You may have heard that a giant blob of sargassum seaweed is heading to Florida, to the dismay of residents and tourists alike. The blob is projected to arrive in July and if history is any guide, seaweed could pile up and rot on Florida’s beaches, making swimming and sunbathing unpleasant. Sargassum is related to rockweed (Ascophyllum nodosum) and bladderwrack (Fucus vesiculosus), two popular brown seaweeds used for culinary and medicinal purposes, skin care, and hydrotherapy. Naturally, this raises the question of whether sargassum is also edible or otherwise useful. Turns out, the answer is a qualified yes.
What is Sargassum, anyways?
Sargassum is the genus name for a group of about 130 or more related species of brown seaweeds in the class Phaeophyceae and the order Fucales. In the 1400’s, Portuguese sailors encountered thick mats of floating seaweed in the North Atlantic within an area of the fabled Bermuda triangle distinctive for its deep blue color, unique gyre currents, and often lack of wind. When Christopher Columbus (who may have been Portuguese and not Italian) sailed through this area, he wrote of seaweed so thick he feared it would trap his ship. This area of the North Atlantic is now known as the Sargasso Sea because sargassum seaweed is so prolific there. The name sargassum itself is said to originate from the Portuguese word sargaço, which either refers to a type of grape or to a wild vine that grows in willow thickets. The seaweed they encountered had berrylike gas-filled bladders that may have reminded them of grapes. Rockweed and bladderwrack have similar bladders to keep them afloat. These two edible species also have round nodes they use for sexual reproduction, which the two Sargassum species responsible for the "seaweed blob" lack.
Sargassum seaweed (left) and bladderwrack (right) are related. Both species have air bladders and their fronds have a similar yellowish color and flattened profile.
The Sargassum species encountered by the Portuguese were most likely S. natans and S. fluitans, which today are commonly referred to as gulf weed or just sargassum. These two species are distinctive in that they are considered to be the only holopelagic macroalgae, meaning their entire life history is spent floating in the water column. S. natans and S. fluitans have lost the ability to reproduce sexually and instead reproduce vegetatively through fragmentation, where pieces break off and grow into new branching individuals. Sargassum can grow very quickly, doubling in biomass every 20 days or so. It plays tremendously important ecological roles in providing food and habitat for other marine organisms, nutrient cycling, and CO2 absorption.
In the open sea sargassum is beneficial, but it can become a problem when winds and currents move vast amounts ashore. Prior to about 2011, the relatively small amounts of sargassum washing up on beaches were a normal and even beneficial aspect of coastal ecology. In 2011, though, researchers began tracking enormous accumulations at sea washing ashore onto African and Caribbean beaches. Scientists speculated that sargassum growth was being accelerated by climate change. According to this hypothesis, changing ocean currents brought about by melting poles and glaciers, combined with increased nutrients from human activities, have helped sargassum reproduce and grow at record speeds. Much of this new sargassum growth has been occurring at the mouth of the Amazon River.
Stretching in a belt nearly 5,000 miles across the North Atlantic, the so-called sargassum blob now currently threatening Caribbean and Florida beaches is estimated to be one of the largest ever observed. More accurately, the sargassum is not actually a solid blob of seaweed but rather a large number of broken up rafts. As sargassum rafts approach shore and start decomposing, they suck oxygen from the water and suffocate the creatures beneath. These influx events are sometimes called “Golden Tides” due to the color of the sargassum. In some years sargassum piles up on beaches in heaps up to 5 or 6 feet deep, where it rots and releases methane and hydrogen sulfide, a gas that smells like rotten eggs and that can cause respiratory problems. It is also suspected that golden tides encourage growth of the microalgae responsible for toxic red tides. Massive sargassum deposits on South Florida beaches in 2018 coincided with one of the biggest red tide events ever seen on that coast. This double-whammy of macroalgae and microalgae was bad for tourism and the health of the ecosystem.
