In 2020, coastal farmers worldwide harvested some 36 million tonnes of macroalgae, also known as seaweed or kelp; 97 per cent of these had grown in specially established algae farms. The seaweeds are used as food, animal feed or fertilizer, primarily in coastal countries. But their components are also traded worldwide because they are needed in the production of food, pharmaceuticals and cosmetics. And industrial companies are increasingly using algal biomass to produce biofuels – for instance in the People’s Republic of China, which now produces 59.5 per cent of the world’s traded macroalgae.
The term “macroalgae” covers organisms from three taxonomic groups: brown algae with some 2000 species, red algae with more than 7200 species and green algae with more than 1800 species. However, globally only 27 species were used for macroalgae farming in 2019, primarily red and brown algae.
Macroalgae are highly productive organisms. They grow quickly and sequester between 91 and 522 grams of carbon per square metre of sea surface, filtering the nutrients (nitrogen and phosphorus) they need to grow from the seawater. They thus not only clean the water and help combat the eutrophication of coastal waters, but they also locally reduce seawater acidification as they absorb carbon dioxide from the water for their photosynthesis and store the carbon in their tissue.

fig. 5.15 > Research is currently underway to determine whether the ocean’s CO₂ uptake could be increased by sinking floating Sargassum algae.

These climate-friendly properties and their comparatively simple cultivation gave experts the idea of taking more carbon dioxide out of the atmosphere by creating huge algae farms in which macroalgae photosynthesize and grow − both near the coast and in the open ocean. The resulting algae forests or mats could then be put to three climate-friendly uses:

• as feedstock for bioenergy production with subsequent carbon dioxide capture and storage (BECCS),
• as feedstock for the production of biochar, which could subsequently be used, among other things, to increase the soil carbon content and water retention capacity of agricultural lands, and
• as biomass to be used for rapid intentional deep-ocean sinking.

The rapid sinking of large amounts of biomass could accelerate the organic biological carbon pump (see Chapter 2), giving marine organisms in the water column less time to consume or decompose the macroalgal biomass. Significantly greater quantities of biomass could reach depths of more than 1000 metres or even the seabed and be decomposed there or permanently stored in the sediment. In both cases, the carbon contained in the macroalgal biomass would be locked away at depth for a long time. For comparison: If biomass sinks to a depth of 500 to 3000 metres, it takes more than 50 years, depending on the ocean region, for the carbon it contains or possible degradation products to rise back to the sea surface.
A rapid expansion of large-scale algae farming is currently not taking place because seaweed farms have so far mainly been operated in coastal waters, where both space and nutrient availability are limited. In addition, coastal waters are warming up with climate change, which makes algae farming even more difficult. Scientists and enterprises in the field are therefore trying to develop cultivation methods for open-ocean macroalgae farming that could be used over thousands of square kilometres. There’s no shortage of ideas. These include, among others:

• free-floating (Sargassum) macroalgae cages that are towed from one nutrient-rich marine region to the next by remote-controlled tug boats to achieve maximum growth rates;
• macroalgae cultivation platforms that float nine metres below the sea surface during the day and are towed down into nutrient-rich deep water at night;
• cultivation platforms that are sunk and unloaded as soon as they are completely covered in macroalgae. The aim here would be to transport the algal biomass to great depths as quickly as possible.

Limits and risks of macroalgae farming

Even though large-scale algae cultivation is one of the so-called nature-based climate solutions, it has disadvantages for both humans and the environment: Where many macroalgae grow, an ecosystem-wide competition for the nutrients dissolved in the ocean water begins. If the algae are harvested and thus removed from the sea, the marine biocoenoses will not only lack an important nutritional basis, but in the long term the ocean’s material cycle will also lack the nutrients contained in the algal biomass. This deficiency applies first and foremost to coastal waters that are not over-fertilized and as a consequence means that the productivity of the marine region in question decreases.
Initially, this dangerous chain reaction would result in less phytoplankton and fewer macroalgae, followed by fewer animals surviving not long after, as they run out of food. In China’s macroalgae-farming areas, experts have been trying to solve this nutrient problem for years. So far, however, every promising approach ultimately resulted in further difficulties in macroalgae cultivation. So there is still no real solution to date.
The natural nutrient deficiency in marine regions such as the subtropical gyres also means that large-scale open-ocean macroalgae farming could not be extended to the entire ocean. It would probably only be promising in the upwelling areas, i.e. those marine regions where nutrient-rich deep water rises to the sea surface, as well as everywhere where humans either succeed in pumping deep water to the sea surface or regularly pull the growth platforms down from the light-filled sea surface layer into nutrient-rich deep water.

fig. 5.16 > In November, the macroalgae farms in the Chinese province of Fujian can already be spotted from a distance. By this time of year, the red and brown algae cultivated here have grown sufficiently and are being hauled in by the fishermen, rope-by-rope.

When researchers recently simulated the effects of large-scale open-ocean macroalgae mariculture and deep-sea sinking in an Earth system model, further consequences and risks for the ocean system became apparent.
The rapid sinking of biomass into water depths of more than 3000 metres and the thus reduced natural decomposition of organic material at medium water depths would decrease the oxygen deficiency zones in this part of the water column. At the same time, however, oxygen consumption would increase at greater depths and on the seabed. There, marine organisms would decompose a large proportion of the algal biomass, resulting in the formation of large oxygen deficient zones in the deep sea; at the same time the deep water would acidify due to the microbial release of carbon dioxide. But that’s not all: as more biomass would also be stored in the seabed, the ocean would lack the nutrients it contains in the long term. This, in turn, would further reduce phytoplankton growth and thus result in less marine life.
In consequence, it is already foreseeable that macroalgae farming will by no means be the sole solution to our climate problem. Instead, it is one of a multitude of methods that we can use to increase the ocean’s carbon dioxide uptake. Its large-scale deployment, however, has drawbacks that first must be thoroughly weighed against potential benefits.