The sea is green in places and brown in others and red in still others. The reason is a set of living things that do not look much like plants, even when they sit where plants would.
They are algae. The big ones are called seaweed, and they are the reason a coast can look like a forest underwater.
The earlier version of this post listed the types and the jobs of marine algae in two short menus and explained almost none of them. It also put a rooted flowering plant in the same sentence as an alga, and credited the ocean with a number it did not quite earn.
This rewrite keeps what is right and fixes what is not.
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| Agiorgio. Seaweed on a rock in the surf. There is no soil here and no root below. The whole organism takes what it needs straight from the water flowing past it. CC BY-SA 3.0 via Wikimedia Commons. |
What Marine Algae Are, and What They Are Not
Marine algae are photosynthetic organisms that live in salt water. They make their own food from light, water and carbon dioxide, just as land plants do. Beyond that one sentence, they are not plants at all.
They come in three main groups, usually sorted by color: green, brown and red. The color is not decoration. It comes from the pigments each group uses to catch light, and that decides where in the water each one can live.
A body with no roots, stems or leaves
A seaweed has none of the three things a plant uses to move water around. It has no roots to pull from the ground, no stems to pipe it upward, and no leaves with veins inside them.
What it has instead is a body called a thallus. It is simple tissue, and water and nutrients enter straight through the whole outer surface from the sea around it.
A holdfast is not a root
A seaweed grips a rock with a holdfast. It looks like a root ball and does none of a root's work. Its only job is to hold on. It absorbs no water and no nutrient.
That fact matters for two reasons at once. Seaweed cannot pull anything out of the sediment, because it never reaches the sediment. And it cannot hold sediment in place, because nothing it grows through binds sand or mud together.
The organisms that do that are rooted plants. Seagrass is a flowering plant with roots and rhizomes under the mud. Salt marsh grasses and mangrove roots do the same job at the shoreline. They look like seaweed in a photograph and they are not seaweed at all.
| Feature | Seaweed (an alga) | Seagrass (a plant) |
| What it is | An alga, one of the three color groups | A flowering plant |
| Roots | None. A holdfast that only anchors | Yes. Roots and rhizomes through the mud |
| Grows on | Rock and other hard surfaces | Sand and mud |
| Holds sediment in place | No | Yes |
| Buries carbon where it grows | No, it lives where sediment cannot build up | Yes, in the sediment below it |
Two green things that look alike in a photograph and do different jobs under the water.
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| Frédéric Ducarme. A seagrass meadow. Flowering plants with roots: this is the vegetation that binds sediment and buries carbon where it grows, which is the job seaweed cannot do. CC BY-SA 4.0 via Wikimedia Commons. |
The Three Types of Marine Algae
Marine algae are commonly grouped into three color divisions. Each one is a whole branch of the algal family tree, not a shade of the same thing.
Green algae (Chlorophyta)
Green algae contain chlorophyll a and b, the same two pigments that give land plants their color. That is not a coincidence: the green algae are the group from which land plants are thought to have descended.
They live mostly in shallow water, where the light is bright enough for the pigments they carry. Two familiar names are Ulva, usually called sea lettuce, and Codium, sometimes called dead man's fingers.
Sea lettuce is a thin green sheet that drapes over rocks in the intertidal zone. Codium is a ropier, spongier green that feels firmer to the hand.
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| Ubiquity. A green macroalga on rock at the coast. Green algae like this live where the light is bright, and their pigments are the same ones land plants use. CC BY 2.5 via Wikimedia Commons. |
Brown algae (Phaeophyceae)
Brown algae are the ones people usually mean by seaweed. They include the kelps, the largest of all algae, and some of them build canopies that reach from the seabed to the surface in a few months.
Their brown color comes from a pigment called fucoxanthin, which catches the green and blue light that penetrates deeper than the red light the green algae prefer.
Two familiar names are Macrocystis, the giant kelp, and Fucus, the rockweed that clings to stones at the tide line.
Brown algae live mostly in colder water. Their size and their pigments both suit that environment.
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| Eric T Gunther. A kelp forest reaching toward the surface. These are the brown algae that build the underwater canopies people mean when they say seaweed forest. CC BY-SA 3.0 via Wikimedia Commons. |
Red algae (Rhodophyta)
Red algae contain a pigment called phycoerythrin, which catches the blue and green light that reaches deepest in the sea. That is why many red algae live deeper than the green and brown ones.
