Scientists Discover World’s Most Abundant Virus in the Ocean

Researchers announced today the discovery of the most abundant virus in the world. Rather than attacking humans however, the virus, called a Pelagiphage, attacks the most abundant microorganism in our oceans, a type of marine bacteria called SAR11, with significant implications for how carbon moves between the atmosphere and our oceans.

The study was published in the scientific journal Nature today.

Dr. Steve Giovannoni, whose lab at Oregon State University made the discovery, said in a press release, "There's a war going on in our oceans, a huge war, and we never even saw it."

Until now.

"This is an important piece of the puzzle in how carbon is stored in the sea," he continued.

As I have covered previously here in the Microblog, microorganisms are the small engines driving the world’s biogeochemical cycles. In the lab of Dr. Giovannoni, where I work, we focus primarily on one group of these microorganisms, called SAR11, which is the most abundant in the oceans. SAR11 bacterioplankton (a term for free-living bacteria in the oceans) are heterotrophs, meaning they turn carbon into CO2, similarly to humans. Their abundance therefore makes them extremely important to the global carbon cycle. While it has been known for a long time that a class of viruses called phage attack and infect bacteria, until now, no one had been able to find a phage that infected SAR11.

A-B) Pelagiphage. C) HTVC010P, the most abundant Pelagiphage. Images courtesy of Nature magazine. Microscopy was done by Thomas Deerinck and Mark Ellisman at the National Center for Microscopy and Imaging Research.

Dr. Yanlin Zhao and Dr. Michael Schwalbach were responsible for isolating Pelagiphage here in Dr. Giovannoni's laboratory. Dr. Ben Temperton then used their genome sequences to look for genetic traces of the phage in ocean samples collected from a variety of sites. The team discovered that the Pelagiphage, HTVC010P is more numerous than any phage ever studied.

"I was amazed it hadn't been found before if it was that abundant," said Temperton.

Part of the reason it had never been seen before is because the DNA sequence of this Pelagiphage is extremely different from any other phage. One of my jobs in this research was to put together the phylogeny of these phage, or a way of measuring how similar they are to other known phage.

D) Pelagiphage. E) An infected SAR11 cell filled with Pelagiphage immediately before lysis (being split open). Images courtesy of Nature magazine. Microscopy was done by Thomas Deerinck and Mark Ellisman at the National Center for Microscopy and Imaging Research.

This discovery is important for a few key reasons. Previously, some had thought that SAR11 might be the most abundant microorganism because it was immune to phage. This hypothesis has been disproven. It also demonstrates the value of culturing microorganisms in the laboratory, not only to study the microbe, but for the purposes of finding their phage. Finally, and perhaps most importantly, having Pelagiphage now allows us to study the effects of this virus on ocean ecology. Since SAR11 is such an important part of the marine carbon cycle, the battle between these microorganisms and their phage is expected to have a direct impact on the rate at which SAR11 organisms convert carbon in the ocean to CO2. Furthermore, phage breaking out of the SAR11 cells can aid in carbon sequestration directly in a process called the "viral shunt" to the biological pump, whereby viral particles sink to the seafloor without being consumed by other organisms. Therefore, studying these Pelagiphage is a critical component of understanding the global carbon cycle.

Dr. Zhao, one of the co-primary authors, who isolated HTVC010P, reflected on the discovery, "I just wanted something new, I didn't expect it would be so abundant. Not everyone has the opportunity to work with SAR11, and the importance of the host determines the importance of the virus, so I feel honored and very thankful."

Reference: Zhao, Yanlin*, Ben Temperton*, J. Cameron Thrash, Michael S. Schwalbach, Kevin L. Vergin, Zachary C. Landry, Mark Ellisman, Tom Deerinck, Matthew B. Sullivan and Stephen J. Giovannoni. (2013) Abundant SAR11 viruses in the ocean. Nature. doi: 10.1038/nature11921. (*Equal contribution)