Fifteen contenders are in the running for Prince William's Earthshot Prize for environmental innovation, which will be awarded in the US city of Boston on Friday.

There are five prizes, each worth £1 million (€1.16 million, $1.21 million), that recognize solutions to some of the planet's greatest environmental challenges.

Among the finalists is a Portuguese group that has developed seaweed farming technology to restore underwater kelp forests.

SeaForester places seaweed spores onto small stones and scatters them into the ocean, where the idea is that the seed new kelp forests. The innovators call the system green gravel, and say it is highly scalable and more cost effective than previous approaches.

Kelp are large brown algae that grow in dense formations like a forest, sequestering carbon and sustaining marine life by providing food and shelter.

But along coastlines from New Zealand to California, kelp forests are being killed off by a variety of factors including climate change and pollution. By planting new kelp forests, the group hopes to reverse ocean acidification, replenish fish stocks, boostbiodiversity, improve water quality and capture carbon.

SeaForester is one of three finalists in the Revive Our Oceans category, competing against a women-led Indigenous Australian group that is helping to protect the Great Barrier Reef and a Dutch initiative that has developed a technique to use a curtain of bubbles to intercept plastics before they reach oceans.

What other categories are there?

Other categories include the prize to Protect and Restore Nature, the prize to Clean Our Air, the prize to Build A Waste-Free World and the prize to Fix Our Climate.

Kheyti, an Indian startup, is in the running for the protecting nature award. Its low-cost Greenhouse-in-a-Box is designed to help small-holder farmers facing the impacts of climate change.

The company says plants grown in its greenhouses use 98% less water, have yields seven-times higher and require fewer pesticides.

So far, 1,000 farms have a greenhouse, but Kheyti is hoping to increase that number to 50,000 by 2027.

A Kenyan group called Mukuru Clean Stoves is in the running for the clean air prize with a cookstove that lives up to its name. The female-founded business says its stoves, which use biomass made from charcoal, wood and sugarcane, are cheaper and far less polluting than open fires or traditional cookstoves.

Vying for the waste-free prize is an Indian business called Phool, which turns floral waste into a leather alternative called Fleather. The group collected tons of waste flowers from temples in India to turn them into products such as incense and essential oils, before realizing they could also be used to make a product similar to leather.

One of three finalists in the climate-fixing category is a British group called Low Carbon Materials that is hoping to reduce emissions from concrete. The construction industry relies heavily on concrete, which is responsible for about 8% of global carbon dioxide emissions — more than double those from flying or shipping, largely due to the energy-intensive process required to create cement.

The other main component is the aggregate. Aggregates form 60% to 75% of concrete, and usually consist of inert granular materials such as sand, gravel, or crushed stone, which are dug up or dredged from water bodies, or made by crushing rocks.

This UK group hopes to replace the aggregate with a product called OSTO, which it says will help make zero-carbon concrete. OSTO is made from a combination of waste materials and by-products.

What is the Earthshot prize?

The Earthshot awards were founded by Prince William in 2021, with the glamourous award event attended by celebrities such as Emma Watson, Ed Sheeran and Dame Emma Thompson.

The name is a reference to US President John F Kennedy's "Moonshot" ambition to place a man on the Moon within 10 years. Much like Kennedy's Moonshot, Earthshot is scheduled to run annually for a decade.

Last year's winners included The Republic of Costa Rica for its forest restoration efforts, an Indian project to turn waste into fertilizer, a coral restoration project in the Bahamas, a food-waste reduction effort run by the city of Milan, and an international project to produce hydrogen with renewable energy.

The winners also receive support and resources from private sector businesses.