Thanks to gene borrowing, viruses may be responsible for a lot of the world’s oxygen. Much of the oxygen in the atmosphere is produced by photosynthetic microbes in the oceans. Some of the viruses that infect them carry their own genes for photosynthesis. When they invade, the viruses take charge of harvesting light. By one rough calculation, 10 percent of all the photosynthesis on Earth is carried out with virus genes. Breathe 10 times, and one of those breaths comes to you courtesy of a virus.
