The Unseen World Of Seaweeds

17:02 minutes

Kelp washed up in shallow water with bulbs in the middle of each thick strand.
Bull Kelp (Nereocystis luetkeana), floating on water surface, Vancouver Island, British Columbia, Canada. Credit: Malcolm Schuyl/Flpa/imageBROKER/Shutterstock (5314714a)
Yellow kelp underwater, flat yellow leaves and bulbs floating
Giant kelp (Macrocystis pyrifera), Islas San Benito, Mexico. Credit: Shutterstock (2861435a)

Chances are you don’t give much thought to seaweed unless you’re at the beach, or perhaps when you’re considering a dinner menu. But the thousands of seaweed species around the world are a key part of our coastal ecosystems. 

Seaweeds photosynthesize, provide food and shelter for marine animals, stabilize the coastlines, and even contribute to making your ice cream creamier (through an ingredient called carrageenans, extracted from red seaweeds in the Rhodophyceae family). Increasingly, they’re also being investigated as a source of biofuels and as biological factories, due to their fast-growing nature. 

Dr. John Bothwell, a phycologist at Durham University in the UK, has written a book in praise of seaweeds. In Seaweeds of the World: A Guide To Every Order, he highlights beautiful, unusual, and important species from each of the three seaweed lineages—green, red, and brown. In this segment, he talks with SciFri’s Charles Bergquist about some of his favorite species, where the seaweeds fit into the web of life, and the importance of seaweeds to the global ecosystem.

Strands of thin, very long, asparagus-like seaweed
Sea-lace (Chorda filum) and Japanese Wireweed (Sargassum muticum) growing in shallow water, Kimmeridge Bay, Dorset, England. Credit: FLPA/Shutterstock (3286841a)

Segment Guests

John Bothwell

Dr. John Bothwell is author of Seaweeds of the World: A Guide to Every Order, and an associate professor in the Department of Biosciences at Durham University, in Durham, UK.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. I’m guessing most of you don’t give much thought to seaweed, unless, of course, you’re at the beach, or maybe when you’re considering the dinner menu. But the thousands of seaweed species around the world are a key part of our coastal ecosystems. Science Friday’s Charles Bergquist is here to tell you more about it. Hey, Charles.

CHARLES BERGQUIST: Hi, Ira. You’re right, I hadn’t really given seaweed that much thought. But recently I saw a book that made me think a lot more about all the types of seaweeds out there.

Dr. John Bothwell is a phycologist. That’s a scientist who specializes in algae and cyanobacteria. He’s an associate professor in the Department of Biosciences at Durham University in the UK. And he’s author of the book Seaweeds of the World– A Guide to Every Order. I asked him why he felt the need to write it.

JOHN BOTHWELL: Because they’re really important. The seaweeds do in the sea, and certainly along the coast, what trees do on land. If we didn’t have seaweeds on our coastline, the coastline would die.

CHARLES BERGQUIST: So this book is beautiful. Listeners can see some pictures on our website at sciencefriday.com/seaweed. There are things that look like clusters of tiny green grapes. There are pink, feathery fronds. There are things that look almost like undersea mushroom caps. How many different kinds of seaweed are there?

JOHN BOTHWELL: Thousands. The main division is between the three major kinds of seaweed– the reds, the greens, and the browns. But within each of those major groups, there are several thousand species. The reds are probably the most diverse, but they’re also the hardest usually to find because they tend to live under the bottom of the tide limit. The browns are usually the ones with which most people will be familiar because they tend to live in what we call the intertidal, so the part of the shore that is exposed when the tide goes out.

CHARLES BERGQUIST: When we talk about the greens, the reds, and the brown seaweeds, is it really that straightforward? If I see something that’s red, it’s a member of the red seaweeds?

JOHN BOTHWELL: It’s pretty much that straightforward. The color does depend on a number of factors. One of the reasons I think why people don’t appreciate seaweeds is they never see them at their best. They only see seaweeds when they’re dried out or they’ve been left to dry out on the shoreline once the tide has gone out. It’s like judging the beauty of a plant by looking at your compost heap or by looking at what’s been cut down in a storm.

In order to see the real beauty of these things, you actually want to go diving. You want to go offshore. You want to look at them in their actual environment when they’re underwater.

CHARLES BERGQUIST: Help me to work out the family tree here. Should I be thinking of seaweeds as something sort of like the grass on my lawn or more like the slime in my fish tank?

JOHN BOTHWELL: Ooh. People often talk about seaweeds as plants that live in the sea. It’s actually the other way around. The land plants that we’re familiar with are seaweeds that about 600 million years ago made them move onto land. So seaweed came first. And the land plants are the descendants.

The division between the slime in your aquarium and the grass in your lawn is actually a really smart thing to point out because there’s two kinds of algae. And the seaweeds are a subdivision of the algae. Algae is a very broad term that means things that photosynthesize that grow in water.

And the big division in the algae is between what we call the cyanobacteria and between what we call the eukaryotic algae. Eukaryotes are basically things that you can see with the naked eye. Anything you can see, pretty much, is a eukaryote. It’s not bacteria.

