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Deep-sea viruses and bacteria battle beneath the waves

The bacteria that live near hydrothermal vents on the ocean floor deal with bone-crushing pressures and noxious plumes of sulfurous water at temperatures above 500 degrees Fahrenheit. As if this environment were not inhospitable enough, scientists at the University of Michigan recently discovered that these bacteria must also battle a group of viruses eager to hijack their molecular machinery for selfish ends.

The microbiologists traveled to the western Pacific Ocean and the Gulf of California, collecting samples from over 6,000 feet deep using an unmanned submarine from the Woods Hole Oceanographic Institution. By sequencing the bits of DNA found near the ocean floor, they determined that a common sulfur-consuming bacterium called SUP05 was accompanied by five predatory viruses previously unknown to science. Although viruses commonly prey on marine bacteria, they had never been found in chemosynthetic organisms, which use mineral compounds for energy instead of light.

What’s more, the viral genomes possessed sequences that closely mirrored the genes used by the bacterium to process sulfur compounds into usable energy. Karthik Anantharaman, first author of the study published online today in Science, said that the viruses inserted these auxiliary metabolic sequences into the bacteria, causing them to speed up their growth and reproduction. “We suspect that these viruses are essentially hijacking bacterial cells and getting them to consume elemental sulfur so the viruses can propagate themselves,” explained Anantharaman.

Although the origins of these viral SUP05-like genes are unclear, the scientists pointed to other studies in which viruses and bacteria swap genetic material, a phenomenon called lateral gene transfer. Said Gregory Dick, coauthor of the study and leader of the research team, “there seems to have been an exchange of genes, which implicates the viruses as an agent of evolution.” Dick added that the viruses could “serve as a reservoir of genetic diversity” from which the bacteria might draw sequences for themselves.

Because these bacteria are important in the cycling of sulfur, understanding how they respond to challenges like viruses is crucial to managing global biogeochemistry, and they also form the basis for a complex ocean floor ecosystem of giant tubeworms, crabs, and molluscs. Additionally, SUP05 bacteria may be important to global climate change, as previous studies have suggested that they produce the greenhouse gas nitrous oxide.