From @earthskyscience

How much do oceans add to world’s oxygen?

Scientists agree that there’s oxygen from ocean plants in every breath we take. Most of this oxygen comes from tiny ocean plants – called phytoplankton – that live near the water’s surface and drift with the currents. Like all plants, they photosynthesize – that is, they use sunlight and carbon dioxide to make food. A byproduct of photosynthesis is oxygen.

Scientists believe that phytoplankton contribute between 50 to 85 percent of the oxygen in Earth’s atmosphere. They aren’t sure because it’s a tough thing to calculate.

Image credit: NASA GFSC