Seaweed is built primarily from complex carbohydrates, with smaller amounts of protein, minerals absorbed from seawater, photosynthetic pigments, and water. Unlike land plants, seaweed has no roots, stems, or leaves. Its entire body is a relatively simple structure called a thallus, held together by unique gel-like polysaccharides that you won’t find in any terrestrial plant.
Three Groups, Three Different Builds
All seaweed falls into one of three color groups: red, brown, and green. These aren’t just cosmetic differences. Each group has a distinct chemical makeup, from the pigments that give it color to the structural fibers that hold its cells together.
Green seaweeds contain the same chlorophyll pigments found in land plants (chlorophyll-a and chlorophyll-b), which is why they look familiar. They tend to grow in shallow, sunlit water. Brown seaweeds get their color from a pigment called fucoxanthin, which is so dominant it masks the chlorophyll underneath. Red seaweeds contain pigments called phycoerythrin and phycocyanin, which absorb blue light efficiently enough that some red species thrive more than 25 meters below the surface where sunlight barely reaches.
Cell Walls: The Gel That Holds It Together
The most distinctive thing about seaweed’s composition is its cell walls. Land plants rely heavily on cellulose for structural support. Seaweed has some cellulose too, but in surprisingly small amounts. In brown seaweeds, cellulose accounts for only 1 to 8 percent of total dry weight. The real structural work is done by specialized gel-forming polysaccharides, and each color group makes its own kind.
Brown seaweeds produce alginate and fucoidan. Alginate is a long-chain sugar that absorbs water and forms a viscous gum, which is why brown seaweeds feel slimy. It’s extracted commercially for use in food thickeners, textile printing, and pharmaceuticals. Red seaweeds produce agar and carrageenan, both gel-forming sugars built from galactose units. Carrageenan contains 15 to 40 percent sulfate groups, giving it a strong negative charge that helps it bind water and form thick gels. You’ll find it in ice cream, plant milks, and processed foods as a stabilizer. Green seaweeds produce a polysaccharide called ulvan, built from a mix of sugars including rhamnose and glucuronic acid.
These cell wall gels are what make seaweed flexible enough to survive constant wave action without snapping. They’re also the reason seaweed has such a different texture from any land vegetable.
Protein Content Varies Widely
Seaweed’s protein levels depend enormously on the species. Red seaweeds are the protein standouts. Nori, the dried sheets used in sushi, contains roughly 32 percent protein by dry weight. Dulse, another red seaweed popular in Northern Europe, comes in around 19 percent. Brown seaweeds carry much less: Irish wakame has about 10 percent protein, and sea spaghetti drops to around 5.5 percent. Red seaweeds also contain higher levels of most individual amino acids compared to brown species.
A Mineral Sponge
Because seaweed sits in mineral-rich seawater its entire life, it accumulates minerals at concentrations far higher than most land vegetables. Calcium, magnesium, iron, manganese, and copper all concentrate in seaweed tissue. Some species of red algae have been measured with calcium levels above 21,000 milligrams per 100 grams of dry material and magnesium above 2,000 milligrams per 100 grams.
Iodine is the mineral seaweed is best known for, and the range between species is enormous. Kombu, a brown kelp used in Japanese cooking, contains between 2,100 and 4,300 micrograms of iodine per gram of dried seaweed. That’s hundreds of times the daily recommended intake in a single gram. Wakame is more moderate at 220 to 280 micrograms per gram. Nori is the lowest of the commonly eaten seaweeds, at roughly 9 to 20 micrograms per gram.
Vitamins and Bioactive Compounds
Nori is one of the few non-animal foods that contains true, bioavailable vitamin B12 rather than inactive lookalikes. A clinical trial in vegetarians found that nori consumption lowered homocysteine, a blood marker that rises when B12 is deficient, confirming that the body can actually use the B12 in nori. This doesn’t apply to all seaweeds. Spirulina and wakame contain B12 analogs that the body can’t use and may even interfere with real B12 absorption.
Brown seaweeds also produce a class of polyphenol compounds called phlorotannins, which are unique to marine algae and have no equivalent in land plants. These compounds show antioxidant, antibacterial, and antiviral properties in lab studies. More broadly, because seawater is rich in halogens like bromine, chlorine, and iodine, seaweed produces halogen-containing organic compounds that land plants simply don’t make. This gives marine algae a chemical profile unlike anything grown on land.
Energy Storage
Just as land plants store energy as starch, seaweeds store energy in their own ways depending on the group. Green seaweeds store starch, similar to land plants. Red seaweeds store a modified form called floridean starch, built from glucose units. Brown seaweeds take a different approach entirely, storing energy as laminarin, a polysaccharide made from glucose and a sugar alcohol called mannitol. These storage carbohydrates fluctuate with the seasons, peaking after periods of strong sunlight.
Heavy Metals and Contaminants
The same ability to absorb minerals from seawater means seaweed also picks up heavy metals. Cadmium, lead, mercury, and arsenic have all been measured in edible seaweed species. Total arsenic levels are a particular concern. In a study of seaweeds from the Salish Sea in the Pacific Northwest, every sample tested exceeded the screening level for inorganic arsenic, though the total arsenic measured includes both harmful inorganic forms and less toxic organic forms. Without testing the specific type of arsenic present, the actual health risk is hard to pin down. Mercury levels in the same study fell below safety thresholds across all samples.
The practical takeaway is that seaweed composition isn’t just about nutrients. Where it grows and what’s dissolved in that water directly shapes what ends up in the tissue you eat. Species, geography, and water quality all matter.