Giant kelp looks like a kind of seaweed, but it is in fact is the largest-known kind of algae. It grows in very long strands up to 65 m in length, and is fastened to the sea-bed with a root-like organ called a holdfast. Small air bladders help to keep the kelp floating upright and spreading across the water’s surface. Kelp is harvested and used for various industrial purposes. One substance extracted from it, is called algin, and is commonly used to make ice cream.
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