Some 300,000 marine species are known to science—about 15 percent of all the species identified on the planet. But the sea is so vast that a million or more as yet unknown species may live in its waters. Most of these aquatic species are tied together through the food web.

The foundation of the sea's food chain is largely invisible. Countless billions of one-celled organisms, called phytoplankton, saturate sunlit upper-ocean waters worldwide. These tiny plants and bacteria capture the sun's energy and, through photosynthesis, convert nutrients and carbon dioxide into organic compounds. On the coast, seaweed and seagrasses do the same thing.

Together, these humble plants play a large role: They are the primary producers of the organic carbon that all animals