"Our findings suggest that viruses in the dark oceans indirectly access vast energy sources in the form of elemental sulfur," said University of Michigan marine microbiologist and oceanographer Gregory Dick, whose team collected DNA from deep-sea microbes in seawater samples from hydrothermal vents in the Western Pacific Ocean and the Gulf of California.
"We suspect that these viruses are essentially hijacking bacterial cells and getting them to consume elemental sulfur so the viruses can propagate themselves," said Karthik Anantharaman of the University of Michigan, first author of a paper on the findings published this week in the journal Science Express.
"Viruses play a cardinal role in biogeochemical processes in ocean shallows," said David Garrison, a program director in the National Science Foundation's (NSF) Division of Ocean Sciences, which funded the research. "They may have similar importance in deep-sea thermal vent environments."
The results suggest that viruses are an important component of the thriving ecosystems--which include exotic six-foot tube worms--huddled around the vents.
"The results hint that the viruses act as agents of evolution in these chemosynthetic systems by exchanging genes with the bacteria," Dick said. "They may serve as a reservoir of genetic diversity that helps shape bacterial evolution."
The samples were taken at depths of more than 6,000 feet, near hydrothermal vents spewing mineral-rich seawater at temperatures surpassing 500 degrees Fahrenheit.
Back in the laboratory, the researchers reconstructed near-complete viral and bacterial genomes from DNA snippets retrieved at six hydrothermal vent plumes.
The genetic data suggest that the viruses prey on SUP05. That's not too surprising, said Dick, since viruses are the most abundant biological entities in the oceans and are a pervasive cause of mortality among marine microorganisms.
"We hypothesize that the viruses enhance bacterial consumption of this elemental sulfur, to the benefit of the viruses," said paper co-author Melissa Duhaime of the University of Michigan. The revved-up metabolic reactions may release energy that the viruses then use to replicate and spread.
"There seems to have been an exchange of genes, which implicates the viruses as an agent of evolution," Dick said.
On Earth's surface, sunlight provides the energy that enables plants to remove carbon dioxide from the air and use it to build sugars and other organic molecules through the process of photosynthesis.
Sulfur was likely one of the first energy sources that microbes learned to exploit on the young Earth, and it remains a driver of ecosystems found at deep-sea hydrothermal vents, in oxygen-starved "dead zones" and at Yellowstone-like hot springs.
Dick said the new microbial findings will help researchers understand how marine biogeochemical cycles, including the sulfur cycle, will respond to global environmental changes such as the ongoing expansion of dead zones.
In addition to Anantharaman, Dick and Duhaime, co-authors of the Science Express paper are John Breir of the Woods Hole Oceanographic Institution, Kathleen Wendt of the University of Minnesota and Brandy Toner of the University of Minnesota.
The project was also funded by the Gordon and Betty Moore Foundation and the University of Michigan Rackham Graduate School Faculty Research Fellowship Program.
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