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Giant kelp (Macrocystis pyrifera). This is the largest of the giant kelps and can grow more than 45 centimetres a day. It is usually found in large beds or forests. The giant kelp uses pnematocysts, gas-filled sacs (orange), as floats to keep its blades near the surface where there is more sunlight for use in photosynthesis. This is an ecologically and economically important species of marine alga. Giant kelp forests are home to hundreds of marine invertebrates, fish and mammals. Giant kelp is also harvested and used as an ingredient in a wide range of household products, including toothpaste, cosmetics and ice cream. Photographed off California, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/GEORGETTE DOUWMA) === ===