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Sargasso Sea, scooping up seawater along the way. Back on Long Island, she carefully extracted the biological material from the seawater, which she coated with metal so that it would show up under the beam of an electron microscope. When Procter finally looked at her samples, she beheld a world of viruses. Some floated freely, while others were lurking inside infected bacterial hosts. Based on the number of viruses she found in her samples, Proctor estimated that every liter of seawater contained up to one hundred billion viruses.
     
    Proctor’s figure was far beyond anything that had come before. It would have surprised few scientists if she had turned out to have added on a few extra zeroes by accident. But when other scientists carried out their own surveys, they ended up with similar estimates. Scientists came to agree that there are somewhere in the neighborhood of 1,000,000,000,000,000,000,000,000,000,000 viruses in the ocean.
     
    It is hard to find a point of comparison to make sense of such a huge number. Viruses outnumber all other residents of the ocean by about fifteen to one. If you put all the viruses of the oceans on a scale, they would equal the weight of seventy-five million blue whales. And if you lined up all the viruses in the ocean end to end, they would stretch out past the nearest sixty galaxies.
     
    These numbers don’t mean that a swim in the ocean is a death sentence. Only a minute fraction of the viruses in the ocean can infect humans. Some marine viruses infect fishes and other marine animals, but by far their most common targets are microbes. Microbes may be invisible to the naked eye, but collectively they dwarf all the ocean’s whales, its coral reefs, and all other forms of marine life. And just as the bacteria that live in our bodies are attacked by phages, marine microbes are attacked by marine phages.
     
    When Felix d’Herelle discovered the first bacteriophage in French soldiers in 1917, many scientists refused to believe thatsuch a thing actually existed. A century later, it’s clear that Herelle had found the most abundant life form on Earth. Ever since Proctor’s discovery of the abundance of marine viruses, scientists have been documenting their massive influence on the planet. Marine phages influence the ecology of the world’s oceans. They leave their mark on Earth’s global climate. And they have been playing a crucial part in the evolution of life for billions of years. They are, in other words, biology’s living matrix.
     
    Marine viruses are powerful because they are so infectious. They invade a new microbe host ten trillion times a second, and every day they kill about half of all bacteria in the world’s oceans. Their lethal efficiency keeps their hosts in check, and we humans often benefit from their deadliness. Cholera, for example, is caused by blooms of waterborne bacteria called Vibrio . But Vibrio are host to a number of phages. When the population of Vibrio explodes and causes a cholera epidemic, the phages multiply. The virus population rises so quickly that it kills Vibrio faster than the microbes can reproduce. The bacterial boom subsides, and the cholera epidemic fades away.
     
    Stopping cholera outbreaks is actually one of the smaller effects of marine viruses. They kill so many microbes that they can also influence the atmosphere across the planet. That’s because microbes themselves are the planet’s great geoengineers. Algae and photosynthetic bacteria churn out about half of the oxygen we breathe. Algae also release a gas called dimethyl sulfide that rises into the air and seeds clouds. The clouds reflect incoming sunlight back out into space, cooling the planet. Microbes also absorb and release vast amounts of carbon dioxide, which traps heat in the atmosphere. Some microbes release carbon dioxide into the atmosphere as waste, warming the planet. Algae and photosynthetic bacteria, on the other hand, suck carbon dioxide in as they grow, making the