I can almost feel the sizzle of energy as sunlight, carbon dioxide, and water work the magic of photosynthesis while I drift, wrapped in a golden curtain of sargassum, just off Bermuda. I revel in the sensation and am thrilled to see tiny bubbles of oxygen, a by-product of photosynthesis, rise to the surface and join the oxygen produced by trillions of diatoms, blue-green bacteria, and other phytoplankton in the surrounding ultraclear water.

As a living laboratory, the Sargasso Sea—with its masses of sargassum and their cargoes of lilliputian creatures—has yielded important findings about how and why the ocean matters to everyone, everywhere, all the time.

It was in 1986 that Prochlorococcus—Earth’s smallest and most numerous photosynthetic organisms—were discovered in the Sargasso. Now known to occur globally, they churn out as much as 20 percent of the oxygen in the atmosphere. Seaweed and microscopic organisms provide oxygen for life in the sea and more than half the oxygen in the air we breathe. The carbon dioxide they capture is transformed with water into sugar, helping to fuel the complex ocean food webs that culminate in tuna, sharks, whales—and us.