You are probably familiar with the scent of rotting sargassum if you have visited a Caribbean beach in recent years. It’s okay, the floating mats have sustained ecosystems for millions of years, but not when it’s drifting offshore in its natural state. It creates hydrogen sulfide at proportions that clear hotel terraces, close beaches, and, at high enough levels, actually cause respiratory discomfort when it washes ashore in the amounts already arriving on beaches from Barbados to the Yucatán, lies in the sun for a few days, and begins to degrade. The scent is overpowering. It is the stench of something going horribly wrong on a large scale.
Over the past ten years, the seasonal phenomena known as the “Sargassum blob,” as it has been somewhat colloquially called in press reports, has grown from a somewhat limited phenomenon to a vast, year-round issue. Sargassum blooms now span thousands of kilometers over the tropical Atlantic and the Gulf of Mexico due to a combination of warmer sea surface temperatures, fertilizer runoff from agricultural land along river systems emptying into the Atlantic, and changing ocean currents. The substance serves as a vital habitat and carbon-absorbing biomass in the open ocean. It’s a different matter when it lands in drifts that are meters thick on a beach.
The AlgaRay, a robot being developed by a UK business called Seaweed Generation, tackles this issue in a way that, when put simply, sounds almost too tidy: gather the seaweed before it reaches the coast, transport it to deep water, and drown it. The manta ray-shaped robot, which is nine feet wide and powered by solar energy and lithium batteries, swims across the ocean’s surface while collecting floating sargassum mats in an open hollow structure. It doesn’t shred the seaweed or filter the water. It simply scoops.
When the robot dives, fascinating physics take place. Sargassum floats because it has gas-filled bladders, which are tiny chambers that function similarly to a life jacket in terms of buoyancy. The seaweed’s ability to rise to the surface is eliminated when water pressure reaches a depth of around 135 meters, compressing and collapsing those bladders. After descending to that compression depth and letting physics take care of the rest, the robot proceeds to a release location below 1,000 meters, where it releases its payload onto the deep ocean floor. The organic material settles there in freezing, almost anoxic water with very slow rates of breakdown; instead of cycling back via the atmosphere, the carbon it received during growth remains trapped in place.
The AlgaRay’s dual-purpose frame is attributed to the carbon sequestration perspective. As it grows, sargassum absorbs CO2, much as all other photosynthetic species. Along with the sulfide gasses, that carbon is released back into the atmosphere as it rots on a beach. The carbon is removed from circulation for periods of time that are significant for climate accounting—possibly centuries, depending on the depth and ocean chemistry—when it is sunk to depth and slowly breaks down in cold deep water. Whether large-scale deep-ocean biomass sinking could serve as a significant carbon removal technique is a topic of actual scientific interest. Currently a prototype, the AlgaRay would need a fleet that operates consistently over vast oceanic expanses in order to scale it to anything that matters for atmospheric carbon budgets.
The honest warning about the entire project is the scaling challenge. Sargassum blooms that are damaging coasts in several nations cannot yet be solved by a single nine-foot robot scooping seaweed mats in the Caribbean. The engineering needed to make it dependable in open-ocean situations, including as wave action, equipment malfunctions, and navigation in fluctuating current patterns, is still being developed. The legislative framework for deliberate large-scale ocean biomass sinking, the economic model for fleet operations, and the question of who pays are all unanswered.
