scum is algae (mostly), the green slime on rocks in a stream is also algae, as is the pea soup green seen in some nutrient-enriched lakes.