Sea Lettuce: A Two-Cell Sheet That Cannot Grow Alone

Sea lettuce grows on rocks in the surf. It has no roots, no stem and no leaves. It has no blood, and it cannot move. It does not flower, and it does not seed.

What it has instead is a flat sheet two cells thick, lying against the rock.

There is a second surprise, and it is the better one. Grown in sterile culture with no bacteria at all, sea lettuce never finishes building itself.

The earlier version of this post listed the taxonomy and moved on. It never mentioned that problem.

W.carter. Ulva on the shore at Brofjorden, Sweden. The sheet lies flat against the rock, which is the whole strategy: no stipe, no lift, nothing to hold the blade up to the light. CC0, via Wikimedia Commons.

What Sea Lettuce Is Made Of, and Where It Lives

The whole body is called a thallus. Botanists use that word for any plant-like body with no true tissues and no true organs. A leaf has layers arranged for specific jobs. A thallus does not.

In Ulva the sheet is usually two cell layers deep. The cells are much alike, and each can photosynthesize, take up water, and pass material to its neighbors.

That thinness is not an accident. It is set by where the alga lives.

The holdfast, and what it does not do

On the shore the sheet spends part of each day under water and part of it in air. A thick body would lose too much water in the sun, and would not photosynthesize at all while covered.

A thicker alga solves that by building support. Kelp grows a stipe, like a stem, and floats its fronds on gas bladders. That structure is expensive, and it comes out of the same budget as everything else.

Structure	What sea lettuce does	What a kelp does instead
Holdfast	A small disc that grips by adhesion	A spreading mass that grips rock hard
Stipe	None at all	A stem-like stalk that lifts the blade
Gas bladders	None at all	Bladders that float the fronds upward
Tissues	Two layers, undifferentiated	Several distinct layers
Body plan	A flat sheet	Branched blades or fronds

Two ways of solving one problem: staying in the light.

Sea lettuce spends none of it. Its holdfast is a small disc, and it does not grip very hard. The blade simply lies flat.

That saves a great deal of material. It also means the blade tears and drifts easily, so the alga rebuilds instead of defending.

The pay-off is speed. With nothing structural to build first, the whole budget can go into new surface.

Snoteleks. A chlorophyte cell. Every cell of a Ulva sheet is close to this shape, and the body is little more than a few thousand of them stacked two deep. CC BY-SA 4.0, via Wikimedia Commons.

Tewy. Giant kelp. A brown alga that spends heavily on structure, with a stipe and gas bladders. Sea lettuce spends nothing, and lies flat instead. CC BY 2.5, via Wikimedia Commons.

It Cannot Finish Without Bacteria

Sea lettuce does not grow on its own. A review of the genus as a research model reports that its growth is symbiotic, and that normal shape development needs close company from bacteria living on the thallus.

These are not passengers. They release substances that trigger cell division and cell specialization in the alga. Supply the right compounds, and Ulva in sterile culture will build a complete thallus.

In one standard laboratory strain, just two bacterial partners are enough to stand in for the whole community. Both belong to groups that live on marine surfaces, and both release the compounds Ulva needs.

The name for those molecules is clumsy but exact. They are algal growth and morphogenesis-promoting factors.

So the thallus a sea lettuce grows is not entirely its own work. It is its own cells, arranged under instructions from the bacteria living on its surface.

That has a second consequence, and it matters for the nutrition section below. Bacteria make vitamin B12. Any B12 measured in sea lettuce biomass is very likely bacterial in origin, which is one reason the figure moves so much between studies.

Aleksandra Rapacka-Zdonczyk, Agata Wozniak, Joanna Nakonieczna and Mariusz Grinholc. A mature bacterial biofilm. The film of bacteria on a Ulva surface is a structure of this kind, and the alga cannot build its normal shape without it. CC BY-SA 4.0, via Wikimedia Commons.

An intertidal alga, not a shallow-water plant

Sea lettuce is an intertidal alga. It lives in the band of shore that is covered at high tide and exposed to air at low tide.

The earlier version of this post gave a depth range of zero to ten meters, which made it sound like a shallow-water plant. It spends much of the day in air, and it has to be built for that.

On salinity it is remarkably relaxed. It grows in open sea and in brackish water, which is a big part of why it reaches estuaries and nearly every temperate and tropical coast.

