Not all viral infections are nasty. Marine bacteria thriving in extreme heat and harsh chemicals spewing from deep-sea hydrothermal vents may get significant help from viruses, researchers report online in the Aug. 21 ISME Journal.
Viruses that inject themselves into the marine bacteria play an “incredibly important” role in shaping these aquatic ecosystems and might even encourage evolution in microbes, the scientists say.
“The real splash is that at the deep sea vents we found consistently high levels of bacteria that seemed to harbor these cryptic viruses,” says study co-author Eric Wommack, a viral ecologist at the University of Delaware in Newark. Virus-wielding bacteria have been found in other marine environments, but this study is the first to investigate the role that viruses play in deep-ocean ecosystems.
What’s more, the vent viruses appear to harbor genes never before encountered in any other organism, says the study’s lead author, environmental virologist Shannon Williamson of the J. Craig Venter Institute in La Jolla, Calif.
Based on that, “I am willing to bet that these viral populations are the largest reservoir of unknown genetic diversity on the planet,” Wommack says.
The sheer abundance of virus-harboring microbes and virus material at these hydrothermal vents suggests their role is pivotal, Williamson says. Analyzing the water column from the sea surface down to the 2,500-meter-deep sea vents of the East Pacific Rise, the team found that, around the hydrothermal vents, virus particles outnumber bacterial cells by a factor of ten.
The viruses inject their DNA into the bacteria, but do not always kill the bacteria. Instead, the viruses can enter the cells and live in a temperate or “cryptic” state where they incorporate their genes into bacterial DNA. When the cells replicate, the virus genomes replicate right along with them.
The viral genes could help the bacteria to survive in the waters around the vent, which can reach 20 to 70 degrees Celsius, the researchers suggest. Surrounding ocean water is as cold as 2º Celsius.
Environmental triggers such as ultraviolet radiation or harsh chemicals can spur viruses into their active, cell-destroying state. Yet in the harsh environment of the hydrothermal vents, where sulfides and iron compounds are prevalent, most of the viruses remained dormant inside the bacteria. “So the next question is,” Wommack says, “are the viruses carrying certain genes vital to the bacteria’s survival?”
The bacteria are “like a ticking time bomb,” he says. Some of the viruses will become active, destroying their host bacteria to distribute their genes. The genes can also be exchanged from one bacterium to another. This swapping around of the beneficial virus genes may not only drive the evolution of the microbes, but also ensure that the viruses maintain their populations, Wommack says.
That’s possible, comments microbiologist Kathleen Scott of the University of South Florida in Tampa. “The authors have discovered something really interesting down there, and it will be exciting to see further sampling.”
In the next study, Williamson and colleagues plan to further analyze the viral and bacterial genomes. And if the researchers find an extraordinarily high proportion of viral genes never before seen elsewhere, “that will be a smoking gun,” Wommack says. “It would mean that the viruses carry certain genes that help the bacteria cope in the harsher vent waters.”
