Seaweeds, those extensive and diverse ecosystems that carpet our coasts, are much more than simple aquatic plants. The report “The State of the World’s Seaweeds 2025” reveals that these organisms are a vital part of our planet, forming habitats that both animals and people depend on. They represent the largest vegetated marine habitats on the planet, far suring seagrasses, coral reefs, mangroves, and salt marshes combined. Their role is fundamental to the functioning of global marine ecosystems, food security, and the livelihoods of millions of people. This new and exhaustive report, prepared by leading scientists, offers us an updated view of their situation, identifying both their immense value – with important roles in combating the climate crisis, plastic pollution, and world hunger – and the growing threats they face, despite often being underestimated.
The report’s authors argue that this perception urgently needs to change. This document arises from the imperative need to protect these ecosystems in the face of the triple planetary crisis: climate change, pollution, and biodiversity loss. Despite their importance, red, green, and brown algae, and the habitats they form, are often overlooked in conservation and environmental management policies. The report seeks to close this gap, laying the groundwork for a “Seaweed Revolution” that will ensure their future.
Dr. Sophie Corrigan, the lead author of the report, emphasizes that “urgent action” is needed to fully assess how threatened seaweeds are, and that a global effort is required to help save them. “Seaweeds sometimes have a bit of a negative reputation as slimy, unpleasant things, but they’re really beautiful and important,” Sophie explains. “Food, medicines, and even the cosmetics industry rely on seaweed for the properties they can offer. We still haven’t realized the full potential of seaweeds and their uses.”
A universe of diversity: Getting to know the world of seaweeds
Seaweeds are a diverse group of macroalgae that have inhabited the Earth for more than a billion years. With ancestors of red seaweeds dating back 1.6 billion years, being some of the oldest known multicellular organisms, followed later by green and brown seaweed species. The report highlights this astonishing diversity and extent. With more than 12,277 species described to date, and an estimate that could reach 24,000, their variety is immense. This biological richness faces the challenge of “cryptic diversity” – species that appear identical but are genetically distinct – and a shortage of specialized taxonomists. “There aren’t a lot of scientists who study seaweed taxonomy, and there are also a lot of seaweeds that are difficult to access,” Dr. Corrigan explains. “We think there are probably many endemic species in areas we haven’t explored yet, especially in our ocean depths.”
Meanwhile, Professor Juliet Brodie, a seaweed expert and co-author of the report, adds that “seaweeds are extraordinary” and need more research to be understood. “This is one of the most challenging times for seaweeds but also one of the most exciting for scientists,” Juliet adds. “We’re in a new age of discovery but a lot of it is disappearing before we can really study it. The ‘State of the World’s Seaweeds’ report brings together our collective knowledge but also highlights what we don’t know.”
Global distribution and major seaweed habitats
Having been present for so long, they are an integral part of Earth’s ecosystems. Collectively covering an area almost the size of Australia, they form the planet’s largest vegetated marine habitats, which many other species depend on. Seaweeds are found in all the world’s oceans, from the poles to the tropics, and the main habitats they form, ordered by their estimated coverage, include:
- Rhodolith beds: Formed by unattached calcareous red algae, they are the most extensive algal habitat, covering about 4.12 million km².
- Fucoid forests: Dominated by intertidal and shallow subtidal brown seaweeds, they span over 2.5 million km².
- Kelp forests: Impressive underwater structures formed by large brown seaweeds, covering approximately 1.47 million km².
- Crustose coralline algae: Adhering to hard substrates, they play a crucial role in reef formation.
- Halimeda meadows: Calcareous green algae that form meadows and sedimentary mounds in tropical and subtropical waters.
- Seaweed turfs: Dense, low-lying communities of small filamentous algae.
- Deep-water communities: “We know about deep-water seaweed habitats that stretch to 200 meters below the surface of the water off Bermuda and in the Gulf of Mexico,” adds Professor Brodie. “There must be other habitats like this around the world, but because they’re hard to study we don’t know about them.”
- Pelagic floating seaweeds: Such as the enormous rafts of Sargassum.
- Cultivated seaweeds: Although artificial, these dense monocultures are increasingly extensive.
It is crucial to highlight that these habitats often coexist and interact. Calcified red algae, for example, cement corals together and allow for the formation of extraordinarily diverse reefs.
The multifaceted importance of seaweeds: Beyond just greenery
The report underscores the contribution of seaweeds to the 17 Sustainable Development Goals (SDGs). Their ecosystem services are classified into four main categories: ing, regulating, provisioning, and cultural.
for biodiversity and fisheries
Seaweeds are ecosystem engineers, creating shelter, food, and breeding grounds for an immense variety of species, including many of commercial importance. Just one square kilometer of kelp forest, for example, is estimated to fish that collectively weigh nearly 240,000 kilograms. It is estimated that one hectare of kelp forest can annually produce almost 2,400 kg of fish biomass, with a harvest value of about $29,900 USD.
Climate regulation and water quality
Seaweeds also play a significant role in cleaning up our world. With their extensive coverage and high productivity, they have a crucial role in the carbon cycle. They absorb carbon dioxide at a much greater rate than terrestrial plants and can help absorb pollution from the oceans. It is estimated that seaweed beds could globally sequester about 175 million tons of carbon per year.