Tolerant of salt, sensitive to light

That tolerance is worth setting next to the caution. Earlier versions called sea lettuce sensitive to changes in water quality, and also said it thrives in nutrient-rich water. Both cannot be the whole truth. It shrugs off swings in salt and temperature. What it is genuinely sensitive to is light, nutrient, and drying out.

Flexible rather than tough

The adaptive trait here is flexibility rather than tolerance. A sea lettuce can rebuild the same body plan from very different starting points, including a loose fragment floating free. That plasticity is why a torn-off blade can start over as a whole new plant.

Eric Guinther. Green algae on an intertidal rock. This is the zone sea lettuce works in: submerged at high tide, exposed to air at low tide. CC BY-SA 3.0, via Wikimedia Commons.

Matthew Herron. Volvox, a colonial green alga. Another route open to the green algae: build a rolling colony instead of a sheet. Ulva takes the flat route, and takes it alone. CC BY-SA 4.0, via Wikimedia Commons.

Two Generations That Look Alike

Most sea lettuce species alternate between two generations. One is haploid and makes gametes. The other is diploid and makes spores by meiosis.

The unusual part is that the two look the same. An adult gametophyte and an adult sporophyte are nearly identical in shape. Biologists call this an isomorphic alternation.

One more detail is specific to the group. The gametes are all the same size. There is no small fast sperm and no large egg. The class Ulvophyceae is named partly for this, and the flagella on the cells are what carry the gametes through the water.

The earlier version had the life cycle nearly right, then broke it in the very next line. It listed zoospore production as asexual reproduction.

Zoospores are the sexual cycle, not a shortcut around it. The diploid sporophyte undergoes meiosis, which produces haploid zoospores. Those grow into haploid gametophytes, and those make the gametes. The original's own next sentence, about a zygote growing into a sporophyte, closes a loop it had just called asexual.

The genuine alternatives to that cycle are fragmentation, and parthenogenesis, where an unfertilized gamete develops on its own. Both happen, and parthenogenesis is why single-clone cultures work at all.

One last caution. Alternation of generations is the textbook description and it suits the genus well enough for most purposes. It is not true of every species. The same review notes that life cycles within Ulva are more varied than the simple summary suggests, with some species having lost a phase.

The Green Tide

Now the one good idea in the earlier version can be paid off.

Sea lettuce has no stored reserves to speak of and no tissue to maintain. Its growth rate is therefore set almost directly by how fast nutrient arrives.

That makes it one of the most nutrient-responsive organisms in coastal water. Add nitrate, and very little else responds faster.

A bloom of Ulva prolifera appeared in the Yellow Sea off China in 2007, and has recurred there since. Researchers report that it suddenly abated in 2017, and that the trigger is still not fully settled. Nutrient load, temperature, light, currents, and interactions with bacteria and other algae all contribute.

The earlier version said the bloom depletes oxygen. That is right, but it puts the mechanism one step too early.

Stage	What is happening	What it costs
Living mat	The sheet spreads over the seabed	Light is cut off, and bottom plants fail
Decay	Bacteria break down the sinking tissue	Oxygen falls and pH drops
Settling	Organic matter smothers the bottom	The benthic community is lost

A green tide charges its cost in two stages, not one.

While the mat is alive, it covers the seabed and cuts off light. The bottom-dwelling plants and animals fail for want of it.

When the mat dies and sinks, bacteria decompose it. Respiration of the decaying tissue consumes dissolved oxygen and releases carbon dioxide, so the water loses oxygen and its pH falls at the same time.

So the living weed is a shading problem and the dead weed is a chemistry problem. The second one is the one that kills.

Sea lettuce is also a useful warning system. Because it answers enrichment faster than almost anything else with a holdfast, a heavy crop in a coastal bay is an early sign of nutrient input.

A bloom can also vanish for good reasons. Nitrogen loads into Narragansett Bay fell by more than half over about a decade, and researchers have reported recovery of the bottom communities afterward. The same signal that flags a problem also clears when the problem is fixed.

Felix Andrews ( Floybix ). Algae in a river. Nutrient arriving from the land is what starts the sequence, and sea lettuce is among the first organisms in coastal water to answer it. CC BY-SA 3.0, via Wikimedia Commons.

