Arriving at a Caribbean beach only to discover it buried beneath a wall of brown, sulfur-smelling seaweed is a certain kind of misery. The rotten-egg smell that permeates thick mats of sargassum piled along the waterline, sometimes extending for miles, reaches you before you see it. Communities in the Caribbean and along the Gulf of Mexico now perceive this as a seasonal calamity rather than an occasional annoyance.
Sargassum blooms have become much more difficult to ignore since about 2011. One of the biggest brown algae masses ever seen crossed the Atlantic in 2023, spanning more than 5,000 miles. Once primarily found in the Sargasso Sea in the central Atlantic, it now frequently washes up on beaches from Barbados to the Yucatán, killing seagrasses, suffocating coral reefs, clogging fishing gear, and sometimes contaminating desalination plants. Over 8,000 cases of acute sargassum toxicity were reported by medical professionals in Guadeloupe and Martinique in 2018. Teachers in Saint Lucia reported that just sitting close to the water made students feel queasy and lightheaded.
The financial cost has also been astounding. In just one year, cleanup expenses across the Caribbean exceeded $120 million. Additionally, the seaweed itself, which is frequently contaminated with heavy metals and arsenic, cannot just be recycled or composted; it is an issue with no clean solution.
The AlgaRay, a solar-powered autonomous robot created by the UK nonprofit Seaweed Generation in collaboration with the University of Exeter, enters this mess. By most accounts, it’s a surprisingly straightforward concept. Sargassum is collected by the machine as it glides across the ocean’s surface before it gets close to land.
When its storage is full, it descends to a depth of about 200 meters, lets go of its load, and then comes back to the surface. The seaweed loses the buoyancy that typically keeps it afloat at that depth because water pressure compresses the air pockets in its leaves. After being released, it slowly descends to the ocean floor, where it may remain for hundreds of years.
Four to six times an hour, the robot completes this cycle. It is powered by solar panels and lithium batteries and uses the Starlink satellite for navigation. A prototype has already undergone testing at Scotland’s Loch Ness and off the coast of Antigua. Additionally, Seaweed Generation and the Antiguan government have reached a 49-year lease agreement for 500 square kilometers of ocean territory, some of which is deeper than 4,000 meters.
The actual design is inspired by nature. AlgaRay is based on the manta ray, which uses its broad, flat body to filter algae as it moves through water. A functional prototype has a width of about nine feet. Up to 16 metric tons of sargassum can be collected in a single operation by a planned offshore version that is more than 30 feet across and can remove roughly two tons of carbon dioxide from the atmosphere. Theoretically, one full-scale AlgaRay operating 12 hours a day for three months could sink about 80,000 tons of seaweed a year. That is about twice as much as Iceland’s largest direct air capture facility currently handles in a year.
Because sargassum absorbs CO2 from the surrounding ocean water during photosynthesis, the carbon angle is important. As the seaweed breaks down on the surface, sinking it stops that carbon from returning to the atmosphere. Oceanographers estimate that it will take at least a century for carbon stored below 1,000 meters to return to the upper ocean. In a way, the deep sea is the most resilient carbon vault because it holds 17 times as much carbon as the surface ocean, land, and atmosphere put together.
When significant amounts of biomass are deposited on the ocean floor, there are valid concerns about what will happen. This is known to Seaweed Generation. In order to track both the deep-sea regions where sargassum settles and the surface zones where it is removed, the company has integrated monitoring robots into its operations. In order to replicate what occurs naturally when sargassum dies and sinks on its own, deposits will be dispersed.
It’s early yet. The company admits that even at its most ambitious scale, the AlgaRay won’t stop the agricultural runoff from the Mississippi and Amazon that contributes to these blooms in the first place or reverse ocean warming. Professor Mike Allen, one of the co-founders, described the sargassum problem as being caused by persistent forces. However, the AlgaRay doesn’t have to solve every problem in order to be significant. Keeping a few hundred kilometers of shoreline unobstructed, providing coral reefs with a breathing season, and discreetly storing carbon in the deep ocean may be sufficient justifications for keeping a close eye on this one.
