Definition

• Algae -- the plural form of alga -- belong to the diverse phylogenic kingdom of Protista. In general, protists share some, but not all, characteristics with the plant and animal kingdoms. Like plants, algae contain chlorophyll, have cells strengthened by cell walls and produce oxygen. They may be either single-celled or multicelled. But unlike true plants, algae do not possess root systems, leaves or seeds. Instead, algae often branch together in floating colonies and reproduce by spores or by fragmentation, in which pieces of an adult alga break off to form a new organism.
  
  

Types

• Up to 30,000 species of algae are said to exist worldwide. Algae fall into eight main phylogenetic groups, but are most commonly characterized by color. Red algae have a red or purple tinge due to pigments known as phycobilins. Most marine seaweed is considered red algae -- though some varieties are subcategorized as brown algae. Blue-green algae are single-celled aquatic bacteria that can actually be red, brown or yellow. Many people are most familiar with green algae, which cause trouble in swimming pools, ponds and fish tanks.
  
  

Accumulation

• Algae typically accumulate for two reasons: colonization and bloom. With colonization, algae cells -- usually green -- group together to form colonies that assist in reproduction. As the cells clump together, the algae can appear as one large, floating plant. Algae bloom occurs when ideal growing conditions enable numerous algae to be produced. More algae being created also means more algae dying off. When algae decay, they remove oxygen from the water and, in the case of blue-green algae, can release sometimes deadly nerve toxins. This can pose a threat to fish, farm animals and humans.
  
  

Growing Conditions

• Algae are an important source of oxygen and food, but they can also be an inconvenience if allowed to overaccumulate. Like land plants, algae require sunlight for photosynthesis and nutrients like nitrogen and phosphorous for growth. It is natural for algae growth to increase during the summer. But in certain conditions, such as near a residential area or farm, fertilizer runoff can lead to an unnatural boom in algae growth. Some forms of green algae can be physically removed with a skimmer or rake. Blue-green algae are difficult to remove because they do not clump together.