Thousands of recently identified RNA viruses in the world’s oceans could exert enormous influence on ecosystems. By reprogramming the hosts they infect, they could actually have a hitherto unsuspected influence on the carbon cycle.
An RNA virus is a virus whose genetic material consists of RNA, a molecular cousin of DNA. Many examples of these pathogens are known on land, some of which are known to infect humans. Coronaviruses and influenza viruses are examples. On the other hand, those that evolve in the oceanic environment are still very little known. How many ? Which hosts do they infect?
Earlier this year, a team of researchers led by Guillermo Dominguez-Huerta of Ohio State University reported finding more than 5,500 previously unidentified RNA viruses in the world’s oceans. For this work, published in early April in the journal Science, the researchers analyzed 35,000 samples of water collected from 121 sites in the five oceans of the world. These water samples teemed with plankton, small organisms at the bottom of the food chain. The latter also often serve as hosts for RNA viruses.
To identify the presence of the pathogens, the team sifted through all the RNA in the plankton cells to find a specific piece of genetic code, called the RdRp gene. It is a coding sequence common to all RNA viruses. Eventually, the researchers had identified so many new RNA viruses that she had proposed double the number of known phyla (large taxonomic categories) to be able to classify them all.
A role in the carbon cycle
Since then, researchers have tried to better understand how these viruses are distributed in the ocean world, but also the hosts they infect. In a new study published in the journal Science, they determined that viral communities can be classified into four main areas : Arctic, Antarctic, temperate and tropical exopelagic zone (near the surface), and temperate and tropical exopelagic zone (between 200 and 1,000 meters). Interesting point: it looked like the variety of viruses greater in the polar areasalthough there is a greater variety of hosts to be infected in warmer waters.
To identify these hosts, the team used several advanced strategies. These analyzes ultimately revealed that many RNA viruses in the ocean infect fungi and protists. Some also infect invertebrates, while a very small fraction infects bacteria.
However, fungi and protists are known, which include algae and amoebas extract carbon dioxide from the atmosphere. Through their actions, these organisms then influence the amount of carbon that ends up being stored in the ocean.
Indeed, the team unexpectedly discovered that many of these viruses carried ‘stolen’ genes from their host cells, thus disrupting their metabolism in one way or another, likely to maximize the production of new viral particles. By infecting these hosts, RNA viruses would then also affect the way the carbon circulates in the ocean. This new study therefore suggests that RNA virus infection of marine organisms could be a hitherto unrecognized factor that should finally be considered in climate change models.