Nostoc is not acutely dangerous to most people, but it does carry real risks depending on the species, the context, and how you’re exposed. Some species of Nostoc produce toxins linked to liver damage and, more controversially, neurodegenerative disease. Others have been safely eaten for centuries in parts of Asia. The difference between harmless and harmful comes down to which Nostoc you’re dealing with and whether it’s contaminating your water, growing on your walkway, or sitting on your plate.
What Nostoc Actually Is
Nostoc is a genus of cyanobacteria, sometimes called blue-green algae. It forms dark green, jelly-like blobs that appear on lawns, sidewalks, and garden beds, especially after rain. You’ll also find it in lakes, rivers, and damp soil. It’s one of the most common cyanobacteria on earth, showing up in environments from Arctic tundra to tropical forests. Nostoc is also the most common cyanobacterial symbiont found living inside lichens.
Despite its alien appearance, Nostoc has been part of human food cultures for a long time. At least five species of Nostoc are traditionally eaten, including Nostoc commune, Nostoc flagelliforme, Nostoc sphaeroides, Nostoc sphaericum, and Nostoc verrucosum. Cyanolichen species containing Nostoc are still used in traditional medicine and eaten by humans in parts of Asia. But “traditionally eaten” and “completely safe” aren’t the same thing.
Toxins Nostoc Can Produce
The two main classes of toxins associated with Nostoc are microcystins (which damage the liver) and BMAA (a neurotoxin). Not every Nostoc species produces these, and production levels vary dramatically between strains and environments, but both pose genuine health concerns.
Microcystins and Liver Damage
Microcystins are small cyclic peptides that powerfully inhibit enzymes your liver cells need to function. They are potent hepatotoxins, meaning they specifically damage liver tissue. A Nostoc strain isolated from the Nile River in Egypt was found to produce a particularly toxic variant called demethylated microcystin-LR. In animal studies, extracts from this strain caused severe liver damage: collapse of cellular structure, inflammation, fat accumulation in liver tissue, and death of liver cells.
Nostoc strains living inside lichens can produce microcystins at concentrations ranging from trace amounts up to 0.2 mg per gram of lichen tissue. In lab cultures, isolated Nostoc strains from lichens produced between 0.2 and 5 mg per gram. Microcystins are also suspected to act as tumor promoters, meaning chronic low-level exposure could increase cancer risk over time.
BMAA and Neurodegenerative Disease
BMAA is a non-protein amino acid first discovered in cycad seeds, where it’s produced by Nostoc bacteria living in the plant’s roots. In laboratory settings, BMAA kills specific populations of motor neurons by overstimulating receptors in the brain. It exists in both a free form and a form bound to proteins, and this protein-bound form may create a slow-release reservoir of the toxin inside the body, leaking out over years.
The most well-known case linking BMAA to human disease involves the Chamorro people of Guam, who historically had extremely high rates of a condition resembling both ALS and Parkinson’s disease. BMAA was found in the brain tissue of Chamorro patients who died of this condition but not in patients who died of unrelated causes. It was also detected in the brains of nine Canadian Alzheimer’s patients, while absent in patients without neurodegenerative disease. People living near lakes contaminated with cyanobacteria have been reported to face higher ALS risk.
That said, the BMAA hypothesis remains debated. A review in The Lancet Neurology noted that animal experiments using large daily doses of BMAA produced weakness but no clear evidence of the kind of progressive motor neuron death seen in ALS. Some researchers argue that even if BMAA is present in brain tissue at low levels, it may not cause harm. The link is plausible and concerning, but not conclusively proven.
Risks From Contaminated Water
For most people, the likeliest route of harmful exposure isn’t eating Nostoc deliberately. It’s drinking or swimming in water where Nostoc or other cyanobacteria have released toxins. The WHO has set a drinking water guideline of 1.0 micrograms per liter for microcystin-LR, the most common and toxic microcystin variant. For recreational water (swimming, kayaking), the limit is 24 micrograms per liter.
The Nile River study is a pointed example. Researchers found a toxin-producing Nostoc strain in the river that serves as Egypt’s main water supply, and concluded that people exposed to water contaminated above WHO guidelines could face serious health problems. Exposure can happen through drinking water directly, through skin contact during recreation, or through eating fish and shellfish that have accumulated the toxins.
Standard municipal water treatment generally removes cyanotoxins effectively, so tap water in countries with modern treatment systems poses low risk. The concern is greater in regions relying on untreated surface water, or during large cyanobacterial blooms that overwhelm treatment capacity.
Is It Safe to Eat?
It depends on the species. Safety evaluations of Nostoc flagelliforme found no adverse effects in toxicity studies, making it one of the better-documented safe options. Nostoc commune, on the other hand, carries a known risk of BMAA production. Other edible species like Nostoc sphaericum have potential antinutritional factors that haven’t been fully studied yet.
If you’re encountering Nostoc in your yard or on a sidewalk, eating it is not advisable. You have no way of knowing the species, let alone whether that particular strain produces toxins. Wild-harvested Nostoc hasn’t been tested, and microcystin content can vary enormously even within the same species depending on growing conditions. The Nostoc sold as food in Asian markets comes from known species with some history of safe use, which is a very different situation from the green blobs in your garden.
The Slipping Hazard on Your Property
Beyond toxins, Nostoc poses a surprisingly practical danger. The colonies are extremely slippery when wet. Michigan State University Extension has flagged Nostoc as a hazard in commercial nurseries and on paved surfaces. Its old folk name “witch’s butter” comes directly from this slick texture. If Nostoc is colonizing your walkways, patio, or driveway, the slip-and-fall risk during rainy weather is real and worth addressing, particularly for older adults. Improving drainage and reducing soil compaction in the area typically discourages Nostoc growth, since it thrives in moist, poorly drained spots.
Putting the Risk in Perspective
Casual contact with Nostoc on your lawn is unlikely to harm you. You’re not going to get sick from touching it or pulling it off your sidewalk. The meaningful risks are chronic: drinking contaminated water over time, regularly eating species that produce toxins, or long-term environmental exposure in areas with heavy cyanobacterial growth in water sources. For the average person finding green jelly in their yard, the biggest realistic concern is slipping on it, not being poisoned by it.