So the division between the cyanobacterial algae and the eukaryotic algae is a really important one. In general, the aquarium slime is the bacterial algae. And a lot of the phytoplankton will be the eukaryotic algae. So seaweeds are a division of three divisions– reds, greens, and browns– of the eukaryotic algae.

CHARLES BERGQUIST: You mentioned land plants being seaweed that managed to crawl out onto the ground and live.

JOHN BOTHWELL: Pretty much.

CHARLES BERGQUIST: Should I be imagining them as something similar to a land plant, with structures like roots and stems and leaves? Or is it completely different? Are we not at that stage yet?

JOHN BOTHWELL: That’s a really good question. Actually, the morphology, the shape the characteristics of a seaweed and of a land plant, are determined largely by the environment in which they find themselves. Seaweeds are in the sea. Land plants are on the land.

They both face common challenges, though. They do need to find nutrients, they need to reproduce, and they need to spread their propagules, their reproductive cells, a long distance. They need to spread their populations.

But in the sea, the nutrients are all around you. They’re in the water. And water is a very good carrier of things. It supports weight. And it will also support your offspring. When you produce your offspring, they’ll be carried away on the tides and with the currents.

And seaweeds have adapted to live in an aquatic environment. So they don’t need much support because the water carries their weight. They don’t need very specialized reproductive organs because the water will carry their offspring away from them.

You mentioned roots. They don’t actually need roots because roots are specialized structures that extract nutrients from the soil. Seaweeds don’t take their nutrients from the soil. They take it from the water. So, in fact, in seaweeds, the thing that looks like a root– it’s called a holdfast– it’s just a device for attaching it to the rocks or to the sand. It doesn’t actually absorb nutrients.

Land plants, on the other hand, all the things we think of– the flowers, leaves, roots, the specialized structures of land plants– they all evolved after ancestors of land plants moved onto land. And they evolved afterwards because land poses particular problems. You have to absorb the nutrients from the soil. So you evolve roots.

You have to spread your offspring a long distance away. So you evolve seeds. You have to lift your leaves up to outcompete other plants to gather light. So you develop lignin and wood. Again, seaweeds don’t have that because the water supports them. So really, really good question.

CHARLES BERGQUIST: Tell me about some of your favorite species. You must have ones that you specifically love.

JOHN BOTHWELL: I do. It’s a very personal question. I’ve got a couple. I have one from each of the major groups– one green, one red, and one brown.

My favorite green– and I’m very biased here– is ulva. Ulva is the Greek word for sedge or grass. And it’s probably the most common green seaweed. If you go down to any Northern Hemisphere certainly beach, you’ll see a layer of green, looks like lettuce leaves on the shoreline. And we call it sea lettuce.

Ulva is my favorite green because my group sequenced the genome of ulva. So we did a lot of work on ulva. And my group currently works on it. It’s a cousin to the land plants. So we can work out a lot of the fundamental evolutionary biology that drove the divergence of land plants and green seaweeds by looking at ulva and comparing it to land plant models.

My favorite brown seaweed is Fucus serratus, which is serrated wrack, which is very common on the shorelines around the North Atlantic. That was the first seaweed I did experimental work on. And one of the nice things about Fucus serratus is that the plants can be either male or female. So if you see a plant that has orange speckles at the tip, that’s male. If they’re green at the tip, that’s female.

And my favorite red is one called Chondrus crispus, which is carrageenan moss in Irish, which is a beautiful little branching seaweed that looks very– again, very common in the shorelines around where I live.

CHARLES BERGQUIST: You mentioned just now the male and female nature of some of these species. Talk to me a little bit about the reproductive cycle in these organisms.

JOHN BOTHWELL: Woo-hoo, you ask easy questions. [LAUGHS] Seaweed reproduction is very complicated. And we’re not quite sure why. So it varies between the greens, the reds, and the browns.

But as a simple overview, seaweeds tend to have at least two lifecycle stages. And one of those lifecycle stages is diploid, which means that it has two copies of every gene. The other of the life cycle stages is haploid, which means it only has one copy of every gene. Humans have one diploid generation, which is us, the forms that we see walking around. That’s the diploid adult stage.

We produce haploid gametes, our sperms and eggs. The sperm and egg fuse to form a diploid zygote. That then grows up into the diploid adult again. So we do produce haploid cells, but only for a very, very short stage of our life cycle.

In a lot of seaweed species, that haploid stage can actually develop into a free-living organism. So it’s as if we could produce a sperm or an egg. And the sperm and egg could, independently of fusing, could just grow up into another person. So there’d be an adult male walking around who was haploid, an adult female walking around who was haploid.

So this is called this alternation of generations. And there are variations on that particular theme in the greens, the red, and the browns. They each do it slightly differently. But this basic alternation of generations is very longstanding. And we’re not entirely sure why it happens.

What it does do is allow seaweeds to grow without having to find a partner. And that’s a very powerful technique, because if you are a species that is buffeted around by the currents, and you produce your sperm or your eggs, and they float off somewhere else where they can’t find a partner to join with, you can still grow up to become an organism. So it allows for much more mobility.