European Union, Copernicus Sentinel-2 imagery. An algal bloom seen from orbit. A green tide is this, in the sea rather than in fresh water, and satellite images pick it up because the color of the surface changes. Attribution, via Wikimedia Commons.

Eating Ulva: What Is Actually In It

Sea lettuce is eaten, in Asia and increasingly elsewhere, fresh in salads and soups and dried as a seasoning. It is also farmed.

It is genuinely nutritious. What it is not is nutritionally neutral.

Iodine is the nutrient to think about first. In a survey of fourteen seaweeds bought for people to eat, iodine ran from 3.4 milligrams per kilogram of dry weight all the way to 7,316. The high end belonged to a brown kelp rather than to Ulva, and green algae sit well below it.

But a range that wide makes the number on a label close to useless. A review of iodine and the thyroid warns that even small amounts of the iodine-rich kelps can affect thyroid function, and that labeling does not reliably make that clear.

Vitamin B12 has a much worse reputation, and that reputation is now out of date.

The old claim was that seaweed carries no usable B12 at all. A 2025 analysis of Ulva fenestrata measured about 681 nanograms per gram of dry weight, high enough to qualify for the European Union's high-in-B12 claim.

That does not settle it. The same paper opens by saying it remains unclear whether the algae contain true B12, and the older worry about inactive analogues has not gone away. It is also specific to one species, and Ulva is not one species.

So the honest position is narrow. Sea lettuce can carry a substantial B12 measurement. Whether that becomes a dependable dietary source is unsettled, and the answer may depend on which bacteria were present at harvest.

Heat matters as well. In the same study, brief blanching cut the B12 in sea lettuce by nearly half, while oven drying and freeze drying kept it.

Here is the finding that undercuts any simple nutritional praise. In the same fourteen-seaweed survey, the sample with the highest copper, nickel, manganese and lead was Ulva lactuca.

That is not a fault peculiar to it. It is the same appetite that takes up nitrate, showing up where nobody wants it. An alga that cannot regulate what crosses its cell membrane will take whatever the water offers, useful or not.

The practical advice is dull and it holds. Gather from water that is not obviously contaminated, because that is where the metals will be. And do not treat any one seaweed as a reliable source of a single vitamin.

The same appetite, on purpose

That last property is why sea lettuce interests people trying to clean up water. Because it takes up surplus nutrient, it can be grown deliberately to strip nitrogen and phosphorus from aquaculture effluent and from wastewater.

The trait that makes a nuisance in a bay is the trait that makes it useful in a tank.

Ben Mills. Iodine in its crystalline form. Iodine is the one seaweed nutrient that is genuinely high and genuinely risky, and the spread across seaweeds is huge. Public domain, via Wikimedia Commons.

Yann Macherez. Harvesting seaweed. Ulva is farmed and gathered for food, and the apparatus is much the same as for the larger algae. CC BY-SA 4.0, via Wikimedia Commons.

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
Thallus	The whole body of an alga, when it has no true tissues and no true organs.
Chlorophyta	The green algae, defined by holding chlorophyll a and b.
Stipe	The stem-like stalk of kelp, which lifts the blade into the light.
Holdfast	The structure that attaches an alga to rock. In Ulva it grips by adhesion and is easily lost.
Epiphyte	An organism living on the surface of another one without taking its food from it.
Symbiosis	Two organisms living together in sustained contact, here to the benefit of both.
Morphogenesis	The shaping of a body during growth.
Gametophyte	The haploid generation, which makes gametes.
Sporophyte	The diploid generation, which makes spores by meiosis.
Meiosis	Cell division that halves the chromosome number, making spores and gametes.
Zoospore	A motile spore with flagella. Part of the sexual cycle, not an alternative to it.
Parthenogenesis	Development from an unfertilized gamete.
Eutrophication	Excess nutrient in water, and the blooms that follow.
Intertidal	The band of shore covered at high tide and exposed at low tide.

Terms used above, in roughly the order they first appear.

