is seaweed living

Seaweeds: Plants or Algae?

  • Camps, Classes & Programs
  • Things to Do & See
  • Learn About the Seashore
  • Support the Seashore

Seaweeds: Plants or Algae?

Did you know seaweeds are technically not plants, but a type of algae? You’ll often find them washed up on the beach, or waving in tidal pools near the shore. Wade into the ocean and you might feel some slippery against your feet and legs. But what are they seaweeds or algae?

What is Seaweed?
Seaweeds inhabit seawaters, and are primitive plants belonging to algae family. However, there is no particular definition for the term seaweed, as there is no one common ancestor to seaweeds, but is a more common term to describe a certain group of plants with distinctive properties. Marine macroalgae, or seaweeds, are plant-like organisms that generally live attached to rock or other hard substrata in coastal areas. They belong to three different groups with over 10,000 species: brown algae (phylum Ochrophyta, class Phaeophyceae), red algae (phylum Rhodophyta), and green algae (phylum Chlorophyta, classes Bryopsidophyceae, Chlorophyceae, Dasycladophyceae, Prasinophyceae, and Ulvophyceae). Red and brown algae are almost exclusively marine, whilst green algae are also common in freshwater (rivers and lakes), and even in terrestrial (rocks, walls, houses, and tree bark in damp places) situations. Many of these algae are very ancient organisms, and although lumped together as “algae”, are not really very closely related, having representatives in 4 of the 5 or 6 kingdoms of organisms. Seaweeds have been useful for humans in many ways viz. food, medicine, fertilizer, and industrial products, as those are rich in vitamins and other nutrients. Carrageenan, agar, and many other gelatinous products come from seaweeds.

What is Algae?
Algae include one of the most primitive organisms on the Earth, with fossils evidences dating back to more than three billion years. Earlier, algae included both prokaryotic and eukaryotic organisms, but now only the eukaryotes are included in the classification. They also do not have a common ancestor. They could be either unicellular or multi-cellular in their structure, and accordingly microscopic as well as macroscopic. They inhabit any aquatic ecosystem including freshwater, saltwater, and brackish water. Almost all the algal species are photosynthetic and show autotrophy. Algae collectively produce the largest amount of energy through photosynthesis. However, they are very simple in cellular structures including the gigantic kelps, without many complex organs (leaves, roots… etc.) as in terrestrial plants. This is an extremely diverse group with an unthinkable number of species. According to the US National Herbarium, there are 320,500 collected specimens, but there is no correct estimation on the number of algal species in the world.

Seaweeds differ from plants in important ways:

  • Seaweeds absorb water and nutrients in all their tissues, directly from the surrounding water. They don’t have the complex system of roots, specialized tissues and leaves that help plants move water and nutrients in their body.
  • Seaweeds can do photosynthesis in all their tissues; most plants photosynthesize only in their leaves.

What is the difference between seaweed and algae?

  • Seaweeds are a group of algae, and have some special characteristics viz. macroscopic, multi-cellular, benthic, and marine.
  • Diversity of algae is extremely high and incomparable with that of seaweeds.
  • Algae could be both unicellular and multi-cellular, whereas seaweeds are necessarily multi-cellular.
  • All the seaweed species are autotrophic, whereas some algal species rely on other external food materials.
  • Algae inhabit both freshwater and marine waters, while seaweeds inhabit only seawaters.
  • Marine algae can distribute over shallow as well as deep waters, while seaweeds mostly inhabit shallow waters.

Below are some resources to help you learn more about this topic and handy reference tools for your next tidepool adventure.

Did you know seaweeds are technically not plants, but a type of algae? You

What You Don’t Know About Seaweed

One of the last things anyone wants to encounter on a beach vacation — right after sharks and jellyfish — is seaweed. Yucky, slimy, clingy seaweed.

You might be surprised to learn, though, that if you run into seaweed on a dip in the ocean, there’s a good chance it won’t be your first seaweed experience of the day. Seaweed was likely in one of the products you used to start your day: toothpaste, soap, a vitamin, medicine or cosmetics. This common use as an extract in everyday products is just one of the things you’ll learn from a conversation with author Susan Hand Shetterly or by reading her book, “Seaweed Chronicles: A World at the Water’s Edge” (Algonquin Books of Chapel Hill).

The book tells the story of how seaweeds are grown and harvested in the world’s oceans and of their importance for a variety of purposes, including an extract in food and other products, helping fight climate change by sequestering carbon, their future use as a possible biofuel, and even the very future of fishing and farming itself. Shetterly, who describes herself as an environmentalist and essayist, anchors her story in the Gulf of Maine, a large area extending from Cape Cod to Nova Scotia, and tells the story of seaweed through the “seaweed people” who live there: aquaculture entrepreneurs, fishermen, marine biologists, conservationists and others. These people connected her from her home in coastal Maine to seafaring communities worldwide — the Philippines, Japan, China, France, England, Scotland, Ireland and Scotland, where different cultures have historically made their living from seaweed — to bring her story about the interconnected global significance of seaweed full circle.

