Sciences. Marine viruses, a “dark matter” that affects the climate

The ocean is dominated by communities of plankton (1) that are essential to support life on Earth. By producing energy and organic matter through photosynthesis, they generate as much oxygen and absorb as much carbon dioxide as trees in forests. Plankton are also at the base of the maritime food chain and therefore indirectly terrestrial.

At the heart of the biological carbon pump

Its impact is fundamental because it controls the biogeochemical cycles of our planet, that is, the processes of transport and transformation of a chemical element (carbon for example) between the great reservoirs of our planet: the earth (geosphere), the air (atmosphere) and the oceans (hydrosphere).

Thus, plankton appear at the heart of the biological carbon pump. Its activity determines whether dissolved carbon dioxide is assimilated into biomass (sequestered in the depths when plankton die), recycled to surface waters, or released into the atmosphere. However, this plankton is sensitive to viral infections…

icon quoteTHE MASSIVE DEATH OF CELLS ADDS A “SNOW” MADE UP OF CARBON, WHICH FALLS TO THE FLOOR OF THE OCEAN. Chris Bowler, biologist

In the world of marine microbiology, DNA viruses are increasingly well known. They influence marine microorganisms and therefore their associated biogeochemical cycles. But other marine viruses, this time RNA viruses (Sars-CoV-2 is, for example, an RNA virus) are much less well known despite being abundant in the ocean.

A living cell remains on the surface.

“We don’t know much about these viruses that we call ‘dark matter’ from the ocean. Its diversity and its role in marine ecosystems are a great unknown”, explains Chris Bowler, head of the algae and plant genomics laboratory at the École Normale Supérieure in Paris. With colleagues from around the world, he participated in a study recently published in the journal “Science” (2), which aims to characterize the first marine RNA viruses.

To do this, the researchers used samples taken by the Tara Ocean Expedition between 2009 and 2013. Why ten years later? “Characterizing RNA viruses is much more complex than characterizing DNA viruses. It is our colleagues in the United States who have developed methods to do this. They have developed protocols to study them”, continues Chris Bowler. So much so that, in his publication, 5,500 different RNA viruses are characterized. By infecting photosynthetic organisms, viruses impact the carbon cycle.
It is through this cycle that the transfer of carbon from the atmosphere to the bottom of the oceans will occur. A living cell remains on the surface. When “the virus kills the host cell, it explodes and the carbon captured during photosynthesis is released into the environment. This massive death of cells can cause an aggregation of organic matter, made up of carbon, and generate what is called “marine snow” that will fall to the bottom of the ocean”, specifies the researcher.

This is, among other things, how RNA viruses affect the carbon cycle and thus climate change. But it is a fact that is still too little known to complete the publication. These characterizations provide a roadmap for studying RNA viruses in nature and their role in the evolution of life on Earth (read “the view” below).

(1) The term plankton designates all organisms that are carried by currents.
(2) “Diversity and ecological footprint of global oceanic RNA viruses”, “Science”, 2022.


The point of view

“Marine RNA viruses are new to science”

Chris Bowler, biologist, director of research at CNRS and ENS, head of the plant and algae genomics laboratory.

How do these RNA viruses behave?

They are close to DNA viruses. Only the genetic inheritance changes. In terms of infectivity, the cycle is very similar. They are no more “nasty” than DNA viruses. In humans, HIV, influenza or Sars-CoV-2 are RNA viruses. They “love” humans. There is no distinction, no bias in interactions with hosts.

Is there a direct link between viruses and CO?two?

No, the link is indirect, positive or negative. By infecting organisms involved in photosynthesis, killing cells, and causing “sea snow,” carbon can be transported to the ocean floor.

If the infected organism is large, the contribution to carbon transport to the ocean floor will potentially be greater because the organisms are heavier and will sink. Thus, diatoms (microalgae) are 1,000 times larger than bacteria. So a virus that infects a bacterium or a diatom will have very different impacts because the amount of carbon released is different.

But, in the same way, by killing the cells, there is less left for photosynthesis and therefore less carbon is absorbed, it is the negative aspect. Although viruses do not photosynthesize, even though viruses are not alive, they have the potential to impact the carbon cycle and therefore climate change.

Are not all organisms infected in the same way?

We lack host/pathogen models to study infections in the laboratory. We have described 5,500 types of RNA viruses, but it is not clear who infects whom. Algae, zooplankton, fish? We still have to develop techniques to better understand these interactions. We could grow diatoms, find the viruses that infect them, characterize them and better understand the processes.

Furthermore, the vast majority of marine RNA viruses are new to science. They probably represent very old lineages, they are different from the “classical” RNA viruses that we know. We see the entire evolution of life on Earth through the study of these viruses.

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