RNA viruses not only trigger infectious diseases such as Covid-19 or flu and rabies. They also play an essential role in the world’s oceans. There, the microbes influence the process of photosynthesis, which is the basis of life for most of life on earth, the carbon cycle and thus also the climate worldwide.

A research team led by Matthew Sullivan, Guillermo Dominguez-Huerta and Ahmed Zayed from Ohio State University concludes this from analyzing the genome of marine RNA viruses that they found in seawater samples. The results of the study have now been published in the journal Science.

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“This study shows once again how little we know about the oceans,” explains Timo Moritz from the German Maritime Museum in Stralsund, who was not involved in the study. “This applies not only to the deep sea, but also to some extent to very fundamental processes in the surface waters.”

The water samples for the analysis were collected during a circumnavigation of the world with the French research ship “Tara” in the years 2009 to 2013. On this expedition, the researchers took water samples from depths of up to 1,000 meters at 210 points. From this, they isolated the genome of many known and thousands of previously unknown RNA viruses. They presented the first results in April in “Science”. In their new publication, the US researchers analyze the role played by these pathogens in the world’s oceans.

At first glance, viruses shouldn’t actually be global players in nature because they can’t reproduce themselves, but need other organisms to do so. But they are omnipresent. According to the new analyses, RNA viruses in the world’s oceans mainly have algae and fungi as hosts. Many of these organisms float in the water with little or no propulsion and are passively transported by currents. They belong to the “plankton”, which comes from the Greek and means “that which is driven about”.

Like land plants, algae and other plankton organisms carry out photosynthesis and form biomolecules from carbon dioxide, water and some trace elements. Plankton produces around half of the biomass on earth. However, the RNA viruses intervene in the metabolism of their host organisms in order to get them to produce new viruses. In doing so, they can also reprogram photosynthesis, essentially changing the basis of life, Sullivan and his team explain. This in turn influences the climate because algae and higher plants use the greenhouse gas carbon dioxide from the air and water to produce biomass.

If the pathogens weaken algae, bioproduction falters and takes less carbon dioxide out of the air. The plankton forms less biomass, from which fewer animals can then feed. The climate balance is also changing: many organisms are decomposed by fungi and microorganisms after they die. Most of the carbon contained in the air is released back into the air in the form of carbon dioxide and heats up the climate. Some of the organisms, however, sink to the bottom of the sea. The carbon transported with them thus disappears from the climate system in the long term.

Sullivan and his team use the frequency of RNA viruses to draw conclusions about this process, known as the “carbon pump”. The RNA viruses in the water therefore have a particularly strong influence on the ecosystem of the polar seas. The team suspects that the viruses could stay there longer in the cold and thus improve their chances of finding a host. In warm water, on the other hand, viruses are decomposed more quickly.

Predatory fish apparently have a strong influence on the marine communities there. Gail Ashton and Gregory Ruiz from the Smithsonian Environmental Research Center in Tiburon, California, and their team also report this in the journal Science.

The group conducted standardized experiments at 36 locations along the Pacific and Atlantic coasts of North, Central and South America, from which they were able to conclude how much the predators affect bottom-seated organisms. Most bottom dwellers disappeared into the warm waters. The team suspects that if climate change continues to heat up the world’s oceans, the influence of predatory fish on ecosystems could increase.