A Planet of Viruses by Carl Zimmer Read Free Book Online

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 permanent part of its host’s genome.
     
    As a host cell manufactures new viruses, it sometimes accidentally adds some of its own genes to them. The new viruses carry the genes of their hosts as they swim through the ocean, and they insert them, along with their own, into the genomes of their new hosts. By one estimate, viruses transfer a trillion trillion genes between host genomes in the ocean every year.
     

    Sometimes these borrowed genes make the new host more successful at growing and reproducing. The success of the host means success for the virus, too. While some species of viruses kill Vibrio , others deliver genes for toxins that the bacteria use to trigger diarrhea during cholera infections. The new infection of toxin-carrying viruses may be responsible for new cholera outbreaks.
     
    Thanks to gene borrowing, viruses may also be directly responsible for a lot of the world’s oxygen. An abundant species of ocean bacteria, called Synechococcus , carries out about a quarter of the world’s photosynthesis. When scientists examine the DNA of Synechococcus samples, they often find proteins from viruses carrying out their light harvesting. Scientists have even found free-floating viruses with photosynthesis genes, searching for a new host to infect. By one rough calculation, 10 percent of all the photosynthesis on Earth is carried out with virus genes. Breathe ten times, and one of those breaths comes to you courtesy of a virus.
     
    This shuttling of genes has had a huge impact on the history of all life on Earth. It was in the oceans that life got its start, after all. The oldest traces of life are fossils