Two familiar names are Porphyra, the seaweed pressed into nori, and Gelidium, one of the algae used to make agar.
Not every red alga looks red in the water. The color can run from green through pink to near-black depending on light and depth, which is why the group is named for its pigment, not its look.
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| Knucklesinkorea. Sheets of nori used in Japanese cuisine. Nori is made from the red alga Porphyra, one of the red algae people meet at the table. CC BY-SA 3.0 via Wikimedia Commons. |
Primary Production: What the Ocean Makes from Light
The ocean makes a great deal of the oxygen on Earth, and algae are part of the reason. Scientists estimate that roughly half of the oxygen production on Earth comes from the ocean.
Most of that production comes from plankton drifting in the surface layer: drifting plants, algae and some bacteria that photosynthesize. One of them, a bacterium called Prochlorococcus, is the smallest photosynthetic organism on Earth and is credited with producing up to 20 percent of the oxygen in the biosphere.
That is a larger share than all of the tropical rainforests on land combined, which is a striking number but not the whole story.
Two cautions follow from the same source. First, the ocean also uses roughly the same amount of oxygen. Marine animals breathe it, and both plants and animals burn it in cellular respiration; dead matter uses it as it decays. Production and use are not far apart.
Second, the half figure is about the ocean as a whole, and algae and phytoplankton together are the main producers inside it, not the whole of it by themselves. The earlier version put the number on algae alone and lost both the plankton and the consumption side.
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| European Space Agency. A phytoplankton bloom swirling around Gotland in the Baltic Sea, seen from space. The drifting algae in blooms like this are part of the ocean production that yields roughly half of Earth's oxygen. CC BY-SA 3.0 IGO via Wikimedia Commons. |
Why the number is hard to pin down
The exact share is not a fixed constant. Plankton numbers change with the season, with nutrients, and with temperature, and satellites can track the greenness from above but cannot see the whole story.
Oxygen also varies with time of day and with the tides at a given place. The point is the scale, not a precise percent.
Carbon: Captured by Algae, Mostly Returned
Marine algae capture carbon when they photosynthesize, and they capture a lot of it. Macroalgae cover a large area of the world's coasts and fix a large amount of carbon each year.
The earlier version called kelp forests and seagrass meadows carbon sinks that help slow climate change, and put them in the same sentence. That sentence mixed two different organisms with two different stories.
Seagrass is a rooted plant that can bury carbon in the sediment under it. Seaweed is an alga that mostly lives on rock, where no sediment builds up to bury anything. Putting them side by side under one label hid that difference.
For seaweed, the better-measured number is the stored share. Of the carbon macroalgae fix, an estimated 11 percent is thought to end up stored in sea-floor sediment or deep water.
The rest is eaten, breathed back as carbon dioxide, or washed ashore to rot. Most of what seaweed fixes is turned over quickly, not locked away where it grows.
Where the stored carbon comes from
Some of the carbon that does get stored arrives from elsewhere. Studies of seagrass sediment have found that a large share of the carbon there came from other producers, seaweed among them. Seaweed can be a donor to long-term stores without being one itself.
Detached fronds can drift offshore and settle, and that export is where most of the lasting seaweed carbon is thought to end up. The carbon rarely stays where the seaweed grew.
Is seaweed a climate tool, then?
It probably helps, and the honest answer is a range rather than a slogan. Researchers have described the macroalgal carbon that reaches long-term storage as large, real, and still hard to account for.
One global estimate puts the annual stored fraction at about 70 million to 220 million tonnes of carbon. That is comparable in size to what mangroves, salt marshes and seagrasses are credited with combined.
The accounting is unfinished. A couple of countries already count farmed seaweed in their blue carbon programs, and most do not. That is a policy choice still being made, not a closed question.
Habitat: The Service Best Documented for Kelp
Where brown algae grow tall, they make a canopy. A kelp forest fills the water between the seabed and the surface with fronds, and animals use that space to feed and to hide.
A global study of six kelp genera found that these forests shelter important fisheries. Abalone, lobsters and reef fish feed and hide in them.
The same study put a dollar figure on the services. Each genus was worth between about $64,000 and $147,000 per hectare per year, adding up to roughly $465 billion to $562 billion worldwide.
