Algae can nourish you, poison you, or end up in your skincare routine, depending on the type and how you encounter it. The word “algae” covers thousands of species, from the microscopic spirulina in your smoothie to the toxic bloom turning a lake green. Some forms are among the most nutrient-dense foods on the planet. Others produce toxins that damage your liver and nervous system. Here’s what the different types actually do to your body.
Edible Algae as a Nutrient Source
Microalgae like chlorella and spirulina pack an unusual amount of nutrition into a small serving. Chlorella, for example, is roughly 59% protein by dry weight, and its protein quality scores higher than soybean protein on the essential amino acid index. It also delivers substantial iron (around 104 mg per 100 g dry weight) and potassium (around 986 mg per 100 g), two minerals tied to preventing anemia and supporting healthy blood pressure.
Chlorella is also unusually rich in folate, containing more per gram than spinach. In a study of 32 pregnant women, taking 6 grams of chlorella daily for 12 to 18 weeks reduced markers of anemia compared to a control group. It’s one of the few plant-based foods that contains genuinely active vitamin B12, which matters if you’re vegetarian or vegan. However, spirulina is a different story: testing of spirulina supplements found they contain mostly pseudo-B12, a form your body can’t actually use. If you’re relying on algae for B12, chlorella is the more reliable choice.
Omega-3 Fatty Acids Without Fish
Algae are the original source of omega-3 fats in the marine food chain. Fish accumulate these fats by eating algae (or eating smaller fish that ate algae), so going straight to the source skips the middleman. A randomized, double-blind clinical trial of 74 adults found that DHA and EPA from microalgal oil supplements were statistically non-inferior to fish oil in terms of bioavailability in blood plasma after 14 weeks. For people who avoid fish due to allergies, diet preferences, or concerns about ocean sustainability, algal oil provides the same functional benefit.
Seaweed, Iodine, and Your Thyroid
Seaweed is one of the richest natural sources of iodine, which your thyroid gland needs to produce hormones that regulate metabolism. But the iodine content varies wildly depending on the type. Red seaweeds like nori and dulse contain 20 to 200 micrograms of iodine per gram of dry weight. You’d need anywhere from a third of a gram to 3 grams to hit the daily recommended intake of 150 micrograms.
Brown seaweeds like kombu (kelp) are a completely different situation. Kombu contains 2,500 to 10,000 micrograms of iodine per gram of dry weight. That means eating just one-hundredth of a gram could meet your daily needs, and a single gram would blow past the European tolerable upper limit of 600 micrograms by several times over. Too much iodine can suppress or overstimulate the thyroid, so if you eat kombu regularly, portion size matters more than you might expect.
Harmful Algal Blooms and Toxic Exposure
Not all algae are food. Cyanobacteria, sometimes called blue-green algae, thrive in warm, nutrient-rich freshwater and produce a class of toxins called microcystins. These toxins target the liver specifically because liver cells have channels that actively transport microcystins inside. Once there, the toxins interfere with enzymes that regulate cell growth and survival, promoting inflammation and, in animal studies, the development of precancerous lesions in the liver and colon.
You don’t have to drink contaminated water to be exposed. Swimming in or even standing near a bloom can bring you into contact with these toxins through skin, inhalation, or accidental swallowing. Researchers have also found that some cyanobacterial extracts cause neurotoxic and cell-damaging effects even when no known toxin can be identified, suggesting there are harmful compounds in blooms that science hasn’t fully characterized yet.
Red Tide and Respiratory Problems
In saltwater, the alga Karenia brevis produces brevetoxins, the compounds behind what’s commonly called red tide. You can inhale these toxins simply by being near the coast during a bloom, as waves and wind carry them in sea spray. Inhaled brevetoxins cause shortness of breath, can trigger asthma attacks, and in more severe cases lead to bronchitis or pneumonia.
Eating shellfish harvested from red tide zones carries a separate set of risks called neurotoxic shellfish poisoning. Symptoms typically appear within 30 minutes to 3 hours and include nausea, vomiting, diarrhea, dizziness, altered hot and cold sensations, and tingling of the lips, tongue, or throat. Heart rhythm changes and blood pressure swings can also occur. These are acute effects, and current research hasn’t clearly established long-term consequences from single exposures.
Algae in Skincare Products
Algae extracts show up in moisturizers, serums, and wound-healing products for practical reasons. Polysaccharides from microalgae stimulate the production of collagen and elastin, the two structural proteins responsible for skin firmness and elasticity. Compounds from species like Porphyridium cruentum work by blocking an enzyme that breaks collagen down, effectively slowing one of the processes behind visible aging.
Algae-derived ingredients like alginate, fucoidan, and laminarin also function as effective humectants, drawing moisture from the environment and locking it into the skin barrier. This makes them common in formulations targeting dry or aging skin. Alginate, sourced from brown algae, doubles as a wound-healing agent by promoting cell growth and collagen production at injury sites.
Carrageenan and Gut Health Concerns
Carrageenan, extracted from red seaweed, is widely used as a thickener and stabilizer in dairy alternatives, deli meats, and processed foods. Its safety has been debated for years. Lab research shows carrageenan can activate inflammatory immune pathways, shift gut bacteria in unfavorable directions, and degrade the protective mucus layer lining the intestines.
Human evidence is mixed. One randomized crossover trial found no evidence that short-term carrageenan use worsened symptoms in people with ulcerative colitis. But a separate study gave carrageenan to ulcerative colitis patients in remission and found it contributed to earlier disease relapse in up to 50% of cases. For people without existing bowel conditions, the amounts used in food products are very small. But if you have inflammatory bowel disease, it’s worth paying attention to.
Heavy Metal Contamination Risks
Because algae absorb minerals directly from their environment, they can also accumulate arsenic, lead, cadmium, and mercury. Seaweed products and carrageenan have been tested by the FDA, and results from a limited sample showed levels well below the safety thresholds set by international food safety bodies. Still, contamination varies by where the algae was harvested and how it was processed. Choosing products from reputable brands that test for heavy metals reduces your risk, particularly if you consume seaweed or algae supplements regularly rather than occasionally.