Meet the climate startup turning a 5,000-mile stretch of seaweed from a plague of coastal communities into a giant carbon store

• Brown seaweed — known as sargassum — causes havoc for coastal communities.
• But it could also provide an opportunity to build massive carbon stores at the bottom of the ocean.
• London-based Seaweed Generation is one such startup dedicated to seaweed sequestration.

Patricia Estridge's birthday celebrations have always coincided with bonfire night on November 5, when the UK marks Guy Fawkes' failed attempt to blow up parliament.

For as long as she can remember, she's watched a torchlit procession of local residents in fancy dress march through her hometown in the southeast of England to an enormous bonfire in an ode to the Gunpowder Plot of 1605.

Nowadays, those bonfires remind Estridge more of the climate crisis than of any botched attempt at regicide.

The former Google staffer and software engineer experienced the brutal reality of the climate crisis firsthand when she lived in California during a wave of destructive wildfires.

"I thought: 'Oh, interesting, someone's having a bonfire. That's unexpected,'" she told Insider. "Then I realized it wasn't a bonfire at all — the state was on fire."

A few weeks of soul searching while locked indoors away from the plumes of smoke set Estridge on a path that ultimately led her to cofound Seaweed Generation, a climate-tech startup dedicated to sequestering carbon dioxide deep in the ocean, in 2021. She's part of a wave of new "blue economy" entrepreneurs taking advantage of seaweed for carbon removal.

Seaweed is an attractive play to investors and environmentalists thanks to its ability to rapidly absorb carbon and the well-established supply chains used to transport it in areas including food, biofuels, and beauty products. Seaweed sequesters an estimated 175 million tons of carbon annually as it sinks into the deep sea — around 0.5% of global emissions.

Sargassum has exploded in recent years

Estridge wants to help transform the Great Atlantic Sargassum Belt — a 5,000-mile stretch of floating seaweed that's visible from space — from a plague on coastal towns to an enormous carbon store rooted to the bottom of the sea.

Sargassum, a catch-all term for brown seaweed, is naturally occurring and isn't a problem in itself, but comes with some considerable drawbacks when it beaches or comes into shallow water.

"Sargassum stinks like sewage when it rots, it's absolutely disgusting," Estridge said. When it rots, it gives off arsenic and a toxic gas called hydrogen sulfide, which is responsible for its egg-like stench. It also attracts insects, can smother sea-turtle nesting sites, and blocks light from reaching coral reefs, Stephen Leatherman, a coastal ecologist, said.

Large deposits of seaweed can also cause problems for locals when they near the coastline. "I've seen it so thick you can't launch a boat in it; people can't get their boats off the beach," Leatherman added.

The belt has appeared in the Caribbean Sea nearly every summer since 2011, according to the Optical Oceanography Lab. The brown tide has been a source of creeping chaos and had a record bloom in March at 13 million tons, causing havoc to coastal communities and tourism across the Caribbean, the Gulf of Mexico, and the state of Florida. The negative impacts of the belt are expected to worsen with its total quantity poised to continue increasing over the next few months.

Sargassum grows faster in warmer water — and oceans hit record temperatures in March — but some scientists pin its explosion to chemical runoff from agriculture in the Amazon that feeds the seaweed.

"Where we see it really becoming so massive is off Brazil, near the Amazon River," Leatherman said. Ocean warming alone can't explain the quantity of sargassum present today, he added.

The seaweed Roomba

Estridge's Seaweed Generation has developed a robot that she described as a "Roomba meets Pac-Man."

"What it does is relatively simple," Estridge said. The robot intercepts seaweed by scooping it into its "mouth" and takes it around 200 meters below the surface where the pressure of the ocean crushes sargassum's air bladders, so it can no longer float. The robot then releases the seaweed so it can sink to the sea floor and comes back to the surface.

Miami-Dade County is spending around $6 million on removing the seaweed this year, while others are scrambling to stop it from shoring in the first place. Mexico even sought help from the Navy. Floating barriers, designed to contain oil spills, are also being used to keep sargassum off beaches, Leatherman said.

Questions remain around permanence and scale

If seaweed is dropped to 1,000 meters and secured on the ocean floor, it is considered a long-term carbon store. "It's so far the biggest kind of carbon sink on Earth," Estridge said. But one of the big challenges in this is proving the seaweed stays there, she added.

This is the sticking point for Brennan Spellacy, the CEO and a cofounder of Patch, which helps companies offset emissions by investing in carbon-removal projects.

"The crux of blue carbon is actually going to be storage and monitoring," he said. "How do you have higher-certainty ways to ensure durability is there? Kelp and algae are phenomenal carbon removers, it is the storage conversation that needs to be sorted out."

Seaweed Generation will work in waters with around 4,000 meters of depth and no upwelling current, which could bring sargassum back to the surface.

At the same time, its device will gather data on relevant data, including GPS location, water depth, temperature, and the volume of seaweed the sargassum sinks. The device will also take a video.

A key question for Leatherman is who will pay for private companies to take on "the Great Sargassum Monster," as he called it. Seaweed Generation has a planned pilot with the government of Antigua, which is invested in protecting its tourism industry.

Long-term, Estridge hopes to generate revenue via carbon credits, one of which would represent one ton of CO2 sequestered from sinking sargassum. Patch said that seaweed-sinking carbon-credit purchases on its platform increased by nearly 200% from the final quarter of last year to the first quarter of 2023.

But the problem is simply too great, Leatherman said. "They'll never get it all," he added. Still, he welcomed any effort that could help and reduce cleanup costs.

In case you missed it, watch a replay of Insider's One Planet virtual event with activist and artist Elijah McKenzie-Jackson.