Blue-green algae are actually a type of bacteria that is known as cyanobacteria. In their aquatic habitat, cyanobacteria are equipped to use the sun's energy to manufacture their own food through photosynthesis . The moniker blue-green algae came about because of the color, which was a by-product of the photosynthetic activity of the microbes, and their discovery as a algal-like scum on the surface of ponds. They were assumed to be algae until their identity as bacteria was determined.
Although the recognition of the bacterial nature of the microbe occurred recently, cyanobacteria are ancient. Fossils of cyanobacteria have been found that date back 3.5 billion years and are among the oldest fossils of any life from thus far discovered on Earth. These microorganisms must have developed very early following the establishment of land on Earth, because the oldest known rocks are only slightly older at 3.8 billion years.
Modern day examples of cyanobacteria include Nostoc, Oscillatoria, Spirulina, Microcystis, and Anabaena
Cyanobacteria were monumentally important in shaping life on this planet. The oxygen atmosphere that supports human and other life was generated by cyanobacterial activity in the distant past. Many oil deposits that were laid down in the Proterozoic Era were due the activity of cyanobacteria. Another huge contribution of cyanobacteria is their role in the genesis of plants. The plant organelle known as a chloroplast , which the plant uses to manufacture food, is a remnant of a cyanobacterium that took up residence in a eukaryotic cell sometime in the Proterozoic or early Cambrian Era. The mitochondrion in eukaryotic cells also arose in this fashion.
The ability of cyanobacteria to photosynthetically utilize sunlight as an energy source is due to a pigment called phycocyanin. The microbes also contain the same chlorophyll a compound used by plants. Some blue-green algae possess a different photosynthetic pigment, which is known as phycoerythrin. This pigment imparts a red or pink color to the cells. An example is Spirulina. The pink color of African flamingos actually results, in part, from their ingestion of Spirulina.
Cyanobacteria tend to proliferate in very slow moving or still fresh water. Large populations can result very quickly, given the appropriate conditions of temperature and nutrient availability. This explosive growth is popularly referred to as a bloom. Accounts of blooms attributable to cyanobacteria date back to the twelfth century. The toxic capabilities of the organism have been known for over 100 years. Some species produce a toxin that can be released into the water upon the death of the microorganism. One of the cyanobacterial toxins is damaging to the liver, and so is designated a hepatotoxin. Another cyanobacterial toxin is damaging to cells of the nervous system, and so is a neurotoxin. Still other cyanobacterial toxins cause skin irritation.
A toxin of particular note is called microcystin. This toxin is produced by Microcystis aeruginosa. The microcystin toxin is the most common in water, likely because of its stability in this environment. One type of microcystin, which is designated microcystin-LR, is found in waters all over the world, and is a common cause of cyanobacterial poisoning of humans and animals.
At low levels, toxins such as microcystin produce more of an uncomfortable feeling than actual damage to the body. However, blue-green algae and their toxins can become concentrated in shallow, slow-moving bodies of water or in fish. Ingestion of the fish or accidental swallowing of the water while swimming can produce nausea, vomiting fever, and diarrhea. Eyes can also become irritated. These symptoms can be more exacerbated in children, because the toxin-to-body-weight ratio is higher in children than in adults. Liver damage can result in children exposed to the toxins.
In contrast to many other toxins, the cyanobacterial toxins can still remain potent after toxin-contaminated water has been boiled. Only the complete removal of the toxin from the water is an assurance of safety. Some success in the removal of toxins has been claimed by the use of charcoal and by techniques that oxidize the water.
Cyanobacteria are one of the few microorganisms that can convert inert atmospheric nitrogen into a usable form, such as nitrate or ammonia. For example, the cyanobacterium Anabaena co-exist with a type of fern called Azolla, where it supplies nitrogen to the plant. The production of rice has benefited from the fertilization capability of this bacterial-plant association. The cyanobacterium Spirulina is a popular, high protein food source.
See also Fossilization of bacteria; Photosynthetic microorganisms