A Planet of Viruses - Carl Zimmer [16]
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. 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