One of nature’s strangest symbioses gives up its secrets

UNLIKE THEIR terrestrial cousins, sea slugs are widely regarded as among the most beautiful animals on Earth. In some cases, part of that beauty comes from subcellular structures called chloroplasts, which they extract intact from the algae they eat and then sequester in gut diverticula for purposes of their own.Chloroplasts are the distant descendants of photosynthetic bacteria that became symbiotic, a bit less than 2bn years ago, with an organism ancestral to today’s algae and plants. From their hosts’ point of view, their purpose is to use energy from sunlight to split water into hydrogen and oxygen, and then react the hydrogen with carbon dioxide to form molecular precursors of glucose. Further reactions turn these basic materials into other biochemicals, too. Rearranging the carbon, hydrogen and oxygen atoms and adding nitrogen, for example, results in amino acids, the building blocks of proteins.