Seaweed is not one kind of living thing but several unrelated ones
Red, brown and green seaweeds look like cousins, yet they share no common many-celled ancestor. What unites them is habitat: salt water, light and something to cling to. Humans farmed nearly 36 million tonnes of them in 2019, and the products end up in sushi, toothpaste, wound dressings and even Danish roofs.
The word seaweed has no formal definition. It usually covers large marine algae visible to the naked eye, sometimes stretching to submerged flowering plants like eelgrass, and the red, green and brown groups form a polyphyletic collection with separate origins. Scientists still argue over the count, though several thousand species is the likely figure. A typical seaweed has a leaf-like blade, a stem-like stipe that is sometimes missing, a holdfast gripping the rock with finger-like haptera, and in some species gas-filled bladders that keep it afloat.
Most seaweeds live near shore on rocky coasts, since they need seawater, enough light for photosynthesis and an anchor point. A few, including Sargassum, float free. Red algae hold the depth record, while rock-pool species endure swings in temperature and salinity and occasional drying out. Shed fronds feed seafloor animals, and drifting pieces carried by currents are eaten by creatures several hundred metres down. Kelp forests shelter young fish, but off California a wasting disease in urchin predators let purple urchins multiply and strip huge areas of kelp.
Of the 35.8 million tonnes produced in 2019, over 97 percent came from Asia. Dried Porphyra becomes nori in Japan, gim in Korea and laverbread in Wales, and in northern Belize seaweed is blended with milk, nutmeg, cinnamon and vanilla into a sweet drink. Agar, alginate and carrageenan thicken sauces and desserts; agar also serves as a culture medium in microbiology, and alginates go into dental moulds. On the Danish island of Læsø, houses were traditionally roofed with seaweed.
Seaweeds line roughly a third of the world's coastlines, and whether their carbon stays locked away long enough to count as blue carbon is still debated. A plan to farm seaweed in the open ocean and sink it has drawn pushback, since open water lacks iron and dumping biomass could disrupt deep-sea life. The Mediterranean, after the Suez Canal opened, now records over 120 introduced seaweed species, more than anywhere else.
Source: Seaweed
