Kelp (Macrocystis) is the largest seaweed on Earth and one of the fastest-growing organisms known, forming vast underwater forests that are among the most productive ecosystems on the planet. A root-like structure that anchors the kelp to rocks (it does not absorb nutrients). A Long, flexible stem that grows upward toward the surface and small, gas-filled floats at the base of each blade keep fronds buoyant and upright in the water column.