Those are potential values: what the services would be worth if they carried a price tag, not money anyone collects. Behind the number is about 900 kilograms of fishery production per hectare per year.
Shelter is also the service that needs the least hedging. A fish hiding in a frond does not depend on how the carbon budget balances.
A canopy without wood
A kelp forest has no trees. The blades stream with the current instead of standing against it, which is how an organism with no wood reaches the surface.
That same flexibility is the reason the canopy is poor at stopping waves. Lifting toward the light and stopping a wave are not the same piece of design, and brown algae do the first well and the second poorly.
Nutrients: Cleaned by Uptake, Removed by Harvest
Seaweed takes up nitrogen and phosphorus from the water quickly. It works like a sponge with no strong preferences, which is why it is grown in farm ponds and in water channels on purpose.
At sea the numbers are best measured in China, which grows about two thirds of the world's seaweed. One hectare of seaweed farm can remove the nutrient load reaching dozens of hectares of coastal water. That is about 18 hectares' worth of nitrogen and over 125 hectares' worth of phosphorus.
Inside those farms, dissolved nutrients measured about half the level in nearby open water. Across the country the farms strip roughly 75,000 tonnes of nitrogen and about 9,500 tonnes of phosphorus from the sea each year.
Then comes the part the earlier version left out. The nutrients leave the water only when the seaweed is cut and used. Uptake holds them in the plant. Harvest takes the plant off the coast.
Seaweed left in the water to decay gives the nitrogen and phosphorus back. The cleaning finishes at the harvest, not at the absorption, and that is why the earlier claim about preventing harmful blooms was too strong on its own.
The crop that leaves with the nutrients
That harvest is an industry. The Food and Agriculture Organization counts about 36.5 million tonnes of farmed algae in 2022, wet weight, worth roughly $17 billion at the farm gate. Production has more than tripled since 2000.
Seaweed goes to food, to thickeners such as alginate and carrageenan, to animal feed and to fertilizer. The people who cut and sell it are the same people removing the nutrients.
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| Andre Engels. Seaweed, including sea lettuce (Ulva lactuca), on rocks at low tide. The intertidal green algae are the closest thing many people ever see to the green algae described above.. CC BY-SA 2.0 via Wikimedia Commons. |
Human Uses: Food, Thickener, Feed and Fuel
Marine algae enter human life in more than one form. The big uses fall into four groups.
Food
Edible seaweed has been a staple in many coastal cuisines for a long time. Nori, made from the red alga Porphyra, is pressed into sheets for sushi. Kombu, made from kelp, is used in broth.
Thickeners
Two products of algae run through ordinary food and industry. Agar comes from red algae such as Gelidium. Alginate comes from brown algae and is used as a thickener and stabilizer.
Feed and fertilizer
Seaweed is also used in animal feed and as a fertilizer or soil conditioner. The same nutrient uptake that cleans coastal water is part of why it is useful on land.
Biofuel
Seaweed is studied as a possible source for biofuel because it grows fast and does not need fresh water or farmland. That use is still largely experimental rather than a large current industry.
Key Terms in Plain English
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Marine algae | Photosynthetic organisms that live in salt water and make their own food from light, water and carbon dioxide. They include the large ones called seaweed. |
| Seaweed | The common name for large marine algae. It is not a plant: it has no true roots, stems or leaves. |
| Thallus | A simple body with no true tissues and no true organs. That is the body plan of every seaweed. |
| Holdfast | The structure that attaches seaweed to rock. It anchors only and absorbs nothing. |
| Chlorophyll a and b | The two green pigments that green algae and land plants share. They catch the red and blue light that is strongest in shallow water. |
| Fucoxanthin | The brown pigment in brown algae. It catches the green and blue light that reaches deeper than red light. |
| Phycoerythrin | The red pigment in red algae. It catches the blue and green light that penetrates deepest in the sea, which is why many red algae live deeper than the others. |
| Phytoplankton | The drifting photosynthesizers of the ocean surface: plants, algae and some bacteria. They are the main source of the ocean's oxygen production. |
| Prochlorococcus | A tiny marine bacterium and the smallest known photosynthetic organism. It is credited with up to 20 percent of the oxygen in the biosphere. |
| Blue carbon | Carbon captured and stored by coastal ecosystems such as marshes, seagrass and mangroves. |
| Rhizome | An underground stem. Seagrass uses it to creep through mud and to hold on. |
| Agar | A gelling agent made from red algae such as Gelidium, used in food and in the laboratory. |
| Alginate | A thickening and gelling agent made from brown algae, used in food and industry. |
Key terms in plain English: what each one means as it first appears above.
