Giant kelp (Macrocystis pyrifera) underwater. Giant kelp is the largest of the Pacific kelps and can grow more than 45 centimetres per day. It is usually found in large beds or forests. Giant kelp is an ecologically and economically important species of marine alga. Giant kelp forests are home to hundreds of marine invertebrates, fish and mammals and are harvested for their algin content. This substance is an ingredient in a wide range of household products, including toothpaste, cosmetics and ice cream. Photographed off the coast of California, USA.