Sargassum lines a Caribbean beach in 2011. Photo by Mark Yokoyama. Used under a CC BY-NC-ND 2.0 license
Just when and where sargassum mats will wash ashore and cause problems is difficult to predict because it depends on variables such as winds and currents. When it does happen, though, beach communities that rely on tourism often expend significant resources to remove the smelly seaweed. This past year, up to 1,600 dump truck loads per day were needed to remove sargassum from beaches in the Barbados. Removal is done by hand when possible because heavy front-loaders can damage the beach and harm wildlife such as turtle egg nests. Clean-up costs were estimated to exceed $120 million for the Caribbean region in 2018. Disposal is another problem. Sargassum can accumulate heavy metals such as arsenic and cadmium from seawater, which can then contaminate groundwater.
Several Sargassum species are edible and at least six are cultured in Korea, Japan, and China, with a few others under development. Farmed sargassum seaweeds are sold dried, salted, or fresh, and consumed in soups, vegetable dishes, salads, or used in various seasonings. S. fusiforme was one of the first Sargassum species to have been cultivated and it is usually marketed as hijiki (or hiziki). Japanese folklore credits regular hijiki consumption with encouraging growth of thick, black, lustrous hair. However, hijiki contains unusually high levels of inorganic arsenic, so health authorities advise against consuming too much.
Although in theory they could also be eaten, S. natans and S. fluitans are not desirable culinary species and their arsenic content makes them even less desirable. All seaweeds accumulate arsenic, but most edible species convert it into various forms of non-toxic organic arsenic, mostly arsenosugars, making them safe to eat. Unfortunately, Sargassum species in general seem to accumulate more total arsenic than other seaweeds and some species, such as S. fusiforme, contain a high proportion of inorganic arsenic, which is toxic when certain levels are exceeded. Arsenic content also makes it challenging, though not impossible, to find other, non-culinary uses for the sargassum blob.
One proposed solution is to compost washed-ashore sargassum and spread it onto gardens. Sargassum contains nitrogen, phosphorous, and potassium, three essential plant nutrients that make it potentially useful as fertilizer. Like other seaweeds, it also contains trace minerals and bioactive compounds that stimulate plant growth. However, if it contains a lot of arsenic, then that presents an obvious risk to consuming any vegetables grown in that garden. Even if the arsenic is mostly present as the non-toxic organic form, during decomposition organic arsenic can break down into the elemental inorganic form. One study reported average total arsenic concentrations in S. fluitans of 74mg/kg, which exceeds the EPA maximum allowable concentration in seaweed for use as fertilizer of 41mg/kg.
Another possibility is to use sargassum as a biofuel. Sargassum, and all seaweed for that matter, is rich in carbohydrates that could potentially be digested by bacteria or though other methods to yield methane for energy production. Another pathway is to use fermentation or other processes to produce bio-ethanol from sargassum polysaccharides such as mannitol. However, producing biofuels from sargassum has its share of scientific skeptics and it is still far from a commercial reality. Process optimization and the establishment of large biorefineries are required in order for sargassum biofuels to be feasible at mass scale, but scientific and commercial interest remains strong despite the skeptics.
Another intriguing possibility is to extract useful substances from sargassum. The Fucales order of brown seaweed, which the Sargassaceae family belongs to, has long been used in Chinese medicine to treat bronchitis, hypertension, infections, and of course thyroid conditions caused by iodine deficiency. Modern science has found bioactive compounds in Fucales species such as rockweed and bladderwrack that show promise for treating cancer, obesity, and diabetes. Some of the phycocolloids and bioactive compounds found in sargassum include alginic acid and polyphenols with potential nutraceutical and medicinal applications. Fucoidan, a sulfated polysaccharide found in the cell walls of all Fucales seaweeds, is one of the more well studied and highly valued of these compounds.
Similar to biofuel production, extracting these substances requires biorefineries. Ideally, extraction would be followed by biofuel production in cascading biorefineries, where all conceivable byproducts are extracted from the sargassum. Biorefineries require significant investment to establish, but in a world of diminishing resources they may ultimately prove to be well worthwhile.
