The New Ocean Cleanup Crew: Seaweed-Sinking Robots

Last summer saw an invasion of epic proportions. A massive island of seaweed sprang up in the Atlantic Ocean and drifted ashore. While mats of brown algae have blanketed the tropical shores of the Caribbean before, 2023 was one of the biggest blooms yet — spanning over 5,000 miles. 

In this case, the brown algae is Sargassum, which naturally clumps together to form free-floating mats. It's popped up in the central Atlantic and Caribbean in higher frequencies since 2011. Once confined to their namesake Sargasso Sea in the central Atlantic, the growing number of Sargassum mats now damage ecosystems and harm economies. 

Yet enterprising companies are tackling the abundance of algae with gusto, transforming it into products like shoes, paper, animal feed, compost and charcoal. One business, Seaweed Generation, takes a different approach to the problem: robots. Its seafaring devices clean up the seas and sequester carbon at the same time.

Ecosystems smothered by seaweed 

Small mats of Sargassum can be beneficial, providing food and protection for sea turtles, fish, crabs and birds. They’re also important nursery areas for commercially-fished species like mahi mahi, amberjacks and jacks, and they help control beach erosion when washed ashore. 

The problem is when Sargassum grows out of control. Large blooms can block sunlight, killing valuable seagrasses below. When the algae die and decompose, they smother corals and deplete oxygen in the water, causing fish kills. They can also pose problems for sea turtles by deterring them from critical nesting habitats and hindering hatchlings from swimming offshore.

To make matters worse, the latest blooms contained large amounts of plastic debris covered in bacteria. These bacteria carried genes causing leaky gut syndrome, which sickens fish, crustaceans and seahorses.

Costs of Sargassum

These Sargassum blooms take a bite out of the economy, too. The piles of brown slime scare off tourists with the smell of rotten eggs that can cause respiratory problems. For instance, in areas affected by large Sargassum blooms in 2018, Barbadian hotels saw their occupancy drop, while those in areas unmarred by seaweed experienced increased occupancy. Tourism in Caribbean islands can generate over $60 billion, contributing up to half of many countries’ GDP. Similarly, tourism is one of the largest industries in Florida, regularly contributing $80 billion to $95 billion toward the state’s GDP and supporting millions of jobs.

To add insult to injury, the clean-up of Sargassum is expensive. In 2018, the Caribbean-wide clean-up cost over $120 million. And what to do with the copious amounts of seaweed is problematic. Since it contains arsenic and other heavy metals, it’s unsuitable for food or fertilizer. Yet leaving it alone can clog desalination and power plants and damage fishing boats, equipment and gear. 

Seaweed solutions

Where others saw a floating mess, Seaweed Generation saw an opportunity. Instead of beach cleanup or product development, they focused their efforts at its source.

“If you want to deal with the Sargassum problem, you have to deal with it offshore,” said Mike Allen, co-founder and chief science officer of Seaweed Generation. “Once it hits that beach, it's too late. And the second major thing is as soon as it hits that beach, it's degrading.”

Inspired by manta rays, they created a seaweed-wrangling robot called the AlgaRay. Gliding slowly across the ocean’s surface, it collects noxious seaweed and dives 200 meters to release it. At that depth, the air sacs keeping the seaweed buoyant compress, and the Sargassum sinks to the ocean floor. It then repeats this sea-cleaning action four to six times per hour.

“What I like about it is its elegance from a carbon perspective, because these robots are solar-powered and the energy footprint is tiny,”  Allen said. “All we're doing is literally moving seaweed biomass about 200 meters. That's all our energy costs.” 

These robotic maneuvers have two main benefits: ridding the surface of algae and sequestering carbon. Sargassum uptakes carbon for photosynthesis from the ocean water surrounding it. And the ocean is a giant carbon sink, absorbing about 31 percent of carbon dioxide emissions from the atmosphere. While seaweed can sink naturally to the ocean floor, the AlgaRay aims to amplify this process. 

At depths below 1,000 meters, the seaweed and the majority of its carbon should stay there for hundreds of years, according to Seaweed Generation’s estimates. 

In fact, the deep sea is something of a black hole for carbon. The amount stored there is 17 times more than that found in the surface ocean, the land and the atmosphere combined, Allen said.

Just one of Seaweed Generation’s offshore models of the AlgaRay should be able to sink 80,000 tons of Sargassum per year, Allen said. That’s equivalent to removing over 8,000 tons of carbon dioxide, which is the same impact as growing over 132,000 tree seedlings for 10 years.

Challenges of seaweed sequestration

Seaweed Generation is off to a great start, but a few obstacles lie on the road ahead. While clearing away Sargassum removes the problem for us, the ocean floor is not devoid of life. However, the company intends to spread out the deposits of seaweed to mimic the natural process of Sargassum sinking, Allen said. 

Further, the AlgaRay's activity is complemented by robots that monitor the ocean surface where Sargassum is removed and the ocean floor where it sinks — measuring the resilience of these deep-sea ecosystems. After preventing a mountain of Sargassum from washing ashore, the company intends to measure the recovery of sensitive coastal ecosystems like coral reefs and mangroves.

Another major challenge lies in reaching customers. “Our problem is the market,” Allen said. “We can scale very quickly, but ultimately we need customers and people who are going to buy carbon dioxide removal, and at the moment it's a voluntary market.” 

Despite this, Seaweed Generation is moving ahead with its plans. “We have an agreement with the Antiguan government, which we’ll be signing soon,” Allen said. “That’s for a 49-year lease on 500 square kilometers in their exclusive economic zone, which is over 4,000 meters deep, so that's the perfect place to sink. And then we're getting lots of interest from hotel groups for much closer to shore. There, we would be intercepting Sargassum and then moving it either to a port where it can be offloaded safely, putting it somewhere else, or taking it out to sea and sinking it.”

Ocean conservation

At the end of the day, algal blooms are not an easy problem to solve. They’re linked to a web of widespread phenomena, including nutrient runoff. 

“The reason why this Sargassum is growing is a combination of a lot of factors, but none of these factors are going away,” Allen said. “You've got deforestation leading to rapid runoff. You’ve got intensive fertilizer use and nutrients running out into the Mississippi Delta, the Amazon and the Congo, and then all coming out into the Atlantic. The oceans are absorbing more and more [carbon dioxide], so there's more [carbon dioxide] available. And you've got ocean warming … It just seems to be nature's response to all those environmental perturbations.”

Addressing the problem head-on, and cleaning up major rivers from the Mississippi to the Orinoco, would help mitigate the blooms. While this would require major efforts, it may be the most permanent solution. In the meantime, companies like Seaweed Generation are doing their best to find Sargassum’s silver lining.