We know there are some populations of seaweeds that reproduce sexually in one region of the ocean but then reproduce asexually in other areas of the ocean. So we think this alternation of generations helps with the spread and survival of seaweeds in what is a very extreme environment.

CHARLES BERGQUIST: How specialized are these species with respect to their niches around the world? Are they generally widespread? Or are there seaweeds that you would only find in, I don’t know, one specific African bay or something?

JOHN BOTHWELL: The answer is a bit of both. There are some species that are what we call cosmopolitan, so spread worldwide. There are some species that fill very, very narrow niches.

One of the problems with seaweeds is we’ve talked already about the simplicity. And so just to give a comparison, we often talk about multicellular organisms as being simple or complex. And that can be defined by the number of different types of cell that an organism can produce. Humans produce a couple of different cell types– red blood cells, various kinds of white blood cells, neurons, et cetera.

A land plant will usually produce maybe 50 different types of cell. Most seaweeds only produce half a dozen types of cell at most. So they’re very, very simple. That allows them an awful lot of plasticity.

So they grow very well in most places. And the clue is in the name. They’re called weeds. They grow like weeds. But it also actually makes them kind of hard to differentiate because when we identify two different plants, we’ll often look at a particular structure on the plant– the flower, for example, or the leaf shape. It’s a lot harder to do that with seaweeds.

So a lot of seaweed species are very difficult to tell apart, which means that we are probably underestimating the number of species that are there. There’s a lot of what we call cryptic diversity. Cryptic diversity is where you have two things that look the same but are actually different species. There are certainly cosmopolitan species. But we are only just really starting to get into a proper species-level description of exactly which species are filling which niches.

Seaweeds which are extremely diverse, but so are the people and cultures that study them and use them. These things are spread worldwide. And particularly in island cultures, seaweeds are much more important to island cultures than they are to inland cultures for obvious reasons.

So there’s a lot of cultural interpretation and cultural importance to different seaweeds. And I think it’s really important to recognize that seaweeds do mean different things to different cultures. And the diversity of seaweeds is increasingly being matched by the diversity of people who are studying these things.

CHARLES BERGQUIST: You’re listening to Science Friday from WNYC Studios. I’m talking with Dr. John Bothwell about the wonderful world of seaweed. Let’s talk a little bit about uses beyond the obvious animal habitat and food. Part of your day job is working in biofuels.

JOHN BOTHWELL: The biomass, that can be burned, just like any other biomass. So humans have been burning plant-like material for thousands of years. And dried seaweed will burn as well as wood.

There’s a lot of interest now in using seaweed as biofuels or as biotechnology precursors or as feed. There are initiatives certainly in Alaska. There’s a lot of kelp farming going on in Alaska. The giant kelps of California have been used for decades. And worldwide, particularly in places like Indonesia, increasingly in Africa, people are growing these things to see if we can use them as a feedstock.

They do have advantages. One of them is we talked earlier about the lack of specialized structures in seaweeds. Specialized structures take a long time to make. Trees grow quite slowly.

Seaweeds, on the other hand, don’t have to make the specialized structures, which means they grow really fast. Seaweeds will grow two, three, four, or five times faster than land plants. So they’re very productive. It’s one of the reasons why people are interested in them. And certainly in kelp farms, they’re just a very, very fast growing form of biomass. So there’s a lot of potential there to grow biomass offshore.

And, of course, one of the problems with certainly a lot of the Global North is there’s a lot of pressure for land. There’s more people. We’re running out of land to grow stuff on. One answer to that is to start moving some of our production offshore. So yeah, there’s potential.

CHARLES BERGQUIST: This is fascinating. And it’s, obviously, something you care a great deal about. Talk to me a little bit more about why people should care about seaweed.

JOHN BOTHWELL: Well, the best way I can explain it is go to the shore sometime. Go to the coast. And stand on the beach. And turn around and look behind you. So look back at the land.

What you’ll see on most shorelines is dunes, grass on the dunes. You’ll see behind that grass on the shoreline. You’ll see trees in the distance. So you’ll see all of these plants that are keeping your environment alive.

Now turn back around and look out at the sea. And you won’t see anything. You’ll just see this flat horizon. You’ll see the water lying there.

But underneath that flat sea is just as much life as was behind you when you were looking back at the land. We can’t see the seaweeds. They’re all under water.

A couple of miles offshore, there’ll be kelp forests. There’ll be red. There’ll be Asparagopsis. There’ll be ulva. They’ll be Caulerpa. There’ll be all sorts of species that you can’t see. But it’s there doing its job, looking after the coastline and looking after you.

CHARLES BERGQUIST: Dr. John Bothwell is author of the book Seaweeds of the World. You can see some images from the book on our website at sciencefriday.com/seaweed. He’s also an associate professor in the Department of Biosciences at Durham University in the UK. Thanks for taking time to talk with me today.

JOHN BOTHWELL: It’s a pleasure. Thank you very much.

Copyright © 2023 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of Science Friday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/.

Meet the Producer

About Charles Bergquist

As Science Friday’s director, Charles Bergquist channels the chaos of a live production studio into something sounding like a radio program. Favorite topics include planetary sciences, chemistry, materials, and shiny things with blinking lights.

Explore More