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The big question, in the mid-21st century, wasn鈥檛 how to transition away from fossil fuels 鈥� that process became self-propelling as renewable energy became ever cheaper 鈥� but how to get carbon dioxide out of the atmosphere.

In the end, achieving the Great Drawdown, as it became known, required many different approaches, including biological, chemical and technological methods of removal. This dispatch examines two solutions that used the under-appreciated, near-magical power of seaweed: one through robotics and the other through bioengineering. The robot ate vast algal blooms; the genetic engineering approach modified a seaweed to self-destruct and sink to the ocean floor. Together, they removed a significant fraction of carbon from the atmosphere and stored it in the deep.

The robot was a design . Mantas are supremely elegant, but swim gormlessly, with their mouths open, devouring billions of tiny organisms a day. This action suggested a way to harvest a monstrous bloom of seaweed that grew each year in the Atlantic.

The was a vast swathe of floating Sargassum seaweed stretching from the Gulf of Mexico to the west coast of Africa, first detected by NASA satellites in 2011. Each year, the algae fed on fertiliser pollution and sewerage runoff and caused ecological damage by swamping corals, choking wildlife and rotting on beaches in huge quantities. In 2019, Mexico declared a 鈥溾��; in 2023, the mass at 12 million tonnes and sprawled across 8000 kilometres.

The solution was to deploy swarms of autonomous ray-like clean-up robots. The were first designed at the University of Exeter, UK, in the 2020s. Deployed in swarms in the 2030s, they glided like gigantic mantas through blooms of Sargassum, inhaling the algae and storing it in their bodies. The mouth of an AlgaRay could be 10 metres wide, and the body could store 20 tonnes of algae at a time. Now, here鈥檚 the clever bit. Once its belly was full, the robot dived to around 200 metres. At that depth, the air bubbles in the seaweed were crushed and it became negatively buoyant. The algal payload was released and sank to the ocean floor, where it was effectively in low-oxygen carbon storage for at least 100 years.

A modified green seaweed, Halimeda, could be selectively deployed, bloom and fix carbon, then sink and die

A single AlgaRay robot, powered by the sun, was able to do this many times a day, sinking over half a million tonnes of algae a year. Cheap to make, a swarm of just a few thousand robots easily cleaned and locked up the biggest annual blooms of seaweed. The ecological impact on the deep ocean was minimal. After all, the ocean contains around 37 trillion tonnes of carbon, more than the terrestrial biosphere. The ecological and financial relief at the surface was immense.

Yet as much as AlgaRay helped remove carbon, we needed to get more out. That鈥檚 where the second Big Seaweed idea came in. It involved the genetic engineering of self-destructive seaweeds that buried themselves after blooming.

Kelp, the large brown seaweed that grows up to a metre a day, was already known as a potential tool for carbon sequestration. One report said it could draw down up to CO₂ per year.

Giant bladder kelp has buoyant bladders of air on its blades. This species was genetically engineered to grow free-floating, and then it could be grown, harvested and sunk in the remote ocean the same way as the Sargassum. Other forms of modification became more ambitious.

Caulerpa is a genus of seaweed that spread invasively in many parts of the world in the early 21st century. In the mid-2030s, a version of the alga was engineered to grow in a free-floating form and to precipitate heavy metals into its body. Blooms of Caulerpa flourished in polluted waters, growing nodules of metal pollutants until they could be harvested by electromagnet-wielding robots and sunk, or plunged to the deep under the weight of their own nodulation.

Modified species were made to clean up 鈥溾�� of phosphorus pollution in the Gulf of Mexico. A green seaweed, Halimeda, for being able to make calcium carbonate, was modified into a free-swimming form. The new Halimeda, genetically engineered with a death switch, could be selectively deployed, bloom and fix carbon, then sink and die.

The as carbon levels fell. The mass farming of kelp forests and other seaweeds boomed, boosting biodiversity, providing nutrient-rich meat substitutes as well as ingredients for alternative plastics and even clothing. Children became able to identify a range of species and ate algae for snacks. Their microbiomes thanked them for it.

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Invention

Atmosphere-restoring seaweeds

Time stamp

2030s

Tagline

The algae that fed the Great Drawdown

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