Viruses aren’t all bad: In the ocean, some help fuel the food web – a new study shows how

Marine viruses play a crucial role in sustaining marine life by infecting microorganisms that form the base of the ocean food web, releasing nutrients and carbon into the water.
A new study found that the rate of virus infection in an oxygen-rich band of the Atlantic Ocean is about 4 times higher than in other parts of the surrounding ocean, with viruses causing massive infections in Prochlorococcus cells.
The viral infection stimulates photosynthesis and the growth of more Prochlorococcus cells, resulting in greater production of oxygen and a ribbon-like structure of oxygen-rich water.
This research highlights the importance of understanding the microscopic world, including the role of viruses in shaping ecosystems and storing carbon in the deep oceans.
The study was made possible by an open-ocean expedition supported by the National Science Foundation, adding to a growing range of studies that demonstrate the central role of viruses in ecosystem function.

A research ship sails in the Atlantic Ocean, where scientists are studying the roles of marine viruses. SW Wilhelm

Virus. The word evokes images of illness and fears of outbreaks. Yet, in the oceans, not all viruses are bad news.

Some play a helpful, even critical, role in sustaining marine life.

In a new study, we and an international team of scientists examined the behavior of marine viruses in a large band of oxygen-rich water just under the surface of the Atlantic Ocean. What we discovered there – and its role in the food web – shows marine viruses in a new light.

Studying something so tiny

Viruses are incredibly small, typically no more than tens of nanometers in diameter, nearly a hundred times smaller than a bacterium and more than a thousand times smaller than the width of a strand of hair.

In fact, viruses are so small that they cannot be seen using conventional microscopes.

Four highly magnified images show a tiny round object, the virus. In two of the images, the tail is visible. — An electron microscope view shows examples of *Prochlorococcus* myoviruses. Images A and D show different viruses with their tails. In B and C, the tail is contracted. The black scale bar indicates a length of 100 nanometers.
MB Sullivan, et al., 2005, PLOS One, CC BY

Decades ago, scientists thought that marine viruses were neither abundant nor ecologically relevant, despite the clear relevance of viruses to humans, plants and animals.

Then, advances in the use of transmission electron microscopes in the late 1980s changed everything. Scientists were able to examine sea water at a very high magnification and saw tiny, circular objects containing DNA. These were viruses, and there were tens of millions of them per milliliter of water – tens of thousands of times greater than had been estimated in the past.

A theory for how viruses feed the marine world

Most marine viruses infect the cells of microorganisms – the bacteria and algae that serve as the base of the ocean food web and are responsible for about half the oxygen generated on the planet.

By the late 1990s, scientists realized that virus activity was likely shaping how carbon and nutrients cycled through ocean systems. We hypothesized, in what’s known as the viral shunt model, that the marine viruses break open the cells of microorganisms and release their carbon and nutrients into the water.

This process could increase the amount of nutrients reaching marine phytoplankton. Phytoplankton provide food for krill and fish, which in turn feed larger marine life across the oceans. That would mean viruses are essential to a food web that drives a vast global fisheries and aquaculture industry producing nearly 200 million metric tons of seafood.

Watching viruses in action

In the new study in the journal Nature Communications led by biologists Naomi Gilbert and Daniel Muratore, our international team demonstrated the viral shunt in action.

The team took samples from a meters-thick band of oxygen that spreads for hundreds of miles across the subtropical Atlantic Ocean. In this region, part of the Sargasso Sea, single-celled cyanobacteria known as Prochlorococcus dominate marine photosynthesis with nearly 50,000 to upwards of 100,000 cells in every milliliter of seawater. These Prochlorococcus can be infected by viruses.

What are Prochlorococcus? Science Magazine.

By sequencing community RNA – molecules that carry genetic instructions within cells – our team was able to look at what nearly all viruses and their hosts were trying to do at once.

We found that the rate of virus infection in this oxygen-rich band of the ocean is about four times higher than in other parts of the surrounding ocean, where cyanobacteria don’t reproduce as quickly. And we observed viruses causing massive infections in Prochlorococcus.

The viruses were attacking cells and spilling organic matter, which bacteria were taking up and using to fuel new growth. The bacteria respired away the carbon and released nitrogen as ammonium. And this nitrogen appears to have been stimulating photosynthesis and the growth of more Prochlorococcus cells, resulting in greater production that generated the ribbon of oxygen.

The viral infection was having an ecosystem-scale impact.

Scientists aboard a research vessel prepare a large device with many tubes for collecting samples once lowered into the ocean. — Scientists aboard a National Science Foundation research expedition in the open Atlantic in 2019 prepare equipment to collect water samples at different depths to analyze the activity of marine viruses.
SW Wilhelm

Understanding the microscopic world matters

Viruses can cause acute, chronic and catastrophic effects on human and animal health. But this new research, made possible by an open-ocean expedition supported by the National Science Foundation, adds to a growing range of studies that demonstrate that viruses are central players in how ecosystems function, including by playing a role in storing carbon in the deep oceans.

We are living on a changing planet. Monitoring and responding to changes in the environment require an understanding of the microbes and mechanisms that drive global processes.

This new study is a reminder of how important it is to explore the microscopic world further – including the life of viruses that shape the fate of microbes and how the Earth system works.

Steven Wilhelm's work on this study was supported by The National Science Foundation, The National Institute of Environmental Health Science, the Simons Foundation and the Allen Family Philanthropies.

Joshua Weitz's work on this study was supported by The National Science Foundation, the Simons Foundation, and the Blaise Pascal Chair of the Île-de-Paris Region.

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Q. What is the common perception of viruses?

A. Viruses are often associated with illness and fears of outbreaks.

Q. How small are marine viruses compared to other microorganisms?

A. Marine viruses are incredibly small, typically no more than tens of nanometers in diameter, which is nearly a hundred times smaller than a bacterium and more than a thousand times smaller than the width of a strand of hair.

Q. What was previously thought about marine viruses?

A. Decades ago, scientists thought that marine viruses were neither abundant nor ecologically relevant.

Q. How did advances in technology change our understanding of marine viruses?

A. Advances in transmission electron microscopes in the late 1980s allowed scientists to examine sea water at a very high magnification and see tiny, circular objects containing DNA, which were later confirmed to be viruses.

Q. What is the role of marine viruses in the ocean food web?

A. Marine viruses infect the cells of microorganisms, such as bacteria and algae, which serve as the base of the ocean food web and are responsible for about half the oxygen generated on the planet.

Q. What is the viral shunt model, and how does it work?

A. The viral shunt model proposes that marine viruses break open the cells of microorganisms and release their carbon and nutrients into the water, which can increase the amount of nutrients reaching marine phytoplankton.

Q. How did the researchers in the study demonstrate the viral shunt in action?

A. The team took samples from a meters-thick band of oxygen-rich water in the Atlantic Ocean and sequenced community RNA to look at what nearly all viruses and their hosts were trying to do at once, finding that virus infection rates were about four times higher than in other parts of the surrounding ocean.

Q. What was observed during the study?

A. The researchers observed massive infections in Prochlorococcus cells caused by viruses, which led to the release of organic matter, taken up by bacteria and used to fuel new growth, resulting in greater production of oxygen.

Q. How does this research contribute to our understanding of ecosystems?

A. This study adds to a growing range of studies that demonstrate that viruses are central players in how ecosystems function, including playing a role in storing carbon in the deep oceans.

Q. Why is it important to explore the microscopic world further?

A. Understanding the microbes and mechanisms that drive global processes is crucial for monitoring and responding to changes in the environment, as we live on a changing planet.