Key Takeaways

• Sea lettuce builds almost nothing: no stipe, no gas bladders, no tissues, and a holdfast that grips by adhesion.
• It cannot finish without help. Bacteria living on the thallus supply the compounds that trigger cell division and shaping.
• Having no structure to build first, it grows as fast as nutrient arrives, which makes it the earliest warning of enrichment in coastal water.
• It is intertidal, not a shallow-water plant, and it tolerates big swings in salt and temperature while staying sensitive to light, nutrient and drying.
• Zoospores are the sexual cycle, not an asexual shortcut. Fragmentation and parthenogenesis are the real alternatives.
• A green tide costs twice: shading while it lives, oxygen and pH loss once it decays.
• On nutrition, iodine varies enormously across seaweeds and can affect the thyroid, and the B12 story is genuinely unsettled rather than settled in either direction.
• Ulva lactuca carried the highest copper, nickel, manganese and lead of the fourteen seaweeds assayed, because an alga that takes up everything takes up metals too.

Frequently Asked Questions

Is sea lettuce the same as lettuce?

No. It is a green alga, a distant relation of land plants, and it has nothing to do with the salad lettuce.

The name describes the shape only. The sheet does look like a leaf of lettuce, which is where the name comes from.

Can I grow it at home?

It needs salt water, light, and nitrogen. In seawater it grows fast enough to be conspicuous within a week.

It also needs its bacteria, which arrive with ordinary seawater. A tank that has never held any marine life may not give a good result, because the thallus will not shape properly on its own.

Is it safe to eat?

Sea lettuce is eaten widely and is generally regarded as safe. The cautions are about where it came from and how much of it you eat.

Gather it from clean water, because algae accumulate metals, and treat any single seaweed as an unreliable source of any one vitamin.

Why does sea lettuce grow so fast?

Because it has almost nothing to build first. It has no stem, no support tissue and no stored reserves, so almost all of its energy goes into making new surface.

That is also why it is useful as an indicator. Feed it nitrogen and it answers immediately, faster than most sea plants can.

Does a green tide have to be bad news?

No. A bloom is a warning, and warnings are allowed to stop. Where nutrient inputs were cut, researchers have reported the bottoms recovering afterward.

What matters is not whether sea lettuce is present, but why.

Key references
• Ulva (genus) and the revision of the family Ulvaceae
• Chlorophyta: chlorophyll a and b, and the position of green algae
• Thallus: a body with no true tissues or organs
• Biofilm: what the epiphytic bacterial community is
• Eutrophication: nutrient enrichment and the blooms it drives
• Algal mat: the floating form a green tide takes
• Seaweed: uses, farming and the broader algal group
• Iodine: dietary sources and excess
• Wichard et al. 2015, Ulva as a model system for morphogenesis, Front Plant Sci 6:72 -- the source for the bacterial symbiosis that the earlier version never mentioned
• Trigo et al. 2025, cobalamin in Ulva fenestrata, Food Chem 483:144302
• Desideri et al. 2016, essential and toxic elements in seaweeds, J Toxicol Environ Health A 79(3):112-122
• Zhang et al. 2019, Ulva prolifera green tides in the Yellow Sea, Natl Sci Rev 6(4):825-838

Figures are from Wikimedia Commons, with the author and license named in each caption. Measurements are from the cited papers.

The Role of Seaweed in Coastal Ecosystems: What It Really Does

Seaweed covers rock along most of the world's coasts. It sways in the surf, grips against the waves, and feeds the animals living around it.

It looks like a plant. It is not one.

The earlier version of this post listed five things seaweed does for the coast and explained none of them. It also credited seaweed with three jobs that belong to other organisms.

This rewrite keeps what holds up in the research and fixes what does not.

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 Seaweed Is, and What It Cannot Do

Seaweed is the common name for thousands of species of large marine algae. They come in three main colors: green, brown and red. None of them has roots, stems or leaves.

What they have instead is a body called a thallus. It is simple tissue with no vessels inside it. Water and nutrients enter through the whole outer surface, directly from the sea.

A holdfast is not a root

A seaweed grips rock with a holdfast. It looks like a root ball and works nothing like one. Its only job is to hold on. It absorbs no water and no nutrient.

That fact settles two claims from the earlier version 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 hold sediment in place are rooted plants. Seagrass is a flowering plant with roots and rhizomes under the mud. Salt marsh grasses and mangrove roots do the same at the shoreline.

Feature	Seaweed	Seagrass
What it is	An alga	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.

Frédéric Ducarme. A seagrass meadow. Flowering plants with roots: this is the vegetation that binds sediment and buries carbon where it grows. CC BY-SA 4.0, via Wikimedia Commons.

