What is Seaweed?

Seaweed is a generic term for multicellular algae that typically live in saltwater or brackish (mildly salty) environments. These can be classified into three main types: green (Chlorophyta), red (Rhodophyta), and brown (Phaeophyta) algae. Cyanobacteria (blue-green algae) may also be called seaweed, but I will not discuss it here as it is a very different organism, being a prokaryote related to bacteria.

Green Algae

Unlike the next two groups (red and brown algae), green algae are part of the Plant Kingdom. Green algae get their bright green color mainly from chlorophyll a and b which are used in photosynthesis and are the same pigments used in “higher plants” (like grass and trees). In addition, they use beta-carotene (yellow) and xanthophylls (brownish) pigments for photosynthesis which leads to a wider variety of colors among green algal seaweeds.

Green algae prefer to be near the surface of the water in order to get maximum sunlight and therefore are often found in shallow waters. Some examples of green algae include sea lettuce and dead man’s fingers.

This group of seaweeds is a significant food source for crustaceans (like crabs), gastropods (snails), and fish. Human uses for green algae include harvesting for beta-carotene which can be used as a food coloring. Green algae also play an important role in absorbing carbon dioxide from the atmosphere as they grow and trapping it on the ocean floor as they die.

Brown Algae

Although they may look like plants and their name suggests that they are “weeds,” most seaweed (and algae in general) belong to the Protist kingdom and are not plants (the exception being the group that we already discussed – the green algae). Their similarities to plants include having cell walls and using chlorophyll for photosynthesis, however these macroscopic (visible without a microscope) algae do not have many of the same structural components that plants do such as roots, flowers, or seeds.

Brown algae are often large, plant-like organisms that range from brown to yellowish-olive in color. They possess a unique pigment called fucoxanthin which gives them their namesake hues. Some common structural features include a holdfast which acts like a root system to anchor them to rocks, blades which look and act like leaves, and air bladders to help them grow upward toward the sunlight. A great example of a brown alga that exhibits these features is kelp.

Unlike green algae, brown algae are often found further from the shore and at much greater depths. They can also grow in a very wide range of water temperatures from the polar zones to the tropics. Kelp forests provide habitat for large numbers of animals, especially invertebrates. Sea urchins, gastropods, and fish feed on brown algae, as well as bottom feeders which scavenge pieces of the seaweed from the ocean floor. Brown algae, like the green algae discussed above, trap carbon dioxide from the atmosphere as they grow.

There are many uses for brown algae. Certain derivatives are being researched for their possible cancer-fighting abilities as well as for boosting our immune systems. Kombu is important in certain Asian cuisines as a flavoring in broths or eaten directly like a vegetable. It can be used while cooking beans to soften them and reduce their… musicality.

Kelp is being explored as a potential source for biomass energy production. Biofuels are those that are grown and converted into usable energy such as bioethanol, biodiesel, biomethanol, biogas, and solid biomass fuels (such as those that might be burned for warmth). Because kelp grows quickly, does not occupy farmland which could be used for foodcrops, and does not need an irrigation system, it may be a viable and promising source of biofuels.

Alginate is a substance derived from kelp and used as a thickening agent in jelly, ice cream, and toothpaste. Kelp is also burned to produce soda ash which can be used in making soap and glass.

Overfishing can cause damage to kelp forests and their biodiversity. Without a substantial fish population, the algae-eating organisms overpopulate and consume a large portion of the kelp, reducing habitat for other organisms and throwing the ecosystem out of balance.

Red Algae

There are more than 6,000 species of red algae and they are typically reddish or purple in color due to the pigment phycoerythrin. This group is similar to the brown algae in that they belong to the kingdom of Protists, but they are unique in that they lack flagella (little tails on cells). Red algae have a global distribution and can be found in deeper waters than other algae because their pigment can absorb blue light which penetrates the deepest.

Red algae are a food source for many of the same organisms as the other two types of algae – fish, crustaceans, and gastropods. Humans also munch on red algae, most commonly in the form of nori which is used for wrapping sushi or eaten by itself as a salty and crunchy snack. Carrageenan (Irish moss) is used as a thickening, stabilizing or gelling additive in ice cream, beer, plant-based milks (like soy or almond), some meat products, and a variety of cosmetics, and is considered a vegan alternative to gelatin. Another important derivative from red algae is agar. Agar is a gel-like substance that is used in scientific research as a culture medium (it’s the stuff that fills petri dishes).


Overall, seaweeds are a diverse and important part of the ecosystem, providing habitat and food for a variety of organisms. They help reduce carbon dioxide in the atmosphere and are a source of biofuels. We can also eat them, use them as flavorings, or extract various components for use as food additives. And they are also fun to view through a scuba mask 🙂

–> Thanks to jkaybay from The Green Stars Project for the suggestion!

One thought on “creatuREport: Seaweed

  1. Thanks for the shout out! And for researching this great post 🙂
    A former colleague has been working on the algal biofuel idea – it would be cool if it worked out.
    Looking forward to the next post!


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