Sargassum’s role in carbon sequestration may turn out to have as much value as any of the above solutions. Like plants, seaweed consumes carbon dioxide through photosynthesis as it grows and incorporates carbon into its cellular structures. In doing so it removes carbon from the oceans, which play a major role in absorbing carbon dioxide from the atmosphere.
This process has led some to declare that seaweed could counteract climate change and help save the planet by sequestering carbon dioxide. However, for carbon to be considered truly sequestered, it has to be locked up for at least 100 years. Seaweed has a short life cycle and as it decays, gets eaten by other organisms, or is used by humans as a biofuel, some or all of the carbon that was incorporated into its tissues gets released and recycled back into the atmosphere. This is not truly carbon sequestration.
In order for sargassum or any seaweed to sequester carbon it has to remain intact in some form for decades or centuries. One way this can occur is if the seaweed falls to the ocean bottom and sinks into the sediment. Sargassum’s buoyant air bladders prevent it from sinking, but a recent paper published in the journal Geophysical Research Letters entitled “Sinking Sargassum” speculated that powerful tropical Atlantic cyclones might break sargassum rafts apart and detach its air-filled vesicles, allowing it to sink. The authors concluded that further research was needed to fully understand sargassum’s role as a carbon sink.
Another way to sequester sargassum carbon is for humans to bury it or incorporate it into long-lasting durable materials. Burying it might be an issue if the arsenic and other heavy metals it contains were to leach into groundwater, but using sargassum as a structural material may hold some promise.
Researchers have been exploring the idea of extracting food-safe sargassum cellulose nanofibers and using the material to manufacture bio-degradable tableware to replace plastic cutlery. Of course, bio-degradable cutlery does not sequester carbon, but sargassum biomass could also potentially be used as a building material in civil construction, where it could remain inert for decades. Sargassum has high fiber content, much of which consists of carbon-rich lignin and cellulose. Seaweed fibers could replace up to 50% of the wood-based fibers in Medium Density Fiberboard (MDF) panels. According to one study, MDF manufactured with sawdust, fibers from Kappaphycus alvarezii (a red seaweed sometimes marketed as sea moss), and 12% adhesive conformed with Japanese Industrial Standards for panels. Using sargassum in building materials would also help preserve trees, which are themselves important for removing and sequestering carbon from the atmosphere.
Commercial interests have taken notice of the possibilities. The Dominican Republic has shipped tons of sargassum to a company in Finland called Origin by Ocean, which is developing a number of useful products made from sargassum and other algae. Carbonwave, with a lab based in Puerto Rico, is another startup company looking to turn Golden Tides into useful materials. The company uses a proprietary extraction process to help remove arsenic and other unwanted elements. SOS Carbon, a spinoff of the Massachusetts Institute of Technology (MIT), has developed a “Littoral Collection Module” to capture sargassum before it reaches the beach. The clean, sand-free sargassum biomass can then be transported to biorefineries for value added processing. Algas Organics, a sargassum fertilizer and biostimulant company founded in St. Lucia by a young entrepreneur named Johanan Dujon, was featured in a Forbes magazine article. Using an enzymatic fermentation process, the company concentrates sargassum bio-actives while removing heavy metals such as arsenic and lead. Rum and Sargassum is a Barbados-based company with plans to turn sargassum, rum distillery wastewater, and manure from the Barbados blackbelly sheep into a bio-based compressed natural gas (Bio-CNG) that can be used as an automotive fuel.
Many other startups have recently formed to capitalize on the regenerative capacity of sargassum seaweed. While not every one will succeed, others will, and they all speak to human ingenuity, entrepreneurship, and above all, capacity for hope. With biomass estimates ranging between 4-20 million metric tons, there seems to be plenty of sargassum to go around. As a small, employee-owned company with over 50 years’ experience selling nourishing and health-giving edible seaweeds, we at Maine Coast Sea Vegetables wish them nothing but success in finding solutions to sargassum!