Shelter: The Claim That Holds Up Best

Where seaweed grows tall it makes a canopy. Kelp can rise from the seabed to the surface in a few months, and the space between the stipes fills with life.

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 money on the services. Each genus was worth between $64,400 and $147,100 per hectare per year, adding up to $465 billion to $562 billion worldwide.

Those are potential values. They are what the services would fetch if they carried a price tag, not money anyone collects. The physical part behind the number is about 904 kilograms of fishery production per hectare per year.

Shelter is also the claim that needs no hedging. A fish hiding in a frond does not care 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 a plant with no wood reaches the surface.

That flexibility does two things at once. It lets the canopy reach the light, and it costs the canopy most of its power to stop a wave. The wave claim is taken up below.

oliver.dodd. Fish moving through a kelp forest. The canopy is shelter and hunting ground at the same time, which is the service kelp is best at and best documented for. CC BY 2.0, via Wikimedia Commons.

Carbon: Captured, Mostly Returned

Seaweed is very good at capturing carbon. Macroalgae cover about 3.4 million square kilometers of coast and fix roughly 1.5 billion tonnes of carbon a year.

The earlier version stopped at that point and called seaweed a natural carbon sink that helps slow global warming. The next number is the one that matters.

Of all that carbon, about 11 percent is estimated 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.

Where seaweed grows, the carbon rarely stays. Most seaweeds live on rock, where no sediment builds up to bury anything. What gets stored is exported instead: currents carry detached fronds offshore until they settle.

Some of that export does arrive for good. Studies of seagrass sediment find that half of the carbon there came from other producers, seaweed among them. Seaweed is a donor to those stores rather than a store itself.

So is it worth anything for the climate?

Probably yes, and the honest answer is a range rather than a slogan. Researchers describe macroalgal carbon as the elephant in the blue carbon room: large, real, and still hard to account for.

A first-order global estimate puts the stored fraction at 173 million tonnes of carbon a year. That is comparable to what mangroves, salt marshes and seagrasses are credited with combined.

The accounting is unfinished. China and Korea already count farmed seaweed in their national blue carbon programs, and most other countries do not.

Eric T Gunther. Kelp canopy at the surface. Fronds that break loose drift away from stands like this one, and that export is where most of the carbon ends up. CC BY-SA 3.0, via Wikimedia Commons.

Nutrients: Cleaned, Then Taken Away by Harvest

Seaweed takes up nitrogen and phosphorus from the water fast. It works like a sponge with no preferences, which is why it is grown in farm ponds and wastewater channels on purpose.

At sea the numbers are measured in China, which grows about two thirds of the world's seaweed. One hectare of farm removes the nutrient load reaching 17.8 hectares of coastal water for nitrogen, and 126.7 hectares for 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 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.

Cut seaweed left to decay gives the nitrogen and phosphorus back to the water. The cleaning finishes at the harvest, not at the absorption.

Scale has limits too. China's seaweed farms remove about 5.6 percent of the nitrogen entering its coastal waters each year, and close to 40 percent of the phosphorus. Useful, and not decisive.

The crop that leaves with the nitrogen

That harvest is an industry. The Food and Agriculture Organization counts 36.5 million tonnes of farmed algae in 2022, wet weight, worth about $17 billion at the farm gate. Production has more than tripled since 2000.

Seaweed goes to food, to thickeners like alginate and carrageenan, to animal feed and to fertilizer. The people who cut it and sell it are the same people removing that nitrogen.

NASA Goddard Space Flight Center from Greenbelt, MD, USA. Seaweed farms off South Korea seen from space. Farms at this scale are visible from orbit, and the nutrient they remove leaves the water only when the crop is harvested. Public domain, via Wikimedia Commons.

Yann Macherez. Seaweed cut by hand at Jambiani, Tanzania. The harvest is the step that completes the cleaning: cut the crop and the nitrogen goes with it. CC BY-SA 4.0, via Wikimedia Commons.

Waves: A Dampener, Not a Barrier

The last claim is the one that failed hardest when it was tested. The earlier version described thick seaweed beds as natural wave barriers that prevent erosion and storm damage.

The researchers who measured this properly were more careful. They placed wave loggers at four sites, each with a kelp bed and a bare patch of the same reef side by side.

