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What is Seaweed?

By Phil Shepley
Updated Mar 05, 2024
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Seaweed is a microorganism that grows in oceans, lakes, rivers and other bodies of water and is comprised of algae. Algae is a plantlike organism that doesn’t contain actual roots, flowers, leaves and stems, but does contain the green pigment known as chlorophyll. This allows the organisms to grow through the process of photosynthesis. Seaweed can include members of green, red or brown algae families, and there are around 10,000 species within many marine habitats around the world.

Some forms appear as long strands and branches, while others look like sheets. A root-like part called the holdfast is used to attach it to rocks and other marine objects, but does not act like a true root since it does not contribute to its growth. This organism can grow in massive quantities in various places from the poles to the equator. It is used by many other marine animals as a source of food as well as a location for mating.

Kelp is a large type of seaweed that has large leaf-like protrusions known as fronds and can grow as long as 200 feet (61 m). Gulfweeds, or sargassum, are a type of brown algae that grow in warmer water and can float in large masses, particularly in an area known as the Sargasso Sea. Varieties comprised of red algae are generally smaller and more delicate, and appear to have many branches.

Seaweed has many uses for different types of products around the world. One of the major uses is for food, as it can be quite rich in nutrients, vitamins and minerals. People living on coasts use it to make noodles, bread, drinks and more. Another popular use is when dried sheets are used to wrap sushi.

Two things that can be extracted from algae are agar and carrageenan, which are used in food, drug, and other industries. Carrageenan is used to help make paper and toothpaste, among other things, while agar can be used as a thickening agent in foods. Agar is also used widely in laboratories to help grow bacterial cultures. Seaweed can also be used in beauty products, medicine, animal food, fertilizer, and more.

Frequently Asked Questions

What exactly is seaweed and is it a plant?

Seaweed refers to diverse marine algae that grow in oceans, rivers, and lakes. Contrary to common belief, seaweed is not a true plant; it's a form of algae. While plants evolved from freshwater green algae, seaweeds belong to several groups of multicellular algae, including red, green, and brown varieties, each with distinct characteristics.

How does seaweed differ from land plants?

Seaweed differs from land plants in several key ways. It lacks the complex structures such as roots, stems, and leaves that characterize terrestrial plants. Instead, seaweed has a holdfast to anchor itself, a stipe resembling a stem, and leaf-like blades. Seaweeds also absorb nutrients directly from the water through their entire surface.

Where can seaweed be commonly found?

Seaweed thrives in a variety of marine environments, from the intertidal zone, where it can be exposed to air during low tide, to the deeper waters, where sunlight can still penetrate. Some species can even be found at depths of 180 meters, showcasing their adaptability to different light conditions and water temperatures.

What ecological roles does seaweed play in marine environments?

Seaweed plays crucial ecological roles in marine environments. It serves as a primary producer, forming the base of the food web, and provides habitat and shelter for a multitude of marine organisms. Seaweed also contributes to the oxygen supply and can help mitigate climate change by absorbing carbon dioxide from the atmosphere and ocean.

Is seaweed beneficial for human use, and if so, how?

Seaweed is highly beneficial for human use, both nutritionally and industrially. It's a rich source of vitamins, minerals, and fiber, and is used in foods like sushi. Industrially, seaweed extracts are used in products such as toothpaste, cosmetics, and as a thickening agent in foods. Seaweed farming is a growing sustainable industry, providing livelihoods and environmental benefits.

How is seaweed harvested and what sustainability concerns exist?

Seaweed is harvested both from wild populations and through aquaculture. Wild harvesting must be managed to prevent overexploitation, while seaweed farming presents a sustainable alternative, with lower environmental impact compared to terrestrial farming. Sustainable practices are essential to maintain the balance of marine ecosystems and ensure the long-term viability of seaweed as a resource.

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Discussion Comments

By orangey03 — On Oct 19, 2012

I use liquid seaweed fertilizer in my yard. It's really powerful, so I only have to use a little. I dilute it with water, so the majority of what the plants are getting is water.

I've used it on my grass, my vegetable garden, and my flowers. It has worked really well. It contains all kinds of nutrients that plants need, and a bottle lasts a long time.

By StarJo — On Oct 19, 2012

@pretzelfish – It can be a bit weird when you first try it, but Nori seaweed is so good for you. It has all kinds of vitamins and minerals, and I've read it can promote weight loss.

Also, I think that it is part of the reason I get satiated when I eat just one sushi roll. It never looks like it will be enough to fill me up, but I stay full for many hours after eating it.

My favorite kind of sushi roll is wrapped in seaweed and then covered in tempura batter and fried. This makes the seaweed easier to chew, and you barely notice the flavor.

By wavy58 — On Oct 19, 2012

There's nothing creepier than natural seaweed. I like to swim in the Gulf of Mexico, and some locations are slightly murky, so you can't tell what's down there.

On more than one occasion, I have gotten seaweed tangled around my arm or leg. Since I can't see it right away, I panic and run out of the water. I am always afraid that a jellyfish has gotten its tentacles wrapped around me or something!

In its natural environment, seaweed is pretty slimy at times. It's gross to get tangled up in it.

By lighth0se33 — On Oct 18, 2012

My friend works at a spa that offers seaweed body wraps. They are supposed to detoxify your body, and they also are promoted as a way to lose weight quickly.

The weight loss thing is kind of a disappointment, because what the wrap does is make you sweat, so you lose water weight only. However, seaweed is supposed to boost your metabolism, so you might get a little benefit from this.

The main thing that it's good for is cleansing your body. If you've been feeling achy and fatigued, you might want to give it a try.

By Valencia — On May 12, 2011

I got a whole basket full of seaweed beauty products for my birthday last year. I was a bit wary of using them at first as it did seem like an odd choice of ingredient. Now I'm a dedicated fan and will be buying more when they run out.

By yumdelish — On May 10, 2011

@Acracadabra - Seaweed is only slimy looking on the beach because of the water! When it is dried it's totally different, and I'm not sure that the type you eat is the same as that washed up on the shore anyhow.

If you want to try cooking with it there are loads of different types. The taste and texture is easier to disguise in recipes and would get you started. Personally I swear by wakame seaweed for it's supposed fat burning qualities.

I like nori too, though it can be a bit salty so watch for that.

By Acracadabra — On May 09, 2011

@pretzelfish - I haven't been brave enough to try eating seaweed, although I like the idea of sushi. I really struggle to get past the smell! In my mind seaweed is strongly associated with walks on a beach, slime and as I said, that strong odor. None of these make me want to actually eat it!

By pretzelfish — On May 09, 2011

The first time I made sushi was really strange, the seaweed has a really interesting texture that works really well with the dish. It's really interesting for me to consider that seaweed is sort of like the lettuce of the sea. It's used in such a variety of foods, and it's a green plant but people tend to be so hesitant to try it.

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