The next frontier in farming? The ocean

For centuries, it's been treasured in kitchens in Asia and neglected almost everywhere else: those glistening ribbons of seaweed that bend and bloom in cold ocean waves.

Today, seaweed is suddenly a hot global commodity. It's attracting new money and new purpose in all kinds of new places because of its potential to help tame some of the hazards of the modern age, not least climate change.

In London, a startup is making a plastic substitute out of seaweed. In Australia and Hawaii, others are racing to grow seaweed that, when fed to livestock, can cut methane from cow burps. Researchers are studying just how much carbon dioxide can be sequestered by seaweed farms as investors eye them as a new source of carbon credits for polluters to offset their greenhouse gas emissions.

And in South Korea, one of the most established seaweed-growing countries in the world,...