Microscopic plant-like organisms that contain chlorophyll. Algae are nourished by sunlight (carrying out photosynthesis). They are introduced by rain and wind and grow in colonies, producing nuisance masses. These organisms include green, blue-green or black, brown, and yellow-green (mustard) algae. Pink/red colored algae-like organisms exist but are bacteria and not algae. There are 21,000 known species of algae.