The Infected Ocean

Marine Phages

Some great discoveries seem at first like terrible mistakes.

In 1986 a graduate student at the State University of New York at Stony Brook named Lita Proctor
decided to see how many viruses there are in seawater. At the time, the general consensus was that
there were hardly any. The few researchers who had bothered to look for viruses in the ocean had
generally found only a scarce supply. Most experts believed that the majority of the viruses they did
find in sea water had actually come from sewage and other sources on land.

But over the years, a handful of scientists had gathered evidence that didn’t fit neatly into the
consensus. A marine biologist named John Sieburth had published a photograph of a marine bacterium
erupting with new viruses, for example. Proctor decided it was time to launch a systematic search.
She traveled to the Caribbean and to the 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 that such 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.