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Sea lettuce

life science Maturity 11-13 Next article →

Sea lettuce is a group of edible green algae belonging to the genus Ulva. These organisms are found widely along the coasts of oceans across the entire world. The name for the genus comes from a type of species called Ulva lactuca. In Latin, the word "lactuca" actually means lettuce. This genus also includes species that were once part of a different group called Enteromorpha. Today, these specific types are commonly known by the name green nori.

Tufts of gutweed in Gullmarsfjorden at Sämstad 1
Tufts of gutweed in Gullmarsfjorden at Sämstad 1

The physical structure of sea lettuce is very specific. Each plant has a leaf-like body called a thallus. This thallus is flattened and can appear in shades from light green to dark green. To stay in place against ocean currents, the algae use a disc called a holdfast. This holdfast acts as an anchor to attach the plant to surfaces. In sheltered areas where the water is calm, the individual blades can grow quite large. A single blade can reach a size of more than 400 mm, which is about 16 inches.

Many different animals rely on sea lettuce as a primary food source. Sea creatures such as manatees and sea slugs, known as sea hares, eat these algae. Humans also consume many species of sea lettuce in various parts of the world. In China and Japan, the algae is known as aosa. People in Scandinavia, Great Britain, and Ireland also use it in their diets. It can be eaten raw in salads or cooked into warm soups. Some cooks even chop it and mix it into dough to fortify bread.

From a nutritional standpoint, sea lettuce is quite dense. It is a high source of protein and soluble dietary fiber. It also provides a variety of essential vitamins and minerals. One of the most notable minerals found in the algae is iron. However, people must be careful about where they collect it. In certain locations, sea lettuce can be contaminated with toxic heavy metals. This contamination makes the algae dangerous for humans to eat.

Scientists continue to discover new species within the Ulva genus. For example, Ulva intestinalis was named by Linnaeus in 1753. In 2014, researchers formally established Ulva paschima as a new species. This species is found only in India and has a tubular thallus. In 2022, ten new species were discovered in New Caledonia. These included Ulva arbuscula, Ulva finissima, and Ulva scolopendra. These discoveries help scientists understand the diversity of life in the ocean.

While sea lettuce is usually helpful, it can sometimes cause environmental issues. In August 2009, massive amounts of algae washed up on beaches in Brittany, France. As the large amounts of seaweed began to decompose, they created a problem. The rotting leaves produced a large quantity of hydrogen sulfide. This is a toxic gas that can be very dangerous to breathe. In one incident, a horse rider lost consciousness and a horse died from the fumes. A lorry driver also died after passing out from the gas while driving a load of decomposing seaweed.

Environmentalists have studied why these massive algae blooms happen. They believe the cause is linked to nitrogenous compounds in the water. These compounds often wash out to sea from industrial farms. This happens through the improper disposal of pig and poultry slurry. Chemical products and slurry used as fertilisers in arable farming also contribute. When these substances enter the ocean, they can cause the seaweed to grow in uncontrolled amounts. This connects the health of our farms directly to the health of our oceans.

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