Scientists discover nearly 200,000 types of viruses from the ocean

Every time you take a sip of seawater when you swim on the beach, you detect as many viruses as there are in North America.

However, despite the staggering abundance of marine viruses – and the key role that these infectious agents appear to play in global processes such as the carbon cycle – scientists still know relatively little about the variety of existing viruses. In 2015, a team listed 5,476 distinct types of viruses in the ocean. In 2016, the same team updated their account to 15,222.

But in a study published today in Cellthis number has increased to 195,728 distinct viral populations, more than twelve times more.

"It's a pretty incredible study," said Louis-Marie Bobay, a microbial genomist from the University of North Carolina at Greensboro, who did not participate in the work. "We know so little about viral ecology in much of the ocean, and it is some of the most impressive and global data ever collected."

The doubling of objectives was made possible by an ambitious global sampling expedition and more sophisticated genomic analysis.

Although the oceans cover 70% of our planet a few years ago, most of the knowledge about marine viral diversity came from a few well-studied sites. This changed with the Tara Oceans project, which aimed at a more comprehensive inventory of microbial and viral diversity by sampling around the world. Schooner Tara pioneered her way around the ocean, collecting samples of the surface at depths and from pole to pole. The new study included samples from 43 Arctic sites that were not used in the 2015 and 2016 studies.

About 40% of the new virus populations came from new Arctic samples. The rest came from reanalysis of Tara samples used for previous studies. "The algorithms we use to assemble viral genomes from pieces of DNA have improved a lot," he said. Ann Gregory, microbial ecologist at the Catholic University of Louvain in Belgium and one of the main authors of the study.

In addition to piecing fragments of DNA from fragments, Gregory and his colleagues had to find a way to categorize the variety of virus genomes that they observed. Defining a viral "species" is controversial because viruses reproduce asexually and frequently exchange DNA between themselves and their hosts. Since viruses do not contain the necessary mechanisms to replicate independently, some biologists do not consider viruses even completely "alive".

Instead of species, Gregory has classified viruses into "populations" in which "there is more gene flow in a group than between groups of viruses." If sequenced viruses shared at least 95% of their DNA, they called them members of the same distinct population. .

This method generated nearly 200,000 populations. About 90% of them could not be associated with any known viral taxonomy, which makes them totally new to science. And, although viruses are not traditionally classified in genres, as Homo for humans or staphylococcus For staphylococcal bacteria, Gregory concluded that the diversity of sampled populations was consistent with that of many new genera.

In addition, the researchers have deduced the existence of five groups of viruses at the community level that have been mapped on distinct marine ecological zones as a function of temperature and depth: Arctic, Antarctic, temperate and tropical surfaces, under – temperate and tropical surfaces and deep ocean. Within the genomes of these communities, researchers have found evidence of genetic adaptation to each ecological zone. "Temperature was the most important predictor of community structure," said Ahmed Zayed, a graduate student at Ohio State University, who led the analysis. Variable temperatures support different types of microbial host communities, Zayed explained, and the viruses adapt accordingly.

Globally, observed biodiversity patterns among viruses are somewhat at odds with established ecological trends. "There is this paradigm that diversity is highest at the equator and decreases as you move towards the poles," Zayed said. The researchers found an increase in diversity at the equator, but also a surprising diversity in the Arctic.

"We were surprised to see the Arctic as a hotspot for biodiversity, which is particularly important as these waters are among the fastest in the world due to climate change," said Matthew Sullivan, a microbiologist at Ohio State and lead author of the study. Gregory said that further research was needed to understand why the Arctic was so diverse, but that might be due to the smaller host cells that live in these cold waters. "Smaller hosts mean more hosts, which could give more opportunities for viruses to diversify."

As for whether researchers are expecting another leap forward in varieties in a few years, Sullivan does not think so. "Do I think there is more to discover? Of course, but I hope at this point that we have largely captured the abundant viruses that we can use with this method, "he added, at least until we entered completely new environments with pressures from totally different selection. "

According to Curtis Suttle, a microbial ecologist at the University of British Columbia, viruses play a major role in global biogeochemical cycles, including the carbon cycle, in which carbon moves between the biosphere and the atmosphere of the world. Earth. "I have been trying for a long time to demonstrate that marine viruses are critically important," said Suttle, who did not participate in the new study. "Communicating this type of data to the community is extremely important in understanding the role of viruses in global processes."

Suttle explained that the oceans currently absorb about half of the carbon emissions caused by humans, and that the amount of carbon dioxide absorbed continues to increase. Viruses affect the level of saturation: according to Suttle, 20 to 40% of the world's bacterial population is killed every day by viruses. When a bacterium is killed by a viral infection, its cell wall explodes. "All the carbon that has caused the bacteria to be released into the oceans," he said, and some of the carbon ends up being sequestered at the bottom of the ocean.

Some scientists have speculated that viruses could one day be used to alter the carbon cycle and reduce the amount of carbon dioxide in the atmosphere, according to Suttle. Zayed, who is interested in viruses as he studies phage therapy as an alternative to antibiotics for treating infections, describes the program as a potentially risky geoengineering for "phage treatment for the environment."

Whether the viral discovery has practical applications or not, Melissa Duhaime, a microbial ecologist at the University of Michigan, is excited about the sheer "cold factor" of the new study. "When you start looking at new data like this, it's like landing on Mars and looking around for the first time," said Duhaime, "but a Mars with little critters has never been described. before you look at yourself. "