Cyanobacteria are microscopic organisms that live in water and use sunlight to produce energy, much like plants do. Often called “blue-green algae,” they aren’t actually algae at all. They’re bacteria, and they happen to be among the oldest life forms on Earth, with a fossil record stretching back roughly 3.5 billion years. That long history matters because cyanobacteria fundamentally changed the planet’s atmosphere, making complex life possible.
How Cyanobacteria Work
Like plants, cyanobacteria perform photosynthesis. They absorb sunlight, pull in carbon dioxide, and release oxygen. They do this using chlorophyll-a, the same green pigment found in leaves. But they also carry extra pigments, blue and red ones, that allow them to capture light energy even in dim conditions. Those blue pigments are what give many species their characteristic blue-green color and inspired the old “blue-green algae” nickname.
Despite the name, cyanobacteria are prokaryotes, meaning their cells lack the internal compartments (like a nucleus) found in true algae and plant cells. They’re structurally much simpler. Over 5,000 species have been identified so far, ranging from single-celled forms to long filaments and colonies visible to the naked eye. They thrive in freshwater lakes, oceans, hot springs, moist soil, and even on rocks in deserts.
The Organisms That Made Earth Breathable
Early Earth had almost no free oxygen in its atmosphere. Around 2.5 to 2.3 billion years ago, cyanobacteria changed that during what scientists call the Great Oxidation Event. By photosynthesizing in enormous quantities, these microbes pumped oxygen into the atmosphere for the first time at scale. This was the single most important atmospheric shift in Earth’s history, because it created the oxygen-rich environment that animals, fungi, and complex plants would eventually need to evolve.
What drove this change wasn’t just the existence of cyanobacteria but their transition to multicellular forms. Multicellular cyanobacteria could move within bacterial mats to find better light, attach more effectively to surfaces, and ultimately grow more abundantly. Greater abundance meant more photosynthesis and more oxygen output. The stromatolites, layered rock structures formed by ancient cyanobacterial mats, are among the oldest evidence of life on Earth.
Nitrogen Fixation and Ecosystem Support
Beyond producing oxygen, certain cyanobacteria perform another trick that few organisms can: they convert atmospheric nitrogen gas into ammonia, a form of nitrogen that other organisms can actually use. This process, called nitrogen fixation, is critical because nitrogen is often the nutrient that limits growth in lakes and oceans. By supplying “new” nitrogen to these ecosystems, cyanobacteria support the base of aquatic food webs.
There’s an inherent contradiction in this process. Nitrogen fixation requires an oxygen-free environment, yet cyanobacteria produce oxygen through photosynthesis. Over billions of years, they’ve evolved clever workarounds. Some species fix nitrogen only at night when photosynthesis shuts off. Others form colonies where interior cells are shielded from oxygen. Perhaps the most sophisticated adaptation is the heterocyst, a specialized cell within a filament that doesn’t photosynthesize at all. It stays oxygen-free internally, handling nitrogen fixation while neighboring cells supply it with energy from photosynthesis.
Spirulina and Other Beneficial Uses
Not all cyanobacteria are harmful. Spirulina, the popular dietary supplement, is actually dried cyanobacteria (specifically, a species called Arthrospira platensis). It contains up to 70% protein by weight, along with B vitamins (including B12), beta-carotene, and iron. It’s sold in tablets and powder form in health food stores worldwide and has a long history as a food source in parts of Africa and Central America. Research has documented its use as a protein and vitamin supplement without significant side effects.
When Cyanobacteria Become Dangerous
Under the right conditions, cyanobacteria can multiply explosively into dense populations called blooms. These blooms typically occur during warm summer months in nutrient-rich water bodies. The water can turn dark green and look like pea soup or spilled paint, though blooms can also appear white, brown, red, or purple. The real concern is that many bloom-forming species produce potent toxins.
Three major groups of cyanobacterial toxins cause the most problems. The first, microcystins, primarily attack the liver. They can also damage the kidneys, gastrointestinal tract, and nervous system. The second, cylindrospermopsins, are broadly toxic to cells throughout the body. They work by blocking protein production and damaging DNA. The third, anatoxins, target the nervous system. They cross the blood-brain barrier and interfere with nerve-to-muscle communication, affecting the brain, muscles, respiratory tract, and cardiovascular system.
What Triggers Harmful Blooms
Blooms don’t appear randomly. They need three things: warm water, excess nutrients, and slow-moving or stagnant conditions. Nitrogen and phosphorus are the key nutrients, typically entering waterways through agricultural runoff, sewage, and stormwater. Research shows that both nitrogen and phosphorus reductions are needed to meaningfully control blooms over the long term.
The form of nitrogen matters too. Cyanobacteria show a strong preference for ammonium over nitrate, and water bodies with high ammonium levels tend to see cyanobacteria outcompete other types of algae like diatoms. When the ratio of nitrogen to phosphorus drops below about 15:1, cyanobacteria gain a competitive advantage because many species can fix their own nitrogen from the atmosphere, something their algal competitors cannot do. Droughts make things worse by increasing water temperatures, reducing flow, and concentrating nutrients.
Symptoms of Exposure
You can be exposed to cyanobacterial toxins by getting contaminated water on your skin, breathing in tiny droplets near a bloom, or swallowing the water. Skin contact and inhalation typically cause rashes, eye irritation, sore throat, nasal irritation, and coughing. Swallowing toxin-contaminated water is more serious and can cause stomach pain, vomiting, diarrhea, headaches, muscle weakness, dizziness, and liver damage.
If the water in a lake or pond looks like green paint, has visible scum on the surface, or smells musty, it’s best to stay out entirely and keep children away from the shoreline.
Risks to Dogs and Other Animals
Cyanobacterial toxins are particularly dangerous for animals. Dogs are at high risk because they drink lake water, wade through scum, and then lick contaminated water off their fur. The toxicity profile in dogs is unusually steep: a dose just below the lethal threshold may produce no visible symptoms at all, but a slightly larger amount can be fatal. Animals can become severely ill or die within minutes to days of swallowing bloom-contaminated water, and there are no known antidotes to these toxins.
Livestock, birds, fish, and marine mammals are also vulnerable. In saltwater environments, the first visible sign of a harmful bloom is often dead fish and wildlife washing ashore. If you see a bloom or notice dead animals near a water body, keep pets leashed and well away from the water’s edge.
Safety Limits for Water
Health agencies have set specific limits for microcystin, the most commonly detected cyanobacterial toxin, in both drinking and recreational water. The World Health Organization sets the drinking water guideline at 1 microgram per liter. The U.S. EPA uses a stricter threshold of 0.3 micrograms per liter for young children and 1.6 micrograms per liter for adults. For recreational water, the WHO considers concentrations between 2 and 4 micrograms per liter a low-risk level, while anything above 20 micrograms per liter represents a high probability of health effects. Many state health departments monitor popular swimming areas during summer and post advisories when toxin levels exceed these thresholds.