Key Takeaways
- Marine algae are not plants: they have no true roots, stems or leaves, and a seaweed body is a thallus that takes water and nutrients straight through its surface.
- The three main groups are green, brown and red algae, and their color comes from the pigments they use to catch light: chlorophyll a and b, fucoxanthin, and phycoerythrin.
- A holdfast is not a root: it anchors seaweed to rock and absorbs nothing, so seaweed neither takes nutrients from sediment nor holds sediment in place. Seagrass does both, because it is a rooted flowering plant.
- The ocean produces roughly half of Earth's oxygen, mostly from plankton in the surface layer, and uses about the same amount. The alga Prochlorococcus alone is credited with up to 20 percent of the biosphere's oxygen.
- Kelp forests are the best-documented service seaweed provides: the six main genera shelter fisheries worth a potential $64,000 to $147,000 per hectare per year.
- Seaweed captures a lot of carbon but stores little of it where it grows. About 11 percent of what it fixes is estimated to reach lasting storage; the rest is eaten, respired or washed ashore.
- Seaweed cleans nutrients only when harvested: China's farms strip about 75,000 tonnes of nitrogen a year, but left to decay, the nutrients return.
- Farming is large and growing: about 36.5 million tonnes of farmed algae in 2022, worth roughly $17 billion at the farm gate, used for food, thickeners, feed and fertilizer.
Frequently Asked Questions
Is seaweed a plant?
No. Seaweed is an alga. Plants have roots, stems and vessels that move water inside them, and seaweed has none of the three. The similarity stops at photosynthesis.
Why are algae green, brown and red?
Each color group uses a different set of pigments to catch light. Green algae use chlorophyll a and b. Brown algae add fucoxanthin. Red algae add phycoerythrin, which catches the blue-green light that reaches deepest in the sea.
Are kelp and seaweed the same thing?
Kelp is a kind of seaweed: the large brown algae of the order Laminariales that build the biggest underwater forests. Seaweed is the common name for all large marine algae, including green and red ones as well.
Does seaweed hold the shore together?
Not the way rooted plants do. A seaweed holdfast grips rock and absorbs nothing, so it never binds sand or mud in place. Seagrass, salt marsh and mangrove are the coastal vegetation that holds sediment.
Do kelp forests stop waves and protect the coast?
Only a little, and not reliably. In a proper measurement at four sites, kelp beds took less energy out of waves than bare reef at three of them. Seaweed is better described as a dampener than as a barrier. The shoreline protection that holds is mostly from rooted vegetation.
Does the ocean really make half the oxygen we breathe?
Roughly, yes, but with two caveats. The ocean produces about half of Earth's oxygen, mostly from plankton, and it also uses about the same amount. Most of the oxygen people breathe has built up in the atmosphere over a very long time, not from this year's photosynthesis.
Is seaweed a carbon sink?
It captures a great deal of carbon and stores a small share of it: about 11 percent of what it fixes is estimated to reach lasting storage. Seagrass is the better sink in place, because it is a rooted plant that buries carbon in sediment. The earlier version put them in the same sentence, which hid that difference.
Does seaweed prevent harmful algal blooms?
Not on its own, and not unconditionally. Seaweed does take up nitrogen and phosphorus, but the nutrients only leave the water when the seaweed is harvested. Left to decay, they return. Absorbing nutrients is not the same as preventing blooms.
Key references
• Marine algae: the three colors and what separates them
• Red algae (Rhodophyta): pigments and deep-water light
• Brown algae (Phaeophyceae): kelp, fucoxanthin, the largest seaweeds
• Green algae (Chlorophyta): chlorophyll a and b, Ulva, Codium
• Seaweed: the macroalgal group and its uses
• Kelp forest: the habitat and its global distribution
• Seagrass: a flowering plant with roots, not an alga
• Holdfast: an anchor with no absorbent function
• How much oxygen comes from the ocean?
• Phytoplankton and the ocean carbon cycle: primary production and the size of the ocean oxygen source
Figures are from Wikimedia Commons, with the author and license named in each caption. Measurements are general biology and oceanography background read for this rewrite.
