First things first, blue-green algae are not algae at all; they are bacteria capable of photosynthesis. A commonly used term for blue-green algae is cyanobacteria, and while it's technically a more accurate name, we'll use the term blue-green algae in this article.
Billions of years ago, early blue-green algae formed dense mats in shallow seas and generated oxygen as a by-product of photosynthesis. This ultimately changed the atmosphere of the planet, allowing the evolution of oxygen-breathing organisms. Fossilized mounds of blue-green algae (called stromatolites) are quite common and can be found in Missouri. Today, there are about 3,000 species of blue-green algae on the planet, and they can be found nearly everywhere. Depending on the species, they can grow in water, on land and symbiotically inside and on other organisms. Some blue-green algae are even eaten by humans (e.g. Spirulina).
Stromatolites (photo: Michael C. Rygel via Wikimedia Commons)
Many blue-green algae are capable of pulling nitrogen from the atmosphere and making it biologically available, a process called nitrogen fixation. One species of nitrogen-fixing blue-green algae lives symbiotically with the aquatic plant Azolla. The Azolla plant gets the benefit of "free" nitrogen and the blue-green algae get carbon from the Azolla. This relationship is put to good use by rice farmers, who for thousands of years have cultivated Azolla (and the associated blue-green algae) to fertilize their paddies. Azolla is commonly found in Missouri.
Many blue-green algae have the ability to move up or down in the water column. The benefit is that the cells can put themselves where they are likely to grow fastest. This is accomplished by the presence of gas vacuoles, or cavities, in the blue-green cells. While in the sunlight and photosynthesizing, all algae (and plants) are producing carbohydrates for respiration. As this happens, the pressure within the cell increases and the gas vacuoles collapse. Then the cells become more dense and start to sink toward the bottom of the lake. In the deeper, darker water the cells consume oxygen to break apart carbohydrate molecules for energy and release carbon dioxide and water (a process called respiration). As the carbohydrates are used up, pressure in the cell is reduced and the vacuoles fill up with gas, bringing the blue-green algae toward the surface again. Massive numbers of blue-green algae rising to the surface at once will lead to a noticeable bloom (as shown in the photo below).
Another interesting ability of some blue-green algae species is the ability to produce toxins, a characteristic that has captured the attention of agricultural, environmental and human health groups across the world. While human fatalities are rare, livestock and wildlife are more commonly affected. Depending on the species of blue-green algae, toxins produced can affect the liver, nervous system or skin. Humans typically experience gastrointestinal problems, nausea or skin irritation when exposed to blue-green algae toxins. Not all species produce toxins, and those that can produce toxins only do so under certain environmental conditions. Because a laboratory test is required to check for the presence of cyanotoxins, it's best to stay out of any waterbody that is experiencing an algae bloom.
The toxin-producing blue-green algae tend to form colonies that are somewhat large (see illustration below). When large colonies are present the probability is high that toxins will also be present. One simple test involves passing water through a 64 µm mesh net. Small individual cells will pass through the net while larger colonies and aggregated cells will be caught in the mesh. A University of Missouri study showed that in lakes where blue-green colonies larger than 64 µm were collected, 98% had detectable microcystins (a common blue-green algal toxin). The study found that microcystin concentrations were generally low across Missouri, but could occasionally cause concern. The World Health Organization recommends that drinking water have less than 1 µg/L of microcystin-LR, the most common blue-green toxin. When detected, maximum concentrations in the Ozarks were 0.05 µg/L, 0.2 µg/L in Missouri's western plains and 2.9 µg/L in its northern plains.
The toxins are released as the cells break apart, or lyse. In-lake chemical treatments will reduce the number of blue-green algae cells, but could ultimately increase the amount of toxin in the water by destroying the cells. Most drinking water filtration plants can remove the toxins at the water plant using activated charcoal. Boiling and other home treatment methods don't work and may actually increase the water's toxicity by breaking apart the blue-green algae cells.