Provision of food, raw materials, and economic opportunities
Today, the global seaweed industry, valued at about $17 billion USD and 97% dominated by aquaculture, is booming. They are used for human and animal food, fertilizers, pharmaceuticals, cosmetics, and are emerging as alternatives to plastic and for construction.
Cultural services and empowerment
Seaweeds are intrinsically linked to the cultural identity and livelihoods of many coastal and indigenous communities. The industry is also an engine of empowerment, especially for women in developing countries.
A threatened ecosystem: Challenges to seaweed survival
The report is clear: seaweeds face multiple threats, exacerbated by the climate crisis. Climate change, pollution, and overharvesting are pushing seaweeds to their limits, yet conservation efforts are sparse. Unfortunately, seaweeds are currently under pressure from all sides.
The climate crisis as a primary threat
Climate change is the greatest threat they face, as it affects the oceans in various ways. “Higher temperatures, marine heatwaves, and ocean acidification are making parts of the ocean unsuitable for seaweeds to live in,” Dr. Corrigan explains. “While some seaweeds may be able to survive by moving to cooler areas, many will not. Kelp forests, which much of our marine biodiversity in temperate regions, are set to lose a significant amount of their habitat by the end of the century.” Climate change also facilitates the spread of diseases and invasive species, multiplying their impact.
Direct anthropogenic pressures
- Overharvesting: Intensive harvesting, especially of slow-growing species like rhodoliths, can be devastating.
- Destructive fishing methods and overfishing: Bottom trawling damages habitats, while overfishing removes predators of herbivores like sea urchins, allowing them to overgraze seaweed beds.
- Pollution and coastal development: Nutrients, untreated wastewater, heavy metals, and sediments degrade water quality. Pollution is also a problem, as it can smother slow-growing seaweeds.
- Pests and diseases: Particularly in aquaculture.
- Invasive non-native species: They compete with and alter native habitats.
Towards conservation and restoration: The path forward and the role of science
Reversing the current decline of seaweeds is no easy task, especially since there is much that scientists still do not understand about these organisms. Of an estimated 24,000 species, only 12,000 have been described.
Deficiencies in knowledge and current protection
Despite their importance, seaweeds lack adequate protection. Even when seaweeds are well-known, protections are limited. Kelp forests are among the best-studied seaweed habitats, yet only 16% are in a protected area, and just 1.6% in regions with the strictest protections. For other seaweed habitats, the level of protection is unknown but probably minimal.
“While interest in kelp forests is rightfully building, other seaweeds have been left behind,” Dr. Corrigan says. “This is something we want to change, and we’re hoping that seaweeds will benefit from the 30 by 30 initiative as more protected areas are created or expanded in the coming years.”
Advances and challenges in restoration
Restoration has focused on kelp forests, with initiatives like the Kelp Forest Alliance (KFA) and its “Kelp Forest Challenge” aiming to protect and restore 4 million hectares by 2040. Methods include herbivore control, transplantation, seeding, and artificial reefs.
Ensuring a sustainable future: The “Seaweed Breakthrough”
The report culminates with a call to action and the presentation of the “Seaweed Breakthrough” initiative. This initiative seeks to catalyze global action to conserve, restore, and sustainably manage seaweed ecosystems. “Safeguarding the future of seaweeds will require a global movement to unite governments, researchers, industries, charities, Indigenous Peoples, and local communities in protecting seaweeds. This report is the first step in that process,” states Dr. Corrigan.
The study’s authors have outlined four high-level goals for 2030: stop loss, protect and conserve 30%, restore millions of hectares, and secure financial investment. These goals, for which “The State of the World’s Seaweeds” report will provide the foundational evidence, will be included in the “Seaweed Breakthrough” to be launched at the UN Oceans Conference in June, giving these organisms the best chance of survival.
Implications for aquaculture
For the aquaculture sector, this report is crucial. The health of wild populations is fundamental to the industry, whether for direct harvesting or as a source of genetic material. Seaweed aquaculture not only provides food and raw materials but can also contribute to restoration and conservation. However, the industry faces challenges such as biosecurity and the need for genetic diversity.
Conclusion: A call to action for our oceans and our Future
“The State of the World’s Seaweeds 2025” is a testament to the extraordinary importance of these ecosystems and the urgent need for action. From their role in climate regulation and biodiversity to their growing relevance in food security and the bioeconomy, seaweeds are a pillar for the health of the planet and human well-being.
The report identifies critical knowledge gaps. But it also offers hope, highlighting scientific advances and collaborative initiatives. For the aquaculture sector, the message is clear: a prosperous and sustainable future is inextricably linked to the conservation of wild marine meadows. Protecting and valuing seaweeds is an investment in our own resilience and prosperity.
Prof Juliet Brodie
PhD Researcher
Natural History Museum, London
Email: [email protected]
Phone: 00 44 (0)207 942 5910
Reference (open access)
Corrigan, S., Cottier-Cook, E.J., Lim, P.-E. & Brodie, J., 2025. The state of the world’s seaweeds. Natural History Museum, London. doi.org/10.5519/4ln9oqk7.
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
