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What Are Cyanobacteria?

Cyanobacteria, also known as blue-green algae, are bacteria which have some of the characteristics of plants. They are found throughout the world on land and in lakes, rivers, and ponds, and in estuaries and seawater (oceans).  Cyanobacteria have an extensive fossil record - the oldest known fossils are found in rocks of Western Australia, dated 3.5 billion years old. Many unicellular cyanobacteria grow in colonies that are often surrounded by a gelatinous or mucilaginous sheath, whereas others grow as thread-like filaments. Morphologies in the group have remained much the same for billions of years. 

Cyanobacteria were previously grouped with algae but are now classified as bacteria after analysis of cell structure and cell division. They differ from other bacteria in that they contain photosynthetic pigments similar to those found in algae and plants. Although they are predominantly photosynthetic (light-dependent) organisms, they are also capable of a using organic compounds as a source of energy. Some cyanobacteria have a specialized structure called a heterocyst that  can fix molecular nitrogen. The ability to fix nitrogen gives these species a competitive advantage over other algae. Many cyanobacteria have gas vacuoles that allow them to remain in suspension and migrate to surface waters where there is plenty of light for photosynthesis. On the surface, colonies may clump together and form a scum which can cause water quality problems in lakes.

What is a cyanobacterial bloom?

A mass of algae in a body of water is called a bloom. Blooms are often found in standing water in lakes, ponds, ditches, lagoons, or embayments of rivers. Because many cyanobacteria species can regulate their buoyancy, they rise to the surface of the water and form a surface scum. A scum is a thin oily-looking film that can become several inches thick. When conditions are good for a bloom, a lake or pond can change from clear to turbid within just a few days.

Most cyanobacterial blooms occur during warm summer months. However, toxic blooms can also occur during the colder winter months. American Lake in Pierce County, Washington, has a history of toxic episodes during the winter at low water temperatures (7-8^oC).  It is possible that a bloom can be found somewhere in Washington nearly any month of the year and at low water temperatures.

What causes a bloom? 

Factors needed for bloom formation - whether toxic or not - are complex.  No individual environmental cause or particular set of conditions clearly controls cyanobacterial bloom formation. Researchers have investigated factors such as light, temperature, percent oxygen saturation, nutrient availability and depletion, wind patterns, internal lake mixing, growth stage and zooplankton predation. 

Three genera of cyanobacteria account for the vast majority of blooms:  Microcystis, Anabaena, and Aphanizomenon. A bloom can consist of one or a mixture of two or more genera of cyanobacteria.

Cyanobacteria cannot maintain an abnormally high population for long and will rapidly die and disappear after 1-2 weeks. If conditions remain favorable, another bloom can replace the previous one in such a way that it may appear as if one continuous bloom occurs for up to several months.

How do I know if a bloom is toxic?

Not all cyanobacterial blooms are toxic. Even blooms caused by known toxin producers may not produce toxins or may produce toxins at undetectable levels. Since cyanobacterial toxins can be lethal to animals in relatively small amounts, caution should always be taken when a bloom occurs. As cells die, toxins are released into surrounding waters. Some toxins, such as microcystins, are very stable and can remain in the water for days or weeks after the bloom has disappeared. Scientists do not know what triggers toxin production by cyanobacteria.