At three of the four sites, waves lost more height over the bare reef than over the kelp. At the fourth there was no difference, except under one wind direction where the kelp did about 10 percent better.

Their conclusion was that evidence for kelp as coastal defense is thin, and that the rock under the kelp and the kelp itself must be measured separately.

The reason is the same flexibility that lifts the canopy. A blade that bends with a wave takes less energy out of it than a reef that stands still.

So seaweed helps a little at the edge. It is not a sea wall. The shoreline claims that hold belong to rooted vegetation: seagrass offshore, salt marsh and mangrove at the line.

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
Macroalgae	Large seaweeds: the red, brown and green algae big enough to see without a microscope.
Thallus	A body with no true tissues and no true organs, which is what every seaweed is built as.
Holdfast	The structure that attaches seaweed to rock. It anchors only, and absorbs nothing.
Kelp	The large brown algae of the order Laminariales, which build the underwater forests.
Canopy	The top layer of a kelp forest, near the surface where the light is.
Blue carbon	Carbon captured and stored by coastal ecosystems such as marshes, seagrass and mangroves.
Eutrophication	Too much nutrient in water, and the algal blooms that follow from it.
Rhizome	An underground stem. Seagrass uses it to creep through mud and to hold on.
Detritus	Dead organic matter. What most of a seaweed crop becomes if nobody harvests it.
Farm gate	The moment a product leaves the farm. Prices quoted at the farm gate ignore transport and processing.

Terms used above, in roughly the order they first appear.

Key Takeaways

• Seaweed is an alga with no roots: the holdfast only anchors, so seaweed cannot take nutrients from sediment or bind sand in place.
• Shelter is the strongest service: kelp forests house abalone, lobster and reef fisheries, and the six main genera carry a potential value of $64,400 to $147,100 per hectare a year.
• Carbon capture is real but mostly temporary: about 11 percent of the carbon seaweed fixes is estimated to reach lasting storage.
• Nutrient cleaning works through harvest: China's farms strip about 75,000 tonnes of nitrogen a year, but only because the crop is cut and removed.
• Wave protection is the weak claim: at three of four measured sites, kelp beds sheltered the shore no better than bare reef did.
• Farming is large and growing: 36.5 million tonnes of algae in 2022, worth about $17 billion at the farm gate.

Frequently Asked Questions

Is seaweed a plant?

No. It is an alga. Plants have roots, stems and vessels that move water inside them, and seaweed has none of the three.

The similarity is real but shallow. Both photosynthesize, and that is where the shared design ends.

Does seaweed hold the shore together?

Not really. Its holdfast grips rock and absorbs nothing, so it never binds sand or mud.

Measured against bare reef, kelp beds took a little more energy out of waves at one site in four and less at three. Seagrass, salt marsh and mangrove are the shoreline holders.

Does seaweed fight climate change?

It captures a great deal of carbon and stores a small share of it. The stored share is estimated at about 11 percent.

That is still a large amount in absolute terms, and it is why two countries count farmed seaweed in their blue carbon programs. The word sink, on its own, hides the rest of the story.

Does seaweed clean polluted water?

It strips nitrogen and phosphorus out of the water very well. Inside Chinese farms, dissolved nutrients measured about half the levels outside.

The cleaning only counts when the seaweed is harvested. Cut and removed, the nutrients leave with it. Left in the water to decay, they come back.

Key references
• Seaweed: the macroalgal group and its uses
• Kelp forest: the habitat and its global distribution
• Seagrass: flowering plants with roots, not to be confused with seaweed
• Holdfast: an anchor with no absorbent function
• Krause-Jensen et al. 2018, macroalgal carbon in the Blue Carbon framework, Biol Lett 14:20180236 -- the source for the 11 percent stored fraction
• Morris et al. 2019, kelp beds as coastal protection, Ann Bot 125:235-246 -- kelp beds measured against bare reef at four sites
• Xiao et al. 2017, nutrient removal by Chinese seaweed aquaculture, Sci Rep 7:46613
• Eger et al. 2023, ecosystem services of global kelp forests, Nat Commun 14:1894
• FAO 2024, State of World Fisheries and Aquaculture: farmed algae output and value

Figures are from Wikimedia Commons, with the author and license named in each caption. Measurements are from the cited papers and the FAO report.