|MadSci Network: Microbiology|
Blue-green algae, also known as cyanobacteria, are primitive autotrophic prokaryotes They have something in common with both plants and bacteria. Like algae, they are photosynthetic. Like other bacteria they lack internal organelles bound by membranes. Also like bacteria, they produce a variety of toxins, whose toxicity is a function of dose, or exposure.
Cyanobacteria exist in many environments including soil, marine and freshwater. Freshwater lakes and ponds are the sites of the most dramatic blooms of these organisms and are responsible for the death of livestock (and people)in many parts of the world.
The cause of the blooms can vary. One well established condition that promotes blooms of cyanobacteria is the relative amounts of phosphorus and nitrogen in the water. Both are required for plant (and cyanobacterial) growth. However, there is an important difference between cyanobacteria and algae such as diatoms. Some blue-greens can fix nitrogen from atmospheric N2 gas. This is a biochemical pathway that is limited to prokaryotes, eukaryotic algae lack the enzymes to do it (there are a few that contain prokaryotic endo-symbionts). In general algal growth in freshwater lakes is limited by the the availability of phosphorus, with plenty of available dissolved nitrogen in the form of nitrate or ammonia. This favors the growth of diatoms and other eukaryotic algae that are good competitors for phosphorus. If the relative amount of nitrogen decreases, resulting in a nitrogen limited pond, cyanobacteria will be favored because they can fix nitrogen from the atmosphere. The decrease in dissolved nitrogen can occur after a bloom of diatoms, or other algae. The bloom can result in very large standing stock of cyanobacteria that discolor the water or result in a surface scum. If present, this is a clear sign not to drink or swim in the water (contact with skin can result in itchiness, rash)
Toxicity is a function of exposure and dose, meaning that the more toxin one contacts, or the longer the contact, the sicker one will become. Just how sick depends of which specific toxin is contacted. A common freshwater, bloom forming blue-green, Aphanizomenon flos-aquae, has been found to produce two or three types of toxin, Anatoxin-a, which causes death in experimental animals in 5-30 min from respiratory paralysis, neosaxitoxin, which also causes paralysis, and lipopolysaccharide endotoxins, which cause liver damage. Of course there are other cyanobacteria that produce similar toxins, such as Microcystis aruginosa and others.
Some of the compounds produced by these organisms have been shown to suppress tumor-growth, but it is my understanding that doses required to suppress tumor growth in humans, such as for cancer patients, would kill the patient. Finding out how the compounds suppress tumor-growth may lead to a compound that can suppress tumors without killing the person involved.
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