SARGASSUM SEAWEED
Sargassum is a large genus of brown seaweed — around 150 species — found in warm and temperate seas worldwide. Some grow attached to rock; others float free in vast drifting mats, most famously in the Sargasso Sea, which is named after them.
Several species are eaten across East and Southeast Asia. One of them, hijiki, carries a genuine safety warning that's worth knowing before anything else.
Hijiki and Arsenic
Sargassum fusiforme — hijiki — accumulates high levels of inorganic arsenic, considerably more than other edible seaweeds. The UK Food Standards Agency advises against eating it, as do the food safety authorities of Canada, Australia, New Zealand and several other countries.
This isn't a trace-contamination issue that better sourcing solves. The seaweed concentrates arsenic naturally. If you're buying Asian seaweed products, check whether hijiki is what you have — it's sold widely and the warning isn't always displayed.
Other Sargassum species don't share this problem to the same degree, but the genus is worth approaching with more care than most seaweeds.
What It Looks Like
Sargassum grows in bushy, branching fronds with small leaf-like blades and distinctive round gas bladders that keep it buoyant — olive-brown to golden, often a metre or more long.
The free-floating species are unusual in completing their entire life cycle without ever attaching to the seabed, drifting as living rafts that support their own ecosystem of fish, crabs and turtles.
What It Tastes Like
Savoury with a nutty edge and some bitterness, particularly in older growth. It's higher in glutamic acid than many seaweeds, which is where the umami comes from. Young fronds are milder; mature ones need blanching to soften both texture and flavour.
How It's Used
- Blanched and dressed — young fronds, briefly boiled and served with vinegar, sesame or citrus.
- Simmered with vegetables — cooked slowly with carrot, soy and mirin, the standard Japanese preparation for this family.
- Dried as seasoning — crumbled over rice or noodles.
- In soups — added to broths for savoury depth.
Sargassum in Britain
Wireweed (Sargassum muticum) arrived in British waters in the 1970s, probably with imported oysters, and has spread widely along the south and west coasts. It's an invasive species that outcompetes native seaweeds and fouls harbours and propellers.
It is edible, and young growth is used in East Asian cooking, so eating it is at least a harmless response to it. Older fronds are tough and not worth the effort.
The Atlantic Sargassum Belt
Since 2011 enormous mats of Sargassum have formed across the tropical Atlantic, drifting into the Caribbean and the coast of West Africa. These blooms now stretch thousands of miles and are increasingly seen as an ecological and economic problem — smothering beaches, releasing hydrogen sulphide as they rot, and damaging tourism and fisheries.
There's considerable interest in whether this biomass can be usefully harvested, for fertiliser, biofuel or building materials. Whether beach-cast Sargassum is safe for any of those uses depends heavily on what it has absorbed while drifting.
Nutrition
Sargassum contains dietary fibre, protein, glutamic acid, iodine and a range of minerals. Iodine and arsenic content both vary significantly by species and location.
Related
Sargassum sits with the brown seaweeds alongside sugar kelp and sea spaghetti. See our guide to whether seaweed can be poisonous, or the full guide to edible seaweed.
Seaweed has been a source of savoury flavour for centuries. Sozyë's sauces are brewed from a blend of wild Scottish seaweeds, harvested off the north coast of Scotland.