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
atmosphere cooler. When microbes in the ocean die, some of their carbon rains down to the sea floor.
Over millions of years, this microbial snow can steadily make the planet cooler and cooler. What’s
more, these dead organisms can turn to rock. The White Cliffs of Dover, for example, are made up of
the chalky shells of single-cell organisms called coccolithophores.
Viruses kill these geoengineers by the trillions every day. As their microbial victims die, they spill
open and release a billion tons of carbon a day. Some of the liberated carbon acts as a fertilizer,
stimulating the growth of other microbes, but some of it probably sinks to the bottom of the ocean. The
molecules inside a cell are sticky, and so once a virus rips open a host, the sticky molecules that fall
out may snag other carbon molecules and drag them down in a vast storm of underwater snow.
Ocean viruses are stunning not just for their sheer numbers but also for their genetic diversity. The
genes in a human and the genes in a shark are quite similar—so similar that scientists can find a
related counterpart in the shark genome to most genes in the human genome. The genetic makeup of
marine viruses, on the other hand, matches almost nothing. In a survey of viruses in the Arctic Ocean,
the Gulf of Mexico, Bermuda, and the northern Pacific, scientists identified 1.8 million viral genes.
Only 10 percent of them showed any match to any gene from any microbe, animal, plant, or other
organism—even from any other known virus. The other 90 percent were entirely new to science. In
200 liters of seawater, scientists typically find 5,000 genetically distinct kinds of viruses. In a
kilogram of marine sediment, there may be a million kinds.
One reason for all this diversity is that marine viruses have so many hosts to infect. Each lineage of
viruses has to evolve new adaptations to get past its host’s defenses. But diversity can also evolve by
more peaceful means. Temperate phages merge seamlessly into their host’s DNA; when the host
reproduces, it copies the virus’s DNA along with its own. As long as a temperate phage’s DNA
remains intact, it can still break free from its host during times of stress. But over enough generations,
a temperate phage will pick up mutations that hobble it, so that it can no longer escape. It becomes a