Kelp was the name given to several varieties of algae native to the oceans of planet Earth. They were also known as seaweed.
By the mid 21st century, kelp had become one of the staple food crops of Earth's Human population, with large agronomy stations established across the Pacific Ocean by companies and organizations like SeaSources and AgroInternational, which cultivated acres of kelp on the ocean surface. This industrial-grade kelp was then processed into a variety of food stuffs, including a type of coffee.
In 2045, a fungal infection known as kelpwilt began to spread and infect a number of kelp farms across the South Pacific. Later that year, agronomist Yoshi Nakamura presented an anti-fungal agent he called "Sorahlaze", which eliminated the kelpwilt within one season, and averted what could have been a global hunger crisis.
By the late 22nd century, large scale kelp production had been phased out in favor of other food production methods. (TOS novel: Strangers from the Sky)