After five years of listening to the stories of seaweed people and researching scholarly papers, the result is a book that’s neither a scientific treatise about seaweed nor an attempt to tell everything there is to know about seaweed. That, Shetterly says, would have resulted in a book the size of “War and Peace” and too heavy to lift. “I wanted the book to be a narrative from which readers could learn about the different aspects of seaweed from the interesting people involved with seaweed and by letting them tell the reader their stories,” she says. Shetterly presents these stories in a beautifully written, novel-like page turner that plumbs the depths of the world’s oceans, from which she dredges up surprising fact after fact about seaweed you likely didn’t know and probably didn’t expect.

Here are some highlights of Shetterly’s deep dive that may give you a new appreciation for the overlooked underwater world of seaweed.

Things you probably don’t know about seaweed

There may be a million species of seaweed. Phycologists, the people who study seaweed, estimate there are between 30,000 and 1 million species of seaweed. Because we are still discovering and learning about seaweeds, Shetterly believes we have much more to learn about their significance to the environment and how to use them to enrich our lives and the lives of the generations to come.

Seaweeds are linked to the oldest organisms on the planet. “There is something called cyanobacteria, a bacterium that suddenly appeared and had the capacity to do photosynthesis,” Shetterly says. “People who studied seaweeds called it a microalgae, a one-celled alga. But people who didn’t study seaweeds called it a bacterium. It was and is a bit of both. Nonetheless, it was the first living thing floating in the ocean. Then it was joined by a microalgae, and what they did was to send little puffs of oxygen into the atmosphere. Without them we wouldn’t have oxygen to breathe.”

Seaweeds are not plants. Shetterly readily acknowledges that people think of seaweeds as plants. She thinks one inescapable reason for that is that “weeds” are part of their common name, and weeds, after all, are plants! But seaweeds are not plants. They are algae, though not the kind of single-celled microscopic algae such as phytoplankton many might associate with algae from their high school biology class. Seaweeds are multicellular algae known as macroalgae — or, simply, “big algae.” In this case, the cells have bonded together into a plant-like appearance.

They are shaped like plants for a reason. To Shetterly’s way of thinking, many seaweeds have the appearance of miniature trees. “They have holdfasts that anchor them to a rock or a hard surface like a shell or piece of wood, they have a stipe that looks like a trunk, they have fronds that look like branches and then they have spores for their reproductive tissues at the top of the fronds.” The reason for this shape, she adds, is photosynthesis so they can make food. “They want to reach as close to the sun as they can so they can to get as much light as they can.”

Calling them weeds does them a disservice. “I think they got the name weed because they were thought of as slippery things of little use that were sort of in the way, and they tripped you up,” Shetterly says. We also tend to use the word weed, she points out, to refer to something that we think has little value. That line of thought brought out one of the favorite quotes from her research. It’s from Paul Molyneaux, who has written about commercial fishing for The New York Times and who won the 2007 Guggenheim Fellowship to study sustainable fisheries in several countries: “We don’t know how to assess the value of species within their ecological community. So, we tend to think of them as worthless rather than priceless.” Seaweeds, as many cultures around the world have known for centuries, have tremendous economic value.

Worldwide seaweed harvests are valued at $6 billion a year. The majority of that, $5 billion, is in food for humans. The rest represents seaweed extracts for a wide range of uses.

35 countries harvest seaweed. China and Indonesia are the largest producers of seaweeds grown in aquaculture farms. The United States and Europe are quickly catching up.

Maine is fast becoming the largest seaweed producer of edible and commercial seaweeds in the United States.

It is almost impossible to go through a day without encountering seaweed. Its uses, Shetterly says, fall into two broad categories: processed foods and processed non-foods.

Many processed food products contain seaweed. Two soft-food examples are puddings and edible oils. Nori, the Japanese name for seaweed, is part of a common daily diet in Japan and is used in such items as rice balls, sushi roils and salads. The Japanese eat more seaweed than any other culture, which some nutritionists say has led to the country’s high life expectancy.

Many processed non-foods contain seaweed. These include toothpaste, cosmetics, soaps, medicines, pet foods, cattle feeds and farm fertilizers. The gel is also used by the printing industry as a component in the gloss or coating in glossy papers, as part of the fluids used in fracking and by medical and other labs in petri dishes to grow tissue cultures, according to Shetterly.

Seaweeds are slippery and slimy for a reason. When Shetterly gives talks about seaweed, something she wants to get out of the way at the beginning is that, yes, seaweeds are slippery and slimy. “Seaweeds have a gel on their outer layer, and there are reasons for that,” she says. “No. 1 is that when seaweeds are thrashing around in the water the gel allows the fronds to easily slide off each other. Without the gel, the fronds would either self-amputate or they would amputate their neighbors. The other thing is that the gel protects seaweeds from sun damage when they are exposed to the sun during low tides. When the tide is extremely low, and we have very low and very high tides here, the seaweed will lie against the rocks. Not only that, but all sorts of animals that live on the fronds are protected as they lay between the fronds and the rocks during low tide. The gel coating protects the seaweed and the seaweed protects the tiny animals from the sun by keeping them damp and briny while they wait for high tide to return.”

Seaweed is very soothing for skin. “A lot of people around here, I am beginning to discover, go to the beach and get some rockweed (the common name for Fucus seaweeds), put it in a kind of baggie with holes in it and stick it in a bathtub of hot water,” Shetterly says. “Then they get in because it is very soothing to the skin. I have not tried that yet.” She says she won’t be surprised, however, if a lot of things made from seaweeds to put in bathwater to soothe the skin become common at farmer’s markets or festivals in coastal communities.

Seaweed is used in wound dressings, especially for burns. Burn hospitals sometimes use dressings that are infused with a form of processed seaweed gel, Shetterly says.

Seaweed plays an important role in protecting the planet from climate change. The world’s oceans absorb approximately 25 percent of the carbon in the atmosphere. In the process, oceans are becoming more acidic. Whereas plants on land absorb carbon from the atmosphere, seaweeds absorb and filter it from the seas. “It used to be thought that when seaweeds pulled loose from the shore and floated out to sea and carried a lot of food in the form of little critters for birds and fish to eat that they would sink, eventually rise again and float into shore where they would release their carbon back into the air,” Shetterly says. “What they may be doing is sinking and staying on the bottom of the ocean and therefore holding onto that carbon. That would be very helpful.” Another thing that happens with carbon sequestration, she adds, occurs when seaweed that sinks to the bottom of the ocean begins to disintegrate. What might be happening in this case is that microscopic pieces of the seaweed go into the water column, and once there are ingested by one-celled microalgae, which then in turn are ingested by something else, perhaps a fish. If, however, seaweed floats toward land and washes up on the shore, it will release its carbon back into the atmosphere. But the seaweed carbon cycle is very complicated, Shetterly says, and scientists are still learning how it works.

Climate change is affecting seaweed. When Shetterly began her research five years ago, tests showed that climate change did not significantly affect seaweed. Six months ago, a scientific article by several phycologists indicated that climate change, which contributes to the warming of the oceans, will affect the seaweed Ascophyllum nodusm, which goes by the common name knotted wrack. “What they found was that as water heats up Ascophyllum growing at its southern edge would cease to thrive,” she says. “That means the genetic diversity within the species would begin to diminish. If ocean warming continues going the way it is now, Ascophyllum will probably start moving north. But the problem with moving north is that at a certain point the winters are just too dark and the summers are too light for Ascophyllum. It would have to adjust to a totally different light regime to survive. Scientists don’t know if it can do that.” Shetterly admits that’s a worst-case scenario, but if it happens she says the impact will be greater than the possible loss of just one seaweed species. “There are so many tiny and essential animals that need Ascophyllum to thrive. What’s going to happen to them? And if Ascophyllum has trouble, other species will most likely have trouble, too.”

Seaweed could become the next ‘big thing.’ The University of Southern Maine has won a $1.3 million research grant to grow sugar kelp on huge platforms in federal waters off the state’s coast. The goal is to establish the United States as a leading producer of macroalgae with a focus on using it as a biofuel for transportation to power cars, planes and trains and to make electricity. “It’s a project that’s still in the imagination of the planners,” Shetterly says. “We don’t know yet if this is going to be a terrible idea or a good idea.”

Seaweed is the future of fishing and farming. “Up here in Maine, we’re very aware that we have plundered our fisheries,” Shetterly says. “Our cod population is commercially extinct right now. It’s heartbreaking. Not only do we lose the richness of our oceans but the richness of the coastal culture, too.” Processes are now in place in Maine through the state Department of Marine Resources and the Legislature to harvest seaweed in a sustainable manner that protects coastal habitats. Shetterly is most encouraged by small inshore businesses where people set up aquaculture projects in the bays where they raise seaweed for food consumption in organic and clean beds. On the other hand, aquaculture is very different in the Pacific Rim. “I am told they have such huge aquaculture farms in China you can see them from space,” Shetterly says. Seaweed farms could become the answer to food crises as the world’s population continues to expand. Without the need for land resources, fresh seaweed has the potential to become one of the most sustainable crops on the planet. “Seaweed gives us an opportunity to do things better than we have in the past,” Shetterly says.

You may not realize it, but seaweed is utterly essential to our world.