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Think of algae and you'll probably think trouble. Algal blooms turned the diving pool green at the 2016 Rio Olympics. Smelly seaweed ruins many a trip to the beach. But Alison Smith, Professor of Plant Biochemistry at the University of Cambridge, argues that we should appreciate algae more. They range in size from giant kelp to microscopic diatoms. They are found all over the world from the Arctic to the Tropics, live in water and make energy from the sun by photosynthesis. Alison Smith talks to Jim al-Khalili about algae's sometimes bizarre biochemistry and how she discovered that they obtain their vitamins from bacteria they live alongside in the sea. They also discuss how we are beginning to farm algae to make all kinds of chemicals, from food stuffs to biofuels. We may become very dependent on them when the oil runs out.