What is the world's largest type of seaweed?

The world's largest type of seaweed is the giant kelp, or Macrocystis pyrifera. It is a species of brown algae that can grow up to 150 feet (45 meters) in length and can be found in cool coastal waters around the world.

Giant kelp forms dense underwater forests that provide habitat for a variety of marine life, including fish, sea urchins, and sea otters. It also has several commercial uses, such as in food, fertilizer, and cosmetic products.