Blue-green algae are one of the most primitive life forms on this planet. This common name is really something of a misnomer. They are not really algae at all; they are a form of bacteria, more properly known as cyanobacteria. They are very much like other bacteria, except they have a special ability to photosynthesize, like green plants. (Read more about their ecology.)

Where do we find cyanobacteria?
They are in our reservoirs, lakes, ponds, and virtually every aquatic source. No doubt, you've noted their presence at some point in time–especially in warm weather. You may have called them “pond scum.” Their individual cells are very small–like other bacteria. But when conditions are right, their numbers can increase very quickly, resulting in what is technically called a bloom. Within a few days, a lake or pond that was quite clear can become extremely cloudy. The color of cyanobacteria blooms—like some of their other characteristics—can vary widely, ranging from greens and blues to yellows, browns, and reds.

Why do we care about cyanobacteria?
Obviously, their unsightly appearance makes them unappealing for any water supply that is used for recreational purposes. But that's just the tip of the iceberg. They are a major headache for those responsible for ensuring the purity and safety of the drinking water supply. Some forms of cyanobacteria release chemicals that produce bad (musty, earthy) tastes and odor in the water. Treating this taste and odor problem can be both difficult and costly. To make matters worse, some species also release toxins, making the water unfit for consumption. Cyanobacteria pose problems not only with human drinking water but also with water supplies used for agricultural